Difference between revisions of "Lithofacies and environmental analysis of clastic depositional systems"

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The geological and reservoir properties of sedimentary rocks depend upon an interplay of tectonics, sea level, sediment supply, physical and biological processes of sediment transport and deposition, and climate. At the basin scale, these processes interact to produce the geometric arrangement of different depositional environments or systems tracts through time, known as the stratigraphic architecture of the basin<ref name=pt06r88>Miall, A. D., 1984, Principles of Sedimentary Basin Analysis: New York, Springer-Verlag, 490 p.</ref>. At smaller scales, these processes control the external geometry and internal “anatomy” of clastic sediment bodies (see “[[Geological heterogeneities]]”). It is at this smaller scale that lithofacies analysis and interpretation of depositional environments become important for reservoir evaluation.
+
The geological and [[Petroleum reservoir fluid properties|reservoir properties]] of sedimentary rocks depend upon an interplay of [[tectonics]], [[Sea level cycle phase|sea level]], [[Depocenter#Siliciclastic vs. carbonate supply|sediment supply]], physical and biological processes of [[sediment transport]] and deposition, and [[Sea level cycles and climate|climate]]. At the basin scale, these processes interact to produce the geometric arrangement of different [[depositional environments]] or [[Definitions of depositional system elements#Systems tracts|systems tracts]] through time, known as the [[stratigraphic architecture]] of the basin.<ref name=pt06r88>Miall, A. D., 1984, Principles of Sedimentary Basin Analysis: New York, Springer-Verlag, 490 p.</ref> At smaller scales, these processes control the external geometry and internal “anatomy” of clastic sediment bodies (see [[Geological heterogeneities]]). It is at this smaller scale that lithofacies analysis and interpretation of depositional environments become important for [[Predicting reservoir system quality and performance|reservoir evaluation]].
  
 
==Data requirements==
 
==Data requirements==
  
−
Basic data requirements for facies analysis of subsurface rocks are listed in Table 1. Data associated with wells are most often used, but seismic data, particularly three-dimensional data, are becoming increasingly important in defining sandstone body geometries (e.g., see <ref name=pt06r17>Brown, A. R., 1986 Interpretation of three-dimensional seismic data: AAPG Memoir 42, 194 p.</ref>; also see the chapter on “[[Three-dimensional seismic method]]s for Reservoir Development” in Part 7). Conventional core is perhaps the most diagnostic for sedimentological interpretation of vertical sequences (see “[[Core description]]”). However, wireline tools such as [[dipmeters]] and formation imaging devices can provide electrical images suitable for sedimentological interpretation with the added ability to determine paleocurrent directions in appropriate cases (see “[[Dipmeters]]” and “[[Borehole imaging devices]]”).
+
Basic data requirements for [[facies analysis]] of subsurface rocks are listed in Table 1. Data associated with wells are most often used, but [[seismic data]], particularly [[3-D seismic: the data cube|three-dimensional data]], are becoming increasingly important in defining [[sandstone]] body geometries.<ref name=pt06r17>Brown, A. R., 1986 Interpretation of three-dimensional seismic data: [http://store.aapg.org/detail.aspx?id=1025 AAPG Memoir 42], 194 p.</ref> Conventional core is perhaps the most diagnostic for sedimentological interpretation of vertical sequences (see [[Core description]]). However, wireline tools such as [[dipmeters]] and [[Borehole imaging devices|formation imaging devices]] can provide electrical images suitable for sedimentological interpretation with the added ability to determine [[paleocurrent]] directions in appropriate cases.
  
 
{| class = "wikitable"
 
{| class = "wikitable"
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! Application
 
! Application
 
|-
 
|-
−
| Slabbed conventional core
+
| [[Overview of routine core analysis#Whole core analysis|Slabbed conventional core]]
 
| Facies, depositional environment
 
| Facies, depositional environment
 
|-
 
|-
−
| Oriented conventional core
+
| [[Core orientation|Oriented conventional core]]
−
| Paleocurrent directions
+
| [[Paleocurrent]] directions
 
|-
 
|-
−
| Core gamma scan
+
| [[Core gamma scan]]
 
| Shift to wireline logs
 
| Shift to wireline logs
 
|-
 
|-
−
| Sidewall cores, well cuttings, thin sections
+
| [[Overview of routine core analysis#Sidewall core analysis|Sidewall cores]], [[Mudlogging: drill cuttings analysis|well cuttings]], [[Thin section analysis|thin sections]], [[sand peel]]s
−
| Mineralogy, lithology
+
| [[Mineralogy]], lithology
 
|-
 
|-
−
| [[Paleontology]] (micro, macro, trace fossil), palynology
+
| [[Paleontology]] ([[Microfossils in exploration|micro]], [[Macrofossils|macro]], [[trace fossils]]), [[palynology]]
−
| Water depth, depositional environment, time lines
+
| [[Paleobathymetry|Water depth]], depositional environment, [[time lines]]
 
|-
 
|-
−
| Dipmeter log
+
| [[Dipmeter]] log
 
| Paleocurrent directions, lithofacies
 
| Paleocurrent directions, lithofacies
 
|-
 
|-
−
| Formation MicroScanner (FMS) log
+
| [[Formation MicroScanner]] (FMS) log
 
| Paleocurrent directions, lithofacies
 
| Paleocurrent directions, lithofacies
 
|-
 
|-
−
| Spontaneous potential log
+
| [[Basic open hole tools#Spontaneous potential|Spontaneous potential]] log
−
| Lithology, curve shape analysis
+
| Lithology, [[Quick-look lithology from logs#Log shapes|curve shape]] analysis
 
|-
 
|-
−
| Gamma ray log
+
| [[Basic open hole tools#Gamma ray|Gamma ray]] log
−
| Lithology, curve shape analysis
+
| Lithology, [[Quick-look lithology from logs#Log shapes|curve shape]] analysis
 
|-
 
|-
−
| Sonic log
+
| [[Basic open hole tools#Sonic|Sonic]] log
−
| [[Porosity]], curve shape analysis
+
| [[Porosity]], [[Sonic log curve shape|curve shape]] analysis
 
|-
 
|-
−
| Caliper log
+
| [[Basic open hole tools#Calipers|Caliper]] log
−
| Borehole condition, quality control
+
| [[Drilling problems#Hole instability|Borehole condition]], quality control
 
|-
 
|-
−
| Neutron log
+
| [[Basic open hole tools#Compensated neutron|Neutron]] log
−
| Porosity, lithology (with sonic, density)
+
| Porosity, [[Quick-look lithology from logs|lithology]] (with sonic, density)
 
|-
 
|-
−
| Density log
+
| [[Basic open hole tools#Density|Density]] log
−
| Lithology (coal), [[porosity]] (with neutron)
+
| Lithology ([[coal]]), [[porosity]] [[Quick-look lithology from logs#Neutron and density logs combined|(with neutron)]]
 
|-
 
|-
−
| Repeat formation test (RFT)
+
| [[Wireline formation testers|Repeat formation test (RFT]])
 
| Pressure (sand body connectedness)
 
| Pressure (sand body connectedness)
 
|}
 
|}
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==Rock description==
 
==Rock description==
  
−
Within any given depositional environment, various physical and biological processes act to transport and deposit sediment. These processes result in various distributions of grain size and sedimentary structures that characterize the deposited sediment. Relating these features back to the processes that produced them is the basic method used by geologists to interpret the depositional environment of sedimentary sequences (Figure 1).
+
[[file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig1.png|300px|thumb|{{figure number|1}}Sedimentary processes, lithofacies, and lithofacies associations for a meandering channel sequence. (The vertical sequence is modified from Walker.<ref name=pt06r147>Walker, R. G., ed., 1984, Facies Models, 2nd ed.: Geoscience Canada Reprint Series 1, 317 p.</ref>)]]
  
−
[[file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig1.png|thumb|{{figure number|1}}Sedimentary processes, lithofacies, and lithofacies associations for a meandering channel sequence. (The vertical sequence is modified from <ref name=pt06r147>Walker, R. G., ed., 1984, Facies Models, 2nd ed.: Geoscience Canada Reprint Series 1, 317 p.</ref>.)]]
+
Within any given depositional environment, various physical and biological processes act to transport and deposit sediment. These processes result in various distributions of [[grain size]] and [[sedimentary structures]] that characterize the deposited sediment. Relating these features back to the processes that produced them is the basic method used by geologists to interpret the depositional environment of sedimentary sequences ([[:file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig1.png|Figure 1]]).
  
 
===Lithofacies===
 
===Lithofacies===
  
−
One of the first steps in the facies analysis of a clastic reservoir is the description and interpretation of available conventional core<ref name=pt06r119>Siemers, C. T., Tillman, R. W., 1981, Recommendations for the proper handling of cores and sedimentological analysis of core sequences, in Siemers, C. T., Tillman, R. W., Williamson, C. R., eds., Deep-Water Clastic Sediments—A Core Workshop: SEPM Core Workshop, n. 2, p. 20–44.</ref>. An important result of [[core description]] is the subdivision of cores into ''lithofacies'', defined as subdivisions of a sedimentary sequence based on lithology, grain size, physical and biogenic sedimentary structures, and stratification that bear a direct relationship to the depositional processes that produced them. Lithofacies and lithofacies associations (groups of related lithofacies) are the basic units for the interpretation of depositional environments.
+
One of the first steps in the [[facies analysis]] of a clastic reservoir is the description and interpretation of available conventional core.<ref name=pt06r119>Siemers, C. T., and R. W. Tillman, 1981, Recommendations for the proper handling of cores and sedimentological analysis of core sequences, in C. T. Siemers, R. W. Tillman, and C. R. Williamson, eds., Deep-Water Clastic Sediments—A Core Workshop: SEPM Core Workshop, n. 2, p. 20–44.</ref> An important result of [[core description]] is the subdivision of cores into ''lithofacies'', defined as subdivisions of a sedimentary sequence based on lithology, grain size, physical and biogenic sedimentary structures, and stratification that bear a direct relationship to the depositional processes that produced them. Lithofacies and lithofacies associations (groups of related lithofacies) are the basic units for the interpretation of depositional environments.
  
 
===Depositional environments===
 
===Depositional environments===
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Interpretation of the environment in which lithofacies were deposited from analysis of cored sequences involves relating the identified lithofacies to the physical and biological processes that produced them. This process-response relationship identifies the specific processes responsible for the sequence and, by inference, the depositional setting in which these processes occurred. The application of the process-response approach relies primarily on depositional models constructed through study of both modern and ancient analogs.
 
Interpretation of the environment in which lithofacies were deposited from analysis of cored sequences involves relating the identified lithofacies to the physical and biological processes that produced them. This process-response relationship identifies the specific processes responsible for the sequence and, by inference, the depositional setting in which these processes occurred. The application of the process-response approach relies primarily on depositional models constructed through study of both modern and ancient analogs.
  
−
Depositional models are important for predicting the distribution of [[permeability]] and porosity within different reservoir types. These models are never exact matches to a reservoir; rather, they serve as guides to aid in the interpretation of any one reservoir<ref name=pt06r147 />). Reservoir properties are generally observed to be correlative with lithofacies types to one degree or another (see “Geological Heterogeneities”). This reflects the fundamental control on permeability and porosity by grain size, sorting, and spatial distribution of different lithofacies types. Even where rocks have experienced later physical and chemical diagenesis, permeability and porosity relationships are controlled, in large part, by the original sedimentary fabric of the rock.
+
Depositional models are important for predicting the distribution of [[permeability]] and [[porosity]] within different reservoir types. These models are never exact matches to a reservoir; rather, they serve as guides to aid in the interpretation of any one reservoir.<ref name=pt06r147 /> Reservoir properties are generally observed to be correlative with lithofacies types to one degree or another (see [[Geological heterogeneities]]). This reflects the fundamental control on permeability and porosity by [[grain size]], [[Core_description#Maturity|sorting]], and spatial distribution of different lithofacies types. Even where rocks have experienced later physical and chemical [[diagenesis]], permeability and porosity relationships are controlled, in large part, by the original sedimentary fabric of the rock.
  
 
===Wireline log calibration and correlation===
 
===Wireline log calibration and correlation===
  
−
Interpretations of depositional environment based on individual well data are transformed into a three-dimensional picture of the reservoir by wireline log correlation and, where possible, by three-dimensional seismic data.
+
[[file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig2.png|300px|thumb|{{figure number|2}}Gamma ray correlation (dip section) of a series of prograding shoreface sandstones. Note the imbricate nature of the sandstone bodies and the “non-layer cake” nature of the correlations.]]
  
−
Wireline logs to be used for facies analysis should, whenever possible, always be calibrated by core. This calibration involves (1) shifting core to log depths (see “Preprocessing of Logging Data”) and “Core-Log Transformations and Porosity-[[Permeability]] Relationships” in Part 5) and (2) establishing a relationship between lithofacies associations and curve shape. Core gamma scans, obtained by passing the core through a device that measures the natural radioactivity of the rock, are particularly useful for shifting cores to logs. The calibration of wireline log shape by core is particularly important for firmly establishing the log response and the identity of vertical sequences on these logs.
+
Interpretations of depositional environment based on individual well data are transformed into a three-dimensional picture of the reservoir by wireline log correlation and, where possible, by [[3-D seismic: the data cube|three-dimensional seismic data]].
  
−
For reservoirs in which no core is available, wireline log shape must be used to interpret sandstone body type and identify depositional environments. If closely spaced cuttings or sidewall cores are available, these can sometimes aid rock to log calibration. Log shapes are deduced from the expected wireline log response of the different environments combined with a knowledge of the paleogeography of the area in which the field is situated. Wireline log shapes are often described as “upward coarsening,” “upward fining,” or “blocky.” However, log shape as determined from a gamma ray or SP log in siliciclastic rocks is related more to argillaceous content than to grain size. Upward coarsening log patterns exhibit an upward decrease in argillaceous content. Upward fining log patterns exhibit the reverse trend. Blocky or cylindrical log patterns exhibit relatively little vertical variation in argillaceous content and are typical of siliciclastic rocks that have low overall argillaceous content. Various publications and reference charts are available to aid in this practice (e.g., <ref name=pt06r132>Spearing, D. R., 1974, Summary sheets of sedimentary deposits: Geological Society of American Publication MC-8.</ref><ref name=pt06r18>Cant, D. J., 1984, Subsurface facies analysis, in Walker, R. G., ed., Facies Models: Geoscience Canada, Reprint Series 1, p. 297–319.</ref><ref name=pt06r110>Rider, M. H., 1986, Geological interpretation of well logs: New York, John Wiley, 175 p.</ref>. However, without core control, curve shape analysis is fraught with hazards (e.g., see <ref name=pt06r125>Snedden, J. W., 1987, Validity of the use of the spontaneous potential curve shape in the interpretation of sandstone depositional environments, in White, B. R., Kier, R. eds., Transactions of the 34th annual meeting of the Gulf Coast Association of Geological Societies and 31st annual meeting of the Gulf Coast Section of SEPM, v. 34, p. 255–263.</ref>; also see the chapter on “Quick-Look Lithology from Logs” in Part 4).
+
Wireline logs to be used for facies analysis should, whenever possible, always be calibrated by core. This calibration involves (1) shifting core to log depths (see [[Preprocessing of logging data]] and [[Core-log transformations and porosity-permeability relationships]]) and (2) establishing a relationship between lithofacies associations and [[Quick-look lithology from logs|curve shape]]. [[Core gamma scans]], obtained by passing the core through a device that measures the natural radioactivity of the rock, are particularly useful for shifting cores to logs. The calibration of wireline log shape by core is particularly important for firmly establishing the log response and the identity of vertical sequences on these logs.
  
−
Correlation sections that will be used for establishing sandstone body geometry should have a depositionally flat datum (such as a bentonite bed, marine shale bed, or laterally persistent limestone). Sections should be oriented parallel and perpendicular to depositional strike, if known, and represent as straight a line as possible given well density and placement.
+
For reservoirs in which no core is available, wireline log shape must be used to interpret sandstone body type and identify depositional environments. If closely spaced cuttings or sidewall cores are available, these can sometimes aid rock to log calibration. Log shapes are deduced from the expected wireline log response of the different environments combined with a knowledge of the [[paleogeography]] of the area in which the field is situated. Wireline log shapes are often described as “upward coarsening,” “upward fining,” or “blocky.” However, log shape as determined from a [[Basic open hole tools#Gamma ray|gamma ray]] or [[Basic open hole tools#Spontaneous potential|SP]] log in siliciclastic rocks is related more to argillaceous content than to [[grain size]]. Upward coarsening log patterns exhibit an upward decrease in argillaceous content. Upward fining log patterns exhibit the reverse trend. Blocky or cylindrical log patterns exhibit relatively little vertical variation in argillaceous content and are typical of siliciclastic rocks that have low overall argillaceous content. Various publications and reference charts are available to aid in this practice (e.g., Spearing,<ref name=pt06r132>Spearing, D. R., 1974, Summary sheets of sedimentary deposits: Geological Society of American Publication MC-8.</ref> Cant,<ref name=pt06r18>Cant, D. J., 1984, Subsurface facies analysis, in Walker, R. G., ed., Facies Models: Geoscience Canada, Reprint Series 1, p. 297–319.</ref> and Rider<ref name=pt06r110>Rider, M. H., 1986, Geological interpretation of well logs: New York, John Wiley, 175 p.</ref>). However, without core control, curve shape analysis is fraught with hazards (e.g., Snedden;<ref name=pt06r125>Snedden, J. W., 1987, Validity of the use of the spontaneous potential curve shape in the interpretation of sandstone depositional environments, in White, B. R., Kier, R. eds., Transactions of the 34th annual meeting of the Gulf Coast Association of Geological Societies and 31st annual meeting of the Gulf Coast Section of SEPM, v. 34, p. 255–263.</ref> also see [[Quick-look lithology from logs]]).
  
−
The only sedimentologically significant correlation horizons are those that approximate time lines within and between sandstone bodies. This style of correlation requires an understanding of the succession of depositional environments and intervening unconformable surfaces. It often leads to nonparallel and nonhorizontal correlations. For example, in shoreface systems, time lines denoted by shale or silt breaks between shingled shoreface sheets and lenses are inclined in a seaward (depositional dip) direction (Figure 2).
+
Correlation sections that will be used for establishing sandstone body geometry should have a depositionally flat datum (such as a [[bentonite]] bed, marine shale bed, or laterally persistent [[limestone]]). Sections should be oriented parallel and perpendicular to depositional [[strike]], if known, and represent as straight a line as possible given well density and placement.
  
−
[[file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig2.png|thumb|{{figure number|2}}Gamma ray correlation (dip section) of a series of prograding shoreface sandstones. Note the imbricate nature of the sandstone bodies and the “non-layer cake” nature of the correlations.]]
+
The only sedimentologically significant correlation horizons are those that approximate time lines within and between sandstone bodies. This style of correlation requires an understanding of the succession of depositional environments and intervening [[Unconformity|unconformable surfaces]]. It often leads to nonparallel and nonhorizontal correlations. For example, in [[Lithofacies and environmental analysis of clastic depositional systems#Shoreline deposits|shoreface]] systems, time lines denoted by shale or silt breaks between shingled shoreface sheets and lenses are inclined in a seaward (depositional [[dip]]) direction ([[:file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig2.png|Figure 2]]).
  
 
This imbrication does not occur in a strike direction. This style of correlation is especially important for reservoir delineation since the large scale (interwell and field) architecture of the sandstone body exerts a control on the movement of fluids through the volume of the reservoir.
 
This imbrication does not occur in a strike direction. This style of correlation is especially important for reservoir delineation since the large scale (interwell and field) architecture of the sandstone body exerts a control on the movement of fluids through the volume of the reservoir.
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==Clastic depositional lithofacies and environments==
 
==Clastic depositional lithofacies and environments==
  
−
Clastic depositional environments range from alpine to abyssal settings (Figure 3 and Table 2). Detailed reviews of these are given by Galloway and Hobday<ref name=pt06r36>Galloway, W. E., Hobday, D. K., 1983, Terrigenous Clastic Depositional Systems Applications to Petroleum, Coal, and Uranium Exploration: New York, Springer Verlag, 423 p.</ref>, Walker<ref name=pt06r147 />), Berg<ref name=pt06r14>Berg, R. R., 1986, Reservoir Sandstones: Englewood Cliffs, N., J., Prentice-Hall, 481 p.</ref>, Reading<ref name=pt06r107>Reading, H. G., ed., 1986, Sedimentary Environments and Facies, 2nd ed.: Boston, MA, Blackwell Scientific Publications, 615 p.</ref>, Beaumont and Foster<ref name=pt06r11>Beaumont, E. A., Foster, N. H., 1987, Reservoirs II—Sandstones: AAPG Treatise of Petroleum Geology Reprint Series, n. 4, 573 p.</ref>, and others. The following review is a cursory summary of the origin, lithofacies, geometry, and reservoir properties of major clastic environments and deposits. The reader should be aware that the remarks offered here for each depositional environment are necessarily of a highly generalized and idealized nature. Siliciclastic reservoirs are typically composed of multiple bodies deposited (and eroded) through time under varying tectonic, sealevel, and climatic conditions. Corresponding geometry, vertical sequence, wireline log character, and [[reservoir quality]] trends for a given reservoir may be, and often are, different from the generalized “single environment” models. In addition, subsequent diagenesis (see “[[Evaluating diagenetically complex reservoirs]]”) may alter the permeability and porosity structure created by depositional (and erosional) processes. However, it has been often observed that in siliciclastic rocks, diagenesis generally follows depositional fabric (see “Geological Heterogeneities”). Complex structural patterns can reduce reservoir continuity as well.
+
[[file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig3.png|300px|thumb|{{figure number|3}}Models of major depositional environments. The curve on the left shows the [[Basic open hole tools#Spontaneous potential|SP]] or gamma ray response and the curve on the right shows the relative grain size profile. The size of the dots next to the vertical profile indicates the relative magnitude of permeability expected in such a sequence. (Parts c and d are from Berg,<ref name=pt06r147 /> and parts f, h, and i are from Galloway and Hobday.<ref name=pt06r36 />]]
  
−
{| class = "wikitable"
+
Clastic depositional environments range from alpine to abyssal settings ([[:file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig3.png|Figure 3]] and Table 2). Detailed reviews of these are given by Galloway and Hobday,<ref name=pt06r36>Galloway, W. E., and D. K. Hobday, 1983, Terrigenous Clastic Depositional Systems Applications to Petroleum, Coal, and Uranium Exploration: New York, Springer Verlag, 423 p.</ref> Walker,<ref name=pt06r147 /> Berg,<ref name=pt06r14>Berg, R. R., 1986, Reservoir Sandstones: Englewood Cliffs, NJ, Prentice-Hall, 481 p.</ref> Reading,<ref name=pt06r107>Reading, H. G., ed., 1986, Sedimentary Environments and Facies, 2nd ed.: Boston, MA, Blackwell Scientific Publications, 615 p.</ref> Beaumont and Foster,<ref name=pt06r11>Beaumont, E. A., and N. H. Foster, 1987, Reservoirs II—Sandstones: AAPG Treatise of Petroleum Geology Reprint Series No. 4, 573 p.</ref> and others. The following review is a cursory summary of the origin, lithofacies, geometry, and reservoir properties of major clastic environments and deposits. The reader should be aware that the remarks offered here for each depositional environment are necessarily of a highly generalized and idealized nature. Siliciclastic reservoirs are typically composed of multiple bodies deposited (and eroded) through time under varying [[Tectonics|tectonic]], [[Sea level cycle phase|sea level]], and [[Biogeography|climatic conditions]]. Corresponding geometry, vertical sequence, [[Quick-look lithology from logs|wireline log character]], and [[reservoir quality]] trends for a given reservoir may be, and often are, different from the generalized “single environment” models. In addition, subsequent [[diagenesis]] (see [[Evaluating diagenetically complex reservoirs]]) may alter the [[permeability]] and [[porosity]] structure created by depositional (and erosional) processes. However, it has been often observed that in siliciclastic rocks, diagenesis generally follows depositional fabric . Complex structural patterns can reduce reservoir continuity as well.
−
|-
 
−
|+ {{table number|2}}Major clastic depositional environments
 
−
|-
 
−
|-
 
−
| Alluvial sediments
 
−
|-
 
−
|    Alluvial fans
 
−
|-
 
−
|    Fan deltas
 
−
|-
 
−
|    Braidplains
 
−
|-
 
−
|    Braided rivers
 
−
|-
 
−
|    Meandering rivers
 
−
|-
 
−
| Lacustrine sediments
 
−
|-
 
−
|    Lake deltas
 
−
|-
 
−
|    Freshwater lakes
 
−
|-
 
−
|    Saline lakes
 
−
|-
 
−
| Eolian sediments
 
−
|-
 
−
|    Dunes
 
−
|-
 
−
|    Interdune deposits
 
−
|-
 
−
|    Sand sheets
 
−
|-
 
−
| Deltaic sediments
 
−
|-
 
−
|    River-dominated deltas
 
−
|-
 
−
|    Wave-dominated deltas
 
−
|-
 
−
|    Tidally dominated deltas
 
−
|-
 
−
| Siliciclastic shoreline sediment
 
−
|-
 
−
|    Wave-dominated shorelines
 
−
|-
 
−
|       Beaches
 
−
|-
 
−
|       Microtidal barrier islands
 
−
|-
 
−
|       Cheniers
 
−
|-
 
−
|    Mixed wave-tide influenced shorelines
 
−
|-
 
−
|       Mesotidal barrier islands
 
−
|-
 
−
|    Tide-dominated shorelines
 
−
|-
 
−
|    Tidal flats
 
−
|-
 
−
|    Estuaries
 
−
|-
 
−
| Shallow siliciclastic seas
 
−
|-
 
−
|    Tidal sand ridges
 
−
|-
 
−
|    Sand shoals
 
−
|-
 
−
|    Sand sheets
 
−
|-
 
−
| Deep marine slope and basin
 
−
|-
 
−
|    Slope channel and gully deposits
 
−
|-
 
−
|    Slope canyon deposits
 
−
|-
 
−
|    Intraslope basin deposits
 
−
|-
 
−
|    Sand spillover sheets
 
−
|-
 
−
|    Slope aprons
 
−
|-
 
−
|    Submarine fans
 
−
|-
 
−
| Glacial sediments
 
−
|-
 
−
|    Supraglacial
 
−
|-
 
−
|    Glaciofluvial
 
−
|-
 
−
|    Glacioeolian
 
−
|-
 
−
|    Glaciolacustrine
 
−
|-
 
−
|    Glaciomarine
 
−
|}
 
  
−
[[file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig3.png|thumb|{{figure number|3}}Models of major depositional environments. The curve on the left shows the SP or gamma ray response and the curve on the right shows the relative grain size profile. The size of the dots next to the vertical profile indicates the relative magnitude of permeability expected in such a sequence. (Parts c and d are from <ref name=pt06r147 />, and parts f, h, and i are from <ref name=pt06r36 />.)]]
+
'''Major clastic depositional environments'''
 +
* [[Alluvial]] sediments
 +
** Alluvial fans
 +
** Fan deltas
 +
** Braidplains
 +
** Braided rivers
 +
** Meandering rivers
 +
* Lacustrine sediments
 +
** Lake deltas
 +
** Freshwater lakes
 +
** Saline lakes
 +
* Eolian sediments
 +
** Dunes
 +
** Interdune deposits
 +
** Sand sheets
 +
* Deltaic sediments
 +
** River-dominated deltas
 +
** Wave-dominated deltas
 +
** Tidally dominated deltas
 +
* Siliciclastic shoreline sediment
 +
** Wave-dominated shorelines
 +
** Beaches
 +
** Microtidal barrier islands
 +
** Cheniers
 +
** Mixed wave-tide influenced shorelines
 +
** Mesotidal barrier islands
 +
** Tide-dominated shorelines
 +
** Tidal flats
 +
** Estuaries
 +
* Shallow siliciclastic seas
 +
** Tidal sand ridges
 +
** Sand shoals
 +
** Sand sheets
 +
* Deep marine slope and basin
 +
** Slope channel and gully deposits
 +
** Slope canyon deposits
 +
** Intraslope basin deposits
 +
** Sand spillover sheets
 +
** Slope aprons
 +
** Submarine fans
 +
* Glacial sediments
 +
** Supraglacial
 +
** Glaciofluvial
 +
** Glacioeolian
 +
** Glaciolacustrine
 +
** Glaciomarine
  
 
===Alluvial fan deposits===
 
===Alluvial fan deposits===
  
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An ''alluvial fan'' is a wedge of clastic detritus that forms at the base of a mountain front as sediments eroding from the mountains are transported downslope by streams or debris flows and deposited at the base (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3e</xref>). The fan-shaped body is generally characterized by a gradation from coarser sediments at the apex to finer sediments at the toe. Alluvial fans are commonly divided into proximal, mid-fan, and distal fan subenvironments.
+
An ''[[alluvial]] fan'' is a wedge of clastic detritus that forms at the base of a mountain front as sediments eroding from the mountains are transported downslope by streams or debris flows and deposited at the base ([[:file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig3.png|Figure 3e]]). The fan-shaped body is generally characterized by a gradation from coarser sediments at the apex to finer sediments at the toe. Alluvial fans are commonly divided into [[Proximal alluvial fan|proximal]], [[Middle alluvial fan|mid-fan]], and [[Distal alluvial fan|distal]] fan subenvironments.
  
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Vertical sequences through the proximal fan are generally dominated by gravelly deposits with subordinate sandy deposits. Sequences through the mid- and distal fan are increasingly sand dominated. Gamma ray, SP, and resistivity log responses throughout a fan can generally be expected to be blocky to irregular, depending on the amount of clay.
+
Vertical sequences through the proximal fan are generally dominated by gravelly deposits with subordinate sandy deposits. Sequences through the mid- and distal fan are increasingly sand dominated. [[Basic open hole tools#Gamma ray|Gamma ray]], [[Basic open hole tools#Spontaneous potential|SP]], and [[Basic open hole tools#Resistivity|resistivity]] log responses throughout a fan can generally be expected to be blocky to irregular, depending on the amount of clay.
  
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Permeability and porosity of alluvial fan deposits vary greatly as a function of depositional process and differential response to diagenesis. In general, streamflow deposits have greater permeability and porosity than debris and mudflow deposits. Finer grained but better sorted distal fan deposits are highly permeable and porous. Because of increased sorting, mid- and distal parts of the fan probably have better and more predictable reservoir quality than proximal parts. Little is known of directional permeability within alluvial fan reservoirs, but paleochannels can be expected to act as preferred pathways of flow.
+
[[Permeability]] and [[porosity]] of alluvial fan deposits vary greatly as a function of depositional process and differential response to [[diagenesis]]. In general, [[streamflow deposits]] have greater permeability and porosity than [[debris flow deposits|debris]] and [[mudflow deposits]]. Finer grained but better [[Core_description#Maturity|sorted]] distal fan deposits are highly permeable and porous. Because of increased sorting, middle and distal parts of the fan probably have better and more predictable reservoir quality than proximal parts. Little is known of directional permeability within alluvial fan reservoirs, but paleochannels can be expected to act as preferred pathways of flow.
  
−
Where alluvial fans prograde into standing bodies of water (that is, oceans or lakes), they are called ''fan deltas''. The distal parts of these fans are generally much better sorted and cleaner as a result of reworking by wave and/or tidal processes. Proximal and mid-fan log responses are the same as alluvial fans. Log response in the distal part depends upon the intensity of wave and tidal processes and on whether the fan is actively prograding or being transgressed. Typically, distal parts will have an upward-coarsening gamma ray, SP, and resistivity log character. Barring adverse diagenetic effects, permeability can be expected to be much greater in marine than more proximal parts of the fan delta due to increased sorting, destruction of compositionally immature grains, and winnowing of fines. Directional permeability trends in distal parts may be different from more proximal locations because of different sand body trends between these different parts of the fan delta.
+
Where alluvial fans [[Well_log_sequence_analysis#Parasequence_stacking_patterns|prograde]] into standing bodies of water (that is, oceans or lakes), they are called ''[[fan deltas]]''. The distal parts of these fans are generally much better sorted and cleaner as a result of reworking by [[wave processes|wave]] and/or [[tidal processes]]. Proximal and mid-fan log responses are the same as alluvial fans. Log response in the distal part depends upon the intensity of wave and tidal processes and on whether the fan is actively prograding or being transgressed. Typically, distal parts will have an upward-coarsening gamma ray, SP, and resistivity log character. Barring adverse diagenetic effects, permeability can be expected to be much greater in marine than in more proximal parts of the fan delta because of increased sorting, destruction of [[grain maturity|compositionally immature grains]], and winnowing of fines. [[Directional permeability]] trends in distal parts may be different from more proximal locations because of different sand body trends between these different parts of the fan delta.
  
 
===Braided and meandering fluvial deposits===
 
===Braided and meandering fluvial deposits===
  
−
Downdip from alluvial fans, rivers typically grade first into braided channels then, farther down the alluvial valley toward the coastal plain, into meandering channels. These different channel types can occur in the same river system and produce distinctly different kinds of sandstone bodies.
+
[[Dip|Downdip]] from [[alluvial]] fans, rivers typically grade first into braided channels then, farther down the alluvial valley toward the coastal plain, into meandering channels. These different channel types can occur in the same river system and produce distinctly different kinds of sandstone bodies.
  
−
''Braided rivers'' and ''braidplains'' form elongate, tabular, sandy and gravelly deposits composed of braided, sand-filled channels and sand and gravel bars (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3c</xref>). They typically consist of coarse sand and gravel with relatively minor amounts of clay. Vertical sequences are composed of stacked, upward-fining channel sands and sand and gravel bars. Lateral trends in these deposits are dominated by an overall tabular geometry bounded by floodplain muds with an internally complex geometry of cross-cutting sands and gravels with subordinate mud-rich beds of varying thickness and dimension. Bar and channel deposits are typically elongate in the paleocurrent direction.
+
''[[Braided rivers]]'' and ''braidplains'' form elongate, tabular, sandy and gravelly deposits composed of braided, sand-filled channels and sand and gravel [http://geonames.usgs.gov/apex/f?p=136:8:0::::: bars] (Figure 3c). They typically consist of coarse sand and gravel with relatively minor amounts of clay. Vertical sequences are composed of stacked, upward-fining channel sands and sand and gravel bars. Lateral trends in these deposits are dominated by an overall tabular geometry bounded by [[floodplain]] muds with an internally complex geometry of cross-cutting sands and gravels with subordinate mud-rich beds of varying thickness and dimension. Bar and channel deposits are typically elongate in the paleocurrent direction.
  
−
''Meandering rivers'' are different in that sand is restricted to a single channel and surrounded by fine-grained sediments (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3d</xref>). Sand is concentrated mainly in the channel bottoms and point bars. A vertical sequence through such a channel system frequently has an upward-fining character, starting from the channel lag at the bottom and grading upward into deposits of the adjacent levee and floodplain. Individual meander belts are built of cross-cutting and stacked individual upward-fining sequences often separated laterally by meander loop cutoffs and clay plugs. Multiple meander belts are built by abandonment of an entire river segment (''avulsion'') and by establishment of a new section in another position on the floodplain.
+
''[[Meandering rivers]]'' are different in that sand is restricted to a single channel and surrounded by fine-grained sediments (Figure 3d). Sand is concentrated mainly in the channel bottoms and point bars. A vertical sequence through such a channel system frequently has an upward-fining character, starting from the [[channel lag]] at the bottom and grading upward into deposits of the adjacent [[levee]] and floodplain. Individual meander belts are built of cross-cutting and stacked individual upward-fining sequences often separated laterally by [[meander loop cutoff]]s and clay plugs. Multiple [[meander belts]] are built by abandonment of an entire river segment (''[[avulsion]]'') and by establishment of a new section in another position on the floodplain.
  
−
Gamma ray, SP, and resistivity logs through braided channel complexes generally have a blocky character, whereas individual meandering channels have an upward-fining signature except where stacked and cross-cut, where they may exhibit more complex wireline log signatures.
+
[[Basic open hole tools#Gamma ray|Gamma ray]], [[Basic open hole tools#Spontaneous potential|SP]], and [[Basic open hole tools#Resistivity|resistivity]] logs through braided channel complexes generally have a blocky character, whereas individual meandering channels have an upward-fining signature except where stacked and cross-cut, where they may exhibit more complex wireline log signatures.
  
−
The upward-fining character of fluvial channels tends to produce sandstone bodies that have their greatest permeability at the base of the body. However, the common stacking and cross-cutting of channels in both braided and meandering river deposits often produces a complex spatial distribution of permeability within the braided or meander belt. Preferred permeability pathways, and consequently, fluid flow, can be expected to follow the paleochannel direction<ref name=pt06r104>Qiu, Yinan, 1984, Depositional model, heterogeneous characteristics, and waterflood performance of sandstone reservoirs in a lake basin case study of oilfields, eastern China: Proceedings of the 11th World Petroleum Congress, v. 3, p. 113–125.</ref>.
+
The upward-fining character of fluvial channels tends to produce sandstone bodies that have their greatest [[permeability]] at the base of the body. However, the common stacking and cross-cutting of channels in both braided and meandering river deposits often produces a complex spatial distribution of permeability within the braided or meander belt. [[Directional permeability|Preferred permeability]] pathways, and consequently, fluid flow, can be expected to follow the paleochannel direction.<ref name=pt06r104>Qiu, Y., 1984, Depositional model, heterogeneous characteristics, and waterflood performance of sandstone reservoirs in a lake basin case study of oilfields, eastern China: Proceedings of the 11th World Petroleum Congress, v. 3, p. 113–125.</ref>
  
 
===Eolian deposits===
 
===Eolian deposits===
  
−
Eolian sands develop in arid settings and commonly form extensive, blanket-like deposits (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3b</xref>). Wind transport removes fines and produces rounded and extremely well sorted grains often leading to favorable reservoir quality.
+
Eolian sands develop in arid settings and commonly form extensive, blanket-like deposits ([[:file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig3.png|Figure 3b]]). Wind transport removes fines and produces rounded and extremely [[Core_description#Maturity|well sorted]] grains often leading to favorable reservoir quality.
  
−
This combination of widespread occurrence and good reservoir properties makes eolian sandstones attractive exploration targets and many hydrocarbon accumulations have been discovered in such deposits (see <ref name=pt06r2>Ahlbrandt, T. S., Fryberger, S. G., 1982, Introduction to eolian deposits, in Scholle, P. A., Spearing, D. eds., Sandstone Depositional Environments: A APG Memoir 31, p. 11–47.</ref>.
+
This combination of widespread occurrence and good reservoir properties makes eolian sandstones attractive exploration targets and many hydrocarbon accumulations have been discovered in such deposits.<ref name=pt06r2>Ahlbrandt, T. S., and S. G. Fryberger, 1982, [http://archives.datapages.com/data/specpubs/sandsto2/data/a058/a058/0001/0000/0011.htm Introduction to eolian deposits], in P. A. Scholle, and D. Spearing, eds., Sandstone Depositional Environments: [http://store.aapg.org/detail.aspx?id=627 AAPG Memoir 31], p. 11–47.</ref>
  
−
Eolian deposits include dune, interdune, sand sheets (marginal to dune complex), and extradune (noneolian) lateral deposits<ref name=pt06r2 />). Dune deposits comprise the major sedimentary bodies in eolian successions. All are characterized by large scale cross stratification in which foreset dips range up to 35°. Associated deposits may include those of wadi (fluvial), playa (lacustrine), and sabkha (arid tidal flat) origin.
+
Eolian deposits include [[dune]], [[interdune]]  (marginal to dune complex), and [[extradune]] (noneolian) [[lateral]] deposits.<ref name=pt06r2 /> Dune deposits comprise the major sedimentary bodies in eolian successions. All are characterized by large scale [[cross-stratification]] in which [[foreset]] dips range up to 35°. Associated deposits may include those of [[wadi]] (fluvial), [[playa]] (lacustrine), and [[sabkha]] (arid tidal flat) origin.
  
−
In the subsurface, eolian sandstones generally comprise thickly bedded sequences with few major interstratified shales. The sequences tend to be uniform and lack discernible coarsening- or fining-upward trends and, thus, exhibit blocky to weakly serrated gamma ray, SP, and resistivity log profiles The well-bedded and high angle cross stratified nature of eolian sandstones promotes reliable results from dipmeter logs. Dune and interdune deposits can often be distinguished and paleowind directions inferred using correctly processed dipmeter data (<ref name=pt06r80>Lupe, R., Ahlbrandt, T. S., 1979, Sediments of the ancient eolian environment—reservoir inhomogeneity, in McKee, E., D., ed., A Study of Global Sand Seas: U., S. Geological Survey Professional Paper 1052, p. 241–252.</ref>.
+
In the subsurface, eolian sandstones generally comprise thickly bedded sequences with few major interstratified shales. The sequences tend to be uniform and lack discernible coarsening- or fining-upward trends and, thus, exhibit blocky to weakly serrated [[Basic open hole tools#Gamma ray|gamma ray]], [[Basic open hole tools#Spontaneous potential|SP]], and [[Basic open hole tools#Resistivity|resistivity]] log profiles The well-bedded and high angle cross stratified nature of eolian sandstones promotes reliable results from [[Dipmeter analysis|dipmeter]] logs. Dune and interdune deposits can often be distinguished and paleowind directions inferred using correctly processed dipmeter data.<ref name=pt06r80>Lupe, R., and T. S. Ahlbrandt, 1979, Sediments of the ancient eolian environment—reservoir inhomogeneity, in E. D. McKee, ed., A Study of Global Sand Seas: [http://pubs.er.usgs.gov/publication/pp1052 U.S. Geological Survey Professional Paper 1052], p. 241–252.</ref>
  
−
Eolian sandstones generally comprise excellent reservoir intervals but often possess complex porosity and permeability variations. They are commonly anisotropic with regard to the flow of fluids and exhibit greater horizontal than vertical permeability because of their pronounced lamination<ref name=pt06r154>Weber, K. J., 1987, Computation of initial well productivities in aeolian sandstone on the basis of a geological model, Leman Gas field, U., K., in Tillman, R. W., Weber, K. J., eds., Reservoir Sedimentology: SEPM Special Publication 40, p. 335–354.</ref>.
+
Eolian sandstones generally comprise excellent reservoir intervals but often possess complex [[porosity]] and [[permeability]] variations. They are commonly anisotropic with regard to the flow of fluids and exhibit greater horizontal than vertical permeability because of their pronounced lamination.<ref name=pt06r154>Weber, K. J., 1987, Computation of initial well productivities in aeolian sandstone on the basis of a geological model, Leman Gas field, UK, in R. W. Tillman, and K. J. Weber, eds., Reservoir Sedimentology: [https://members.sepm.org/iMISpublic/Core/Orders/Default.aspx? SEPM Special Publication 40], p. 335–354.</ref>
  
 
===Deltas===
 
===Deltas===
Line 243: Line 194:
 
Deltaic bodies are generally classified into three major categories or end-members on the basis of the dominant sediment transport process that influences their facies constituents and external geometries. These three end-members are as follows:
 
Deltaic bodies are generally classified into three major categories or end-members on the basis of the dominant sediment transport process that influences their facies constituents and external geometries. These three end-members are as follows:
  
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* ''River- or fluvially dominated deltas'' (such as the Mississippi River delta) are those in which wave and tidal energy is low and river transport processes dominate. Sandstone bodies in these systems tend to form well-developed sand bars at the mouths of distributary channels. River-dominated systems periodically abandon their lower course and begin deposition in an adjacent area resulting in the deposition of sandstone bodies over a fairly large area.
+
* ''River- or [[fluvially dominated delta]]s'' (such as the Mississippi River delta) are those in which [[wave processes|wave]] and [[tidal processes|tidal]] energy is low and river transport processes dominate. Sandstone bodies in these systems tend to form well-developed sand bars at the mouths of [[distributary channel]]s. River-dominated systems periodically abandon their lower course and begin deposition in an adjacent area resulting in the deposition of sandstone bodies over a fairly large area.
−
* ''Wave-dominated deltas'' (such as the Nile and Rhone deltas) are those in which wave energy at the coast exceeds either the fluvial or the tidal energy. Wave reworking causes sand to be formed into shore-parallel bodies that are cuspate at distributary mouths.
+
* ''[[Wave-dominated deltas]]'' (such as the Nile and Rhone deltas) are those in which wave energy at the coast exceeds either the fluvial or the tidal energy. Wave reworking causes sand to be formed into shore-parallel bodies that are cuspate at distributary mouths.
−
* ''Tidally dominated deltas'' (such as the Gulf of Papua delta) are those in which the tidal energy exceeds that of either wave or fluvial processes. Sand deposited by the distributaries is reworked by tidal currents into elongate sand ridges that are generally perpendicular to the regional coastline.
+
* ''[[Tidally dominated deltas]]'' (such as the Gulf of Papua delta) are those in which the tidal energy exceeds that of either wave or fluvial processes. Sand deposited by the distributaries is reworked by tidal currents into elongate sand ridges that are generally perpendicular to the regional coastline.
  
−
Rarely do deltas conform perfectly to these end-members. In general they are transitional, giving rise to complexity and variability in the geometry and reservoir heterogeneities of resulting sandstone bodies (e.g., <ref name=pt06r130>Sneider, R. M., Tinker, C. N., Meckel, L. D., 1978, Deltaic environment reservoir types and their characteristics: Journal of Petroleum Technology, Nov., p. 1538–1546.</ref>.
+
Rarely do deltas conform perfectly to these end-members. In general they are transitional, giving rise to complexity and variability in the geometry and reservoir heterogeneities of resulting sandstone bodies (e.g., Sneider et al.<ref name=pt06r130>Sneider, R. M., C. N. Tinker, and L. D. Meckel, 1978, Deltaic environment reservoir types and their characteristics: Journal of Petroleum Technology, Nov., p. 1538–1546.</ref>).  For more information on deltas, see also [[Deltaic environments]],  [[Delta plain, upper]], [[Delta plain, lower]], and [[Delta plain, subaqueous]].
  
−
Distributary mouth bars and channel deposits (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3h</xref>) comprise the best reservoir quality bodies within a delta system. The general upward-coarsening character of distributary mouth bars tends to produce sandstone bodies that have their greatest permeability at the top. Conversely, distributary channel sandstone bodies are usually upward-fining and have their greatest permeability at the base<ref name=pt06r130 />). Preferred orientation of flow may be expected to follow paleochannel trends.
+
[[Distributary mouth bar]]s and channel deposits ([[:file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig3.png|Figure 3h]]) comprise the best reservoir quality bodies within a delta system. The general upward-coarsening character of distributary mouth bars tends to produce sandstone bodies that have their greatest [[permeability]] at the top. Conversely, distributary channel sandstone bodies are usually upward-fining and have their greatest permeability at the base.<ref name=pt06r130 /> Preferred orientation of flow may be expected to follow paleochannel trends.
  
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Distributary mouth bars typically contain a high percentage of interstratified clay that reduces vertical permeability. Hartman and Paynter<ref name=pt06r50>Hartman, J. A., Paynter, D. D., 1979, Drainage anomalies in Gulf Coast Tertiary sandstones: Journal of Petroleum Technology, Oct., p. 1313–1322.</ref> document an example of such behavior from a Gulf Coast deltaic reservoir undergoing natural waterdrive. After several years of production, the better quality channel sands watered out, whereas oil remained in the poorer quality delta fringe deposits. In this field, by-passed oil was accessed by recompleting wells only in the delta fringe interval.
+
Distributary mouth bars typically contain a high percentage of interstratified clay that reduces [[Directional permeability|vertical permeability]]. Hartman and Paynter<ref name=pt06r50>Hartman, J. A., and D. D. Paynter, 1979, [https://www.onepetro.org/journal-paper/SPE-7532-PA Drainage anomalies in Gulf Coast Tertiary sandstones]: Journal of Petroleum Technology, Oct., p. 1313–1322.</ref> document an example of such behavior from a Gulf Coast deltaic reservoir undergoing natural [[Drive mechanisms and recovery#Water drive|water drive]]. After several years of production, the better quality channel sands watered out, whereas oil remained in the poorer quality delta fringe deposits. In this field, bypassed oil was accessed by recompleting wells only in the delta fringe interval.
  
−
Wave modification acts to winnow delta mouth bar sandstones thus increasing their reservoir quality. In addition, wave processes and longshore currents enhance overall reservoir potential in deltas by redistributing sand as beach and chenier deposits in the interdistributary areas. Tidal reworking can affect reservoir quality at the delta mouth either by acting to winnow fines from the sands or by reducing effective permeability in distributary channels by the introduction of increased amounts of interstratified clay.
+
Wave modification acts to winnow delta mouth bar sandstones thus increasing their reservoir quality. In addition, wave processes and [[longshore currents]] enhance overall reservoir potential in deltas by redistributing sand as beach and [[chenier]] deposits in the interdistributary areas. Tidal reworking can affect reservoir quality at the delta mouth either by acting to winnow fines from the sands or by reducing effective permeability in distributary channels by the introduction of increased amounts of interstratified clay.
  
 
===Lacustrine deposits===
 
===Lacustrine deposits===
  
−
Lakes occur in a wide variety of geological settings. They are often very important during the early rifting phase of basin formation on continental crust. Major hydrocarbon-bearing lake deposits are associated with very large and long-lived Tertiary lakes such as those of the western United States, Indonesia, and China. These deposits are characterized by siliciclastic, carbonate, and organic-rich sediments deposited under generally low energy conditions, often by suspension deposition. Other processes include turbidity flows in the lake interior and wave and current reworking along the lake margin.
+
Lakes occur in a wide variety of geological settings. They are often very important during the early [[rift]]ing phase of basin formation on [[continental crust]]. Major hydrocarbon-bearing lake deposits are associated with very large and long-lived [[Tertiary]] lakes such as those of the western United States, Indonesia, and China. These deposits are characterized by siliciclastic, carbonate, and organic-rich sediments deposited under generally low energy conditions, often by [[suspension]] deposition. Other processes include [[turbidity flow]]s in the lake interior and wave and current reworking along the lake margin.
  
−
Lacustrine rocks are generally the source rocks for hydrocarbons found in alluvial fan, fluvial, eolian, and deltaic rocks rather than the reservoirs. However, sandstone bars, beaches, turbidites, and fan deltas associated with lake margins can be reservoirs sourced by open lake deposits. The core and log response characteristics of these deposits are similar to those described from analogous marine environments.
+
Lacustrine rocks are generally the source rocks for hydrocarbons found in [[alluvial]] fan, fluvial, eolian, and deltaic rocks rather than the reservoirs. However, sandstone [http://geonames.usgs.gov/apex/f?p=136:8:0::::: bars], [[beaches]], [[turbidites]], and [[fan deltas]] associated with lake margins can be reservoirs sourced by open lake deposits. The core and log response characteristics of these deposits are similar to those described from analogous marine environments.
  
 
===Shoreline deposits===
 
===Shoreline deposits===
  
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In shoreline systems adjacent to active deltas, the geometry and internal anatomy of sandstone bodies are controlled by an interplay of tidal and wave processes. Clastic, nondeltaic shorelines with a tidal range of 0–2 m (microtidal) tend to be wave-dominated. Resulting sand bodies are elongate barrier islands and strandplains. A tidal range of 2–4 m (mesotidal) tends to produce short (“drum stick”) barrier islands with extensive tidal flats and ebb tidal deltas. A tidal range of 4–6 m (macrotidal) tends to produce estuarine linear tidal sand ridges that are perpendicular to shoreline with associated extensive tidal flats.
+
In shoreline systems adjacent to active deltas, the geometry and internal anatomy of sandstone bodies are controlled by an interplay of tidal and wave processes. Clastic, nondeltaic shorelines with a tidal range of 0–2 m (microtidal) tend to be wave-dominated. Resulting sand bodies are elongate [[barrier islands]] and [[strandplain]]. A tidal range of 2–4 m (mesotidal) tends to produce short (“drum stick”) barrier islands with extensive [[tidal flats]] and [[ebb tidal deltas]]. A tidal range of 4–6 m (macrotidal) tends to produce [[Estuaries|estuarine]] linear tidal sand ridges that are perpendicular to shoreline with associated extensive [[tidal flat]]s.
  
−
Barrier islands (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3f</xref>) illustrate the spatial variability in facies that affect reservoir properties. Sands in the beach or foreshore are very well sorted, lack interstratified clay, and exhibit excellent reservoir properties where not cemented. Tidal inlet and flood tidal delta deposits comprise another important grouping of reservoir quality rocks, particularly because they are most often preserved in the rock record.
+
Barrier islands (Figure 3f) illustrate the spatial variability in facies that affect reservoir properties. Sands in the beach or foreshore are [[Core_description#Maturity|very well sorted]], lack interstratified clay, and exhibit excellent reservoir properties where not cemented. [[Tidal inlets|Tidal inle]]t and [[Flood tidal deltas|flood tidal delta]] deposits comprise another important grouping of reservoir quality rocks, particularly because they are most often preserved in the rock record.
  
−
Wireline log shapes through barrier island sequences vary depending on exactly where a well intersects the barrier island complex. Gamma ray, SP, and resistivity logs through the barrier core have an upward-coarsening motif (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3f</xref>). Logs through the back barrier and lower shoreface are typically highly serrate and often lack a well-defined upward-coarsening motif. Logs through the barrier inlet may exhibit upward fining.
+
Wireline log shapes through barrier island sequences vary depending on exactly where a well intersects the barrier island complex. [[Basic open hole tools#Gamma ray|Gamma ray]], [[Basic open hole tools#Spontaneous potential|SP]], and [[Basic open hole tools#Resistivity|resistivity]] logs through the barrier core have an upward-coarsening motif (Figure 3f). Logs through the [[back barrier]] and [[lower shoreface]] are typically highly serrate and often lack a well-defined upward-coarsening motif. Logs through the [[barrier inlet]] may exhibit upward fining.
  
−
In general, barrier islands have the best reservoir quality rocks at the top of the sequence. [[Reservoir quality]] drops off as one moves either seaward down the foreshore and shoreface into muds of the marine shelf or landward into the lagoon. High reservoir quality is also developed within the tidal inlet sandstones. Two major trends in directional permeability are suggested by (1) the shore-parallel nature of foreshore and shoreface sandstones and (2) shore-perpendicular tidal inlet and delta sandstones. In coastlines dominated by tidal processes, extensive interbedded mud and sand “flats” occur in the intertidal area of the coast and sand bars in estuarine channels in the subtidal area. The reservoir quality of tidal flat environments varies as a function of sand to mud ratio of the deposits. Reservoir quality of estuarine channel deposits also varies as a function of sand to mud ratio and degree of bioturbation.
+
In general, barrier islands have the best reservoir quality rocks at the top of the sequence. [[Reservoir quality]] drops off as one moves either seaward down the [[foreshore]] and [[shoreface]] into muds of the marine [[shelf]] or landward into the [Lagoons|[lagoon]]. High reservoir quality is also developed within the tidal inlet sandstones. Two major trends in directional permeability are suggested by (1) the shore-parallel nature of foreshore and shoreface sandstones and (2) shore-perpendicular tidal inlet and delta sandstones. In coastlines dominated by tidal processes, extensive interbedded mud and sand “flats” occur in the intertidal area of the coast and sand bars in estuarine channels in the [[subtidal]] area. The reservoir quality of tidal flat environments varies as a function of sand to mud ratio of the deposits. Reservoir quality of estuarine channel deposits also varies as a function of sand to mud ratio and degree of [[bioturbation]].
  
 
===Shallow marine clastic deposits===
 
===Shallow marine clastic deposits===
  
−
The marine shelf is an environment affected by storm- and tidal-driven waves and currents and sometimes by oceanic currents. Although shelf sand ridges of either storm or tidal origin formed during transgression are the best known examples (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3g</xref>), sand bodies associated with the marine shelf also include reworked delta front and barrier sands, amalgamated storm sheets, and oceanic current deposits<ref name=pt06r10>Barwis, J. H., 1989, The explorationist and shelf sand models—where do we go from here?: 7th Annual Research Conference Proceedings, Gulf Coast SEPM, p. 1–14.</ref>.
+
The marine shelf is an environment affected by [[Storm processes|storm]]- and [[Tidal processes|tidal]]-driven waves and [[currents]] and sometimes by [[oceanic currents]]. Although [[shelf sand ridges]] of either storm or tidal origin formed during [[transgression]] are the best known examples ([[:file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig3.png|Figure 3g]]), sand bodies associated with the marine [http://education.nationalgeographic.com/education/encyclopedia/continental-shelf/?ar_a=1 shelf] also include reworked [[Delta front sands|delta front]] and [[Barrier islands|barrier sands]], amalgamated [[Storm deposits|storm sheets]], and [[oceanic current deposits]].<ref name=pt06r10>Barwis, J. H., 1989, The explorationist and shelf sand models—where do we go from here?: 7th Annual Research Conference Proceedings, Gulf Coast SEPM, p. 1–14.</ref>
  
−
Most marine sand bodies are upward coarsening with the best reservoir quality rocks at the top of the body. Gamma ray, SP, and resistivity logs have a corresponding upward-coarsening character. In the case of storm-deposited sheet sands either attached or detached from the shoreface, amalgamation of individual storm deposits at the top of the bodies produces the greatest permeability and porosity and the most laterally continuous units<ref name=pt06r8>Atkinson, C. D., Goesten, B. G., Speksnijder, A., vander Vlugt, W., 1986, Storm-generated sandstone in the Miocene Miri Formation, Seria Field, Brunei (N., W. Borneo), in Knight, R. J., McLean, J. R., eds., Shelf Sands and Sandstones: Canadian Society of Petroleum Geologists Memoir 11, p. 213–240.</ref><ref name=pt06r38>Gaynor, G. C., Scheihing, M. H., 1988, Shelf depositional environments and reservoir characteristics of the Kuparuk River Formation (Lower Cretaceous), Kuparuk field, North Slope, Alaska, in Lomando, A. J., Harris, P. M., eds., Giant oil and gas fields—A core workshop: Society of Economic Paleontologists and Mineralogists Core Workshop 12, p. 333–389.</ref>. In the case of tidal- and storm-generated shelf sand ridges, best reservoir quality is also at the top in the form of several different types of large scale cross bedding.
+
Most marine sand bodies are upward coarsening with the best reservoir quality rocks at the top of the body. [[Basic open hole tools#Gamma ray|Gamma ray]], [[Basic open hole tools#Spontaneous potential|SP]], and [[Basic open hole tools#Resistivity|resistivity]] logs have a corresponding upward-coarsening character. In the case of storm-deposited sheet sands either attached or detached from the [[shoreface]], amalgamation of individual storm deposits at the top of the bodies produces the greatest [[permeability]] and [[porosity]] and the most laterally continuous units.<ref name=pt06r8>Atkinson, C. D., B. G. Goesten, A. Speksnijder, and W. van der Vlugt, 1986, Storm-generated sandstone in the Miocene Miri Formation, Seria Field, Brunei (NW Borneo), in R. J. Knight and J. R. McLean, eds., Shelf Sands and Sandstones: Canadian Society of Petroleum Geologists Memoir 11, p. 213–240.</ref><ref name=pt06r38>Gaynor, G. C., and M. H. Scheihing, 1988, Shelf depositional environments and reservoir characteristics of the Kuparuk River Formation (Lower Cretaceous), Kuparuk field, North Slope, Alaska, in A. J. Lomando, and P. M. Harris, eds., Giant oil and gas fields—A core workshop: Society of Economic Paleontologists and Mineralogists Core Workshop 12, p. 333–389.</ref> In the case of tidal- and storm-generated shelf sand ridges, best reservoir quality is also at the top in the form of several different types of large scale [[Cross-stratification|cross bedding]].
  
 
===Deep water marine deposits===
 
===Deep water marine deposits===
  
−
Reservoir quality sand bodies form on both the continental slope and at the base of the slope. Slope environments include sand bodies formed within submarine canyons and gullies cut into the slope and as spillover sheets<ref name=pt06r120>Slatt, R. M., 1986, Exploration models for submarine slope sandstones: Transactions of the 36th Annual Meeting of the Gulf Coast Association of Geological Societies, Continental Slope—Frontier of the 80's, p. 295–304.</ref>. Sands can also accumulate on tectonically formed small basins within the slope itself.
+
Reservoir quality sand bodies form on both the [http://www.thefreedictionary.com/continental+slope continental slope] and at the base of the slope. Slope environments include sand bodies formed within [[submarine canyons]] and gullies cut into the slope and as [[spillover sheets]].<ref name=pt06r120>Slatt, R. M., 1986, Exploration models for submarine slope sandstones: Transactions of the 36th Annual Meeting of the Gulf Coast Association of Geological Societies, Continental Slope—Frontier of the 80's, p. 295–304.</ref> Sands can also accumulate on [[Tectonics|tectonically]] formed small basins within the slope itself.
  
−
Submarine fans may form at the base of slopes that have a delta-like appearance in plan view (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3i</xref>). Internal facies vary from channelized sand and gravel bodies to sheet-like, thin, graded beds deposited by turbidity flows in distal parts of the fan. Vertical sequences through channelized portions of the fan typically show an upward-fining character accompanied by an upward-fining wireline log motif. Vertical sequences through more distal parts of the fan show an alternation between sandstone and mudstone beds, so that wireline logs are typically interdigitate and irregular. Reservoir quality varys accordingly. Many variations of morphologies and internal facies configurations occur in submarine fans as a function of sediment supply, sea level, type of continental margin, and local tectonic features.
+
[[Submarine fans]] may form at the base of slopes that have a [[ Lithofacies and environmental analysis of clastic depositional systems#Deltas|delta]]-like appearance in plan view ([[:file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig3.png|Figure 3i]]). Internal facies vary from channelized sand and gravel bodies to sheet-like, thin, graded beds deposited by [[turbidity flow]]s in distal parts of the fan. Vertical sequences through channelized portions of the fan typically show an upward-fining character accompanied by an upward-fining wireline log motif. Vertical sequences through more distal parts of the fan show an alternation between sandstone and [[mudstone]] beds, so that wireline logs are typically interdigitate and irregular. Reservoir quality varys accordingly. Many variations of morphologies and internal facies configurations occur in submarine fans as a function of [[Depocenter#Siliciclastic vs. carbonate supply|sediment supply]], [[Sea level cycle phase|sea level]], type of [[continental margin]], and local [[Tectonics|tectonic]] features.
  
 
==See also==
 
==See also==
 
* [[Introduction to geological methods]]
 
* [[Introduction to geological methods]]
−
* [[Monte carlo and stochastic simulation methods]]
 
−
* [[Subsurface maps]]
 
−
* [[Flow units for reservoir characterization]]
 
−
* [[Effective pay determination]]
 
−
* [[Multivariate data analysis]]
 
−
* [[Geological cross sections]]
 
−
* [[Evaluating structurally complex reservoirs]]
 
−
* [[Conversion of well log data to subsurface stratigraphic and structural information]]
 
−
* [[Evaluating tight gas reservoirs]]
 
−
* [[Correlation and regression analysis]]
 
 
* [[Reservoir quality]]
 
* [[Reservoir quality]]
−
* [[Carbonate reservoir models: Facies, diagenesis, and flow characterization]]
+
* [[Carbonate diagenesis]]
−
* [[Fluid contacts]]
 
 
* [[Geological heterogeneities]]
 
* [[Geological heterogeneities]]
−
* [[Evaluating fractured reservoirs]]
 
 
* [[Evaluating stratigraphically complex fields]]
 
* [[Evaluating stratigraphically complex fields]]
 
* [[Evaluating diagenetically complex reservoirs]]
 
* [[Evaluating diagenetically complex reservoirs]]
−
* [[Statistics overview]]
+
* [[Deltaic environments]]
 +
* [[Delta plain, upper]]
 +
* [[Delta plain, lower]]
 +
* [[Delta plain, subaqueous]]
  
 
==References==
 
==References==
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[[Category:Geological methods]]
 
[[Category:Geological methods]]
 +
[[Category:Petrophysics and well logs]]
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[[Category:Sedimentation and stratigraphy - Clastics]]
 +
[[Category:Basin modeling]]

Latest revision as of 22:11, 15 January 2024

Development Geology Reference Manual
Series Methods in Exploration
Part Geological methods
Chapter Lithofacies and environmental analysis of clastic depositional systems
Author Mark H. Scheihing, Christopher D. Atkinson
Link Web page
Store AAPG Store

The geological and reservoir properties of sedimentary rocks depend upon an interplay of tectonics, sea level, sediment supply, physical and biological processes of sediment transport and deposition, and climate. At the basin scale, these processes interact to produce the geometric arrangement of different depositional environments or systems tracts through time, known as the stratigraphic architecture of the basin.[1] At smaller scales, these processes control the external geometry and internal “anatomy” of clastic sediment bodies (see Geological heterogeneities). It is at this smaller scale that lithofacies analysis and interpretation of depositional environments become important for reservoir evaluation.

Data requirements[edit]

Basic data requirements for facies analysis of subsurface rocks are listed in Table 1. Data associated with wells are most often used, but seismic data, particularly three-dimensional data, are becoming increasingly important in defining sandstone body geometries.[2] Conventional core is perhaps the most diagnostic for sedimentological interpretation of vertical sequences (see Core description). However, wireline tools such as dipmeters and formation imaging devices can provide electrical images suitable for sedimentological interpretation with the added ability to determine paleocurrent directions in appropriate cases.

Table 1 Types of data commonly used in facies analysis of clastic rocks
Data Type Application
Slabbed conventional core Facies, depositional environment
Oriented conventional core Paleocurrent directions
Core gamma scan Shift to wireline logs
Sidewall cores, well cuttings, thin sections, sand peels Mineralogy, lithology
Paleontology (micro, macro, trace fossils), palynology Water depth, depositional environment, time lines
Dipmeter log Paleocurrent directions, lithofacies
Formation MicroScanner (FMS) log Paleocurrent directions, lithofacies
Spontaneous potential log Lithology, curve shape analysis
Gamma ray log Lithology, curve shape analysis
Sonic log Porosity, curve shape analysis
Caliper log Borehole condition, quality control
Neutron log Porosity, lithology (with sonic, density)
Density log Lithology (coal), porosity (with neutron)
Repeat formation test (RFT) Pressure (sand body connectedness)

Rock description[edit]

Figure 1 Sedimentary processes, lithofacies, and lithofacies associations for a meandering channel sequence. (The vertical sequence is modified from Walker.[3])

Within any given depositional environment, various physical and biological processes act to transport and deposit sediment. These processes result in various distributions of grain size and sedimentary structures that characterize the deposited sediment. Relating these features back to the processes that produced them is the basic method used by geologists to interpret the depositional environment of sedimentary sequences (Figure 1).

Lithofacies[edit]

One of the first steps in the facies analysis of a clastic reservoir is the description and interpretation of available conventional core.[4] An important result of core description is the subdivision of cores into lithofacies, defined as subdivisions of a sedimentary sequence based on lithology, grain size, physical and biogenic sedimentary structures, and stratification that bear a direct relationship to the depositional processes that produced them. Lithofacies and lithofacies associations (groups of related lithofacies) are the basic units for the interpretation of depositional environments.

Depositional environments[edit]

Interpretation of the environment in which lithofacies were deposited from analysis of cored sequences involves relating the identified lithofacies to the physical and biological processes that produced them. This process-response relationship identifies the specific processes responsible for the sequence and, by inference, the depositional setting in which these processes occurred. The application of the process-response approach relies primarily on depositional models constructed through study of both modern and ancient analogs.

Depositional models are important for predicting the distribution of permeability and porosity within different reservoir types. These models are never exact matches to a reservoir; rather, they serve as guides to aid in the interpretation of any one reservoir.[3] Reservoir properties are generally observed to be correlative with lithofacies types to one degree or another (see Geological heterogeneities). This reflects the fundamental control on permeability and porosity by grain size, sorting, and spatial distribution of different lithofacies types. Even where rocks have experienced later physical and chemical diagenesis, permeability and porosity relationships are controlled, in large part, by the original sedimentary fabric of the rock.

Wireline log calibration and correlation[edit]

Figure 2 Gamma ray correlation (dip section) of a series of prograding shoreface sandstones. Note the imbricate nature of the sandstone bodies and the “non-layer cake” nature of the correlations.

Interpretations of depositional environment based on individual well data are transformed into a three-dimensional picture of the reservoir by wireline log correlation and, where possible, by three-dimensional seismic data.

Wireline logs to be used for facies analysis should, whenever possible, always be calibrated by core. This calibration involves (1) shifting core to log depths (see Preprocessing of logging data and Core-log transformations and porosity-permeability relationships) and (2) establishing a relationship between lithofacies associations and curve shape. Core gamma scans, obtained by passing the core through a device that measures the natural radioactivity of the rock, are particularly useful for shifting cores to logs. The calibration of wireline log shape by core is particularly important for firmly establishing the log response and the identity of vertical sequences on these logs.

For reservoirs in which no core is available, wireline log shape must be used to interpret sandstone body type and identify depositional environments. If closely spaced cuttings or sidewall cores are available, these can sometimes aid rock to log calibration. Log shapes are deduced from the expected wireline log response of the different environments combined with a knowledge of the paleogeography of the area in which the field is situated. Wireline log shapes are often described as “upward coarsening,” “upward fining,” or “blocky.” However, log shape as determined from a gamma ray or SP log in siliciclastic rocks is related more to argillaceous content than to grain size. Upward coarsening log patterns exhibit an upward decrease in argillaceous content. Upward fining log patterns exhibit the reverse trend. Blocky or cylindrical log patterns exhibit relatively little vertical variation in argillaceous content and are typical of siliciclastic rocks that have low overall argillaceous content. Various publications and reference charts are available to aid in this practice (e.g., Spearing,[5] Cant,[6] and Rider[7]). However, without core control, curve shape analysis is fraught with hazards (e.g., Snedden;[8] also see Quick-look lithology from logs).

Correlation sections that will be used for establishing sandstone body geometry should have a depositionally flat datum (such as a bentonite bed, marine shale bed, or laterally persistent limestone). Sections should be oriented parallel and perpendicular to depositional strike, if known, and represent as straight a line as possible given well density and placement.

The only sedimentologically significant correlation horizons are those that approximate time lines within and between sandstone bodies. This style of correlation requires an understanding of the succession of depositional environments and intervening unconformable surfaces. It often leads to nonparallel and nonhorizontal correlations. For example, in shoreface systems, time lines denoted by shale or silt breaks between shingled shoreface sheets and lenses are inclined in a seaward (depositional dip) direction (Figure 2).

This imbrication does not occur in a strike direction. This style of correlation is especially important for reservoir delineation since the large scale (interwell and field) architecture of the sandstone body exerts a control on the movement of fluids through the volume of the reservoir.

Wireline log correlation is an exercise in pattern recognition combined with the geometry suggested by the interpreted depositional environment(s). The depositional interpretation of a reservoir exerts a major impact on the methods and style of correlation and mapping. For example, correlation in fluvial systems applies very different assumptions about sandstone continuity than does correlation in shoreface or shelf systems.

Clastic depositional lithofacies and environments[edit]

Figure 3 Models of major depositional environments. The curve on the left shows the SP or gamma ray response and the curve on the right shows the relative grain size profile. The size of the dots next to the vertical profile indicates the relative magnitude of permeability expected in such a sequence. (Parts c and d are from Berg,[3] and parts f, h, and i are from Galloway and Hobday.[9]

Clastic depositional environments range from alpine to abyssal settings (Figure 3 and Table 2). Detailed reviews of these are given by Galloway and Hobday,[9] Walker,[3] Berg,[10] Reading,[11] Beaumont and Foster,[12] and others. The following review is a cursory summary of the origin, lithofacies, geometry, and reservoir properties of major clastic environments and deposits. The reader should be aware that the remarks offered here for each depositional environment are necessarily of a highly generalized and idealized nature. Siliciclastic reservoirs are typically composed of multiple bodies deposited (and eroded) through time under varying tectonic, sea level, and climatic conditions. Corresponding geometry, vertical sequence, wireline log character, and reservoir quality trends for a given reservoir may be, and often are, different from the generalized “single environment” models. In addition, subsequent diagenesis (see Evaluating diagenetically complex reservoirs) may alter the permeability and porosity structure created by depositional (and erosional) processes. However, it has been often observed that in siliciclastic rocks, diagenesis generally follows depositional fabric . Complex structural patterns can reduce reservoir continuity as well.

Major clastic depositional environments

  • Alluvial sediments
    • Alluvial fans
    • Fan deltas
    •  Braidplains
    •  Braided rivers
    •  Meandering rivers
  • Lacustrine sediments
    •  Lake deltas
    •  Freshwater lakes
    •  Saline lakes
  • Eolian sediments
    •  Dunes
    •  Interdune deposits
    •  Sand sheets
  • Deltaic sediments
    •  River-dominated deltas
    •  Wave-dominated deltas
    •  Tidally dominated deltas
  • Siliciclastic shoreline sediment
    •  Wave-dominated shorelines
    •  Beaches
    •  Microtidal barrier islands
    •  Cheniers
    •  Mixed wave-tide influenced shorelines
    •  Mesotidal barrier islands
    •  Tide-dominated shorelines
    •  Tidal flats
    •  Estuaries
  • Shallow siliciclastic seas
    •  Tidal sand ridges
    •  Sand shoals
    •  Sand sheets
  • Deep marine slope and basin
    •  Slope channel and gully deposits
    •  Slope canyon deposits
    •  Intraslope basin deposits
    •  Sand spillover sheets
    •  Slope aprons
    •  Submarine fans
  • Glacial sediments
    •  Supraglacial
    •  Glaciofluvial
    •  Glacioeolian
    •  Glaciolacustrine
    •  Glaciomarine

Alluvial fan deposits[edit]

An alluvial fan is a wedge of clastic detritus that forms at the base of a mountain front as sediments eroding from the mountains are transported downslope by streams or debris flows and deposited at the base (Figure 3e). The fan-shaped body is generally characterized by a gradation from coarser sediments at the apex to finer sediments at the toe. Alluvial fans are commonly divided into proximal, mid-fan, and distal fan subenvironments.

Vertical sequences through the proximal fan are generally dominated by gravelly deposits with subordinate sandy deposits. Sequences through the mid- and distal fan are increasingly sand dominated. Gamma ray, SP, and resistivity log responses throughout a fan can generally be expected to be blocky to irregular, depending on the amount of clay.

Permeability and porosity of alluvial fan deposits vary greatly as a function of depositional process and differential response to diagenesis. In general, streamflow deposits have greater permeability and porosity than debris and mudflow deposits. Finer grained but better sorted distal fan deposits are highly permeable and porous. Because of increased sorting, middle and distal parts of the fan probably have better and more predictable reservoir quality than proximal parts. Little is known of directional permeability within alluvial fan reservoirs, but paleochannels can be expected to act as preferred pathways of flow.

Where alluvial fans prograde into standing bodies of water (that is, oceans or lakes), they are called fan deltas. The distal parts of these fans are generally much better sorted and cleaner as a result of reworking by wave and/or tidal processes. Proximal and mid-fan log responses are the same as alluvial fans. Log response in the distal part depends upon the intensity of wave and tidal processes and on whether the fan is actively prograding or being transgressed. Typically, distal parts will have an upward-coarsening gamma ray, SP, and resistivity log character. Barring adverse diagenetic effects, permeability can be expected to be much greater in marine than in more proximal parts of the fan delta because of increased sorting, destruction of compositionally immature grains, and winnowing of fines. Directional permeability trends in distal parts may be different from more proximal locations because of different sand body trends between these different parts of the fan delta.

Braided and meandering fluvial deposits[edit]

Downdip from alluvial fans, rivers typically grade first into braided channels then, farther down the alluvial valley toward the coastal plain, into meandering channels. These different channel types can occur in the same river system and produce distinctly different kinds of sandstone bodies.

Braided rivers and braidplains form elongate, tabular, sandy and gravelly deposits composed of braided, sand-filled channels and sand and gravel bars (Figure 3c). They typically consist of coarse sand and gravel with relatively minor amounts of clay. Vertical sequences are composed of stacked, upward-fining channel sands and sand and gravel bars. Lateral trends in these deposits are dominated by an overall tabular geometry bounded by floodplain muds with an internally complex geometry of cross-cutting sands and gravels with subordinate mud-rich beds of varying thickness and dimension. Bar and channel deposits are typically elongate in the paleocurrent direction.

Meandering rivers are different in that sand is restricted to a single channel and surrounded by fine-grained sediments (Figure 3d). Sand is concentrated mainly in the channel bottoms and point bars. A vertical sequence through such a channel system frequently has an upward-fining character, starting from the channel lag at the bottom and grading upward into deposits of the adjacent levee and floodplain. Individual meander belts are built of cross-cutting and stacked individual upward-fining sequences often separated laterally by meander loop cutoffs and clay plugs. Multiple meander belts are built by abandonment of an entire river segment (avulsion) and by establishment of a new section in another position on the floodplain.

Gamma ray, SP, and resistivity logs through braided channel complexes generally have a blocky character, whereas individual meandering channels have an upward-fining signature except where stacked and cross-cut, where they may exhibit more complex wireline log signatures.

The upward-fining character of fluvial channels tends to produce sandstone bodies that have their greatest permeability at the base of the body. However, the common stacking and cross-cutting of channels in both braided and meandering river deposits often produces a complex spatial distribution of permeability within the braided or meander belt. Preferred permeability pathways, and consequently, fluid flow, can be expected to follow the paleochannel direction.[13]

Eolian deposits[edit]

Eolian sands develop in arid settings and commonly form extensive, blanket-like deposits (Figure 3b). Wind transport removes fines and produces rounded and extremely well sorted grains often leading to favorable reservoir quality.

This combination of widespread occurrence and good reservoir properties makes eolian sandstones attractive exploration targets and many hydrocarbon accumulations have been discovered in such deposits.[14]

Eolian deposits include dune, interdune (marginal to dune complex), and extradune (noneolian) lateral deposits.[14] Dune deposits comprise the major sedimentary bodies in eolian successions. All are characterized by large scale cross-stratification in which foreset dips range up to 35°. Associated deposits may include those of wadi (fluvial), playa (lacustrine), and sabkha (arid tidal flat) origin.

In the subsurface, eolian sandstones generally comprise thickly bedded sequences with few major interstratified shales. The sequences tend to be uniform and lack discernible coarsening- or fining-upward trends and, thus, exhibit blocky to weakly serrated gamma ray, SP, and resistivity log profiles The well-bedded and high angle cross stratified nature of eolian sandstones promotes reliable results from dipmeter logs. Dune and interdune deposits can often be distinguished and paleowind directions inferred using correctly processed dipmeter data.[15]

Eolian sandstones generally comprise excellent reservoir intervals but often possess complex porosity and permeability variations. They are commonly anisotropic with regard to the flow of fluids and exhibit greater horizontal than vertical permeability because of their pronounced lamination.[16]

Deltas[edit]

Deltaic bodies are generally classified into three major categories or end-members on the basis of the dominant sediment transport process that influences their facies constituents and external geometries. These three end-members are as follows:

  • River- or fluvially dominated deltas (such as the Mississippi River delta) are those in which wave and tidal energy is low and river transport processes dominate. Sandstone bodies in these systems tend to form well-developed sand bars at the mouths of distributary channels. River-dominated systems periodically abandon their lower course and begin deposition in an adjacent area resulting in the deposition of sandstone bodies over a fairly large area.
  • Wave-dominated deltas (such as the Nile and Rhone deltas) are those in which wave energy at the coast exceeds either the fluvial or the tidal energy. Wave reworking causes sand to be formed into shore-parallel bodies that are cuspate at distributary mouths.
  • Tidally dominated deltas (such as the Gulf of Papua delta) are those in which the tidal energy exceeds that of either wave or fluvial processes. Sand deposited by the distributaries is reworked by tidal currents into elongate sand ridges that are generally perpendicular to the regional coastline.

Rarely do deltas conform perfectly to these end-members. In general they are transitional, giving rise to complexity and variability in the geometry and reservoir heterogeneities of resulting sandstone bodies (e.g., Sneider et al.[17]). For more information on deltas, see also Deltaic environments, Delta plain, upper, Delta plain, lower, and Delta plain, subaqueous.

Distributary mouth bars and channel deposits (Figure 3h) comprise the best reservoir quality bodies within a delta system. The general upward-coarsening character of distributary mouth bars tends to produce sandstone bodies that have their greatest permeability at the top. Conversely, distributary channel sandstone bodies are usually upward-fining and have their greatest permeability at the base.[17] Preferred orientation of flow may be expected to follow paleochannel trends.

Distributary mouth bars typically contain a high percentage of interstratified clay that reduces vertical permeability. Hartman and Paynter[18] document an example of such behavior from a Gulf Coast deltaic reservoir undergoing natural water drive. After several years of production, the better quality channel sands watered out, whereas oil remained in the poorer quality delta fringe deposits. In this field, bypassed oil was accessed by recompleting wells only in the delta fringe interval.

Wave modification acts to winnow delta mouth bar sandstones thus increasing their reservoir quality. In addition, wave processes and longshore currents enhance overall reservoir potential in deltas by redistributing sand as beach and chenier deposits in the interdistributary areas. Tidal reworking can affect reservoir quality at the delta mouth either by acting to winnow fines from the sands or by reducing effective permeability in distributary channels by the introduction of increased amounts of interstratified clay.

Lacustrine deposits[edit]

Lakes occur in a wide variety of geological settings. They are often very important during the early rifting phase of basin formation on continental crust. Major hydrocarbon-bearing lake deposits are associated with very large and long-lived Tertiary lakes such as those of the western United States, Indonesia, and China. These deposits are characterized by siliciclastic, carbonate, and organic-rich sediments deposited under generally low energy conditions, often by suspension deposition. Other processes include turbidity flows in the lake interior and wave and current reworking along the lake margin.

Lacustrine rocks are generally the source rocks for hydrocarbons found in alluvial fan, fluvial, eolian, and deltaic rocks rather than the reservoirs. However, sandstone bars, beaches, turbidites, and fan deltas associated with lake margins can be reservoirs sourced by open lake deposits. The core and log response characteristics of these deposits are similar to those described from analogous marine environments.

Shoreline deposits[edit]

In shoreline systems adjacent to active deltas, the geometry and internal anatomy of sandstone bodies are controlled by an interplay of tidal and wave processes. Clastic, nondeltaic shorelines with a tidal range of 0–2 m (microtidal) tend to be wave-dominated. Resulting sand bodies are elongate barrier islands and strandplain. A tidal range of 2–4 m (mesotidal) tends to produce short (“drum stick”) barrier islands with extensive tidal flats and ebb tidal deltas. A tidal range of 4–6 m (macrotidal) tends to produce estuarine linear tidal sand ridges that are perpendicular to shoreline with associated extensive tidal flats.

Barrier islands (Figure 3f) illustrate the spatial variability in facies that affect reservoir properties. Sands in the beach or foreshore are very well sorted, lack interstratified clay, and exhibit excellent reservoir properties where not cemented. Tidal inlet and flood tidal delta deposits comprise another important grouping of reservoir quality rocks, particularly because they are most often preserved in the rock record.

Wireline log shapes through barrier island sequences vary depending on exactly where a well intersects the barrier island complex. Gamma ray, SP, and resistivity logs through the barrier core have an upward-coarsening motif (Figure 3f). Logs through the back barrier and lower shoreface are typically highly serrate and often lack a well-defined upward-coarsening motif. Logs through the barrier inlet may exhibit upward fining.

In general, barrier islands have the best reservoir quality rocks at the top of the sequence. Reservoir quality drops off as one moves either seaward down the foreshore and shoreface into muds of the marine shelf or landward into the [Lagoons|[lagoon]]. High reservoir quality is also developed within the tidal inlet sandstones. Two major trends in directional permeability are suggested by (1) the shore-parallel nature of foreshore and shoreface sandstones and (2) shore-perpendicular tidal inlet and delta sandstones. In coastlines dominated by tidal processes, extensive interbedded mud and sand “flats” occur in the intertidal area of the coast and sand bars in estuarine channels in the subtidal area. The reservoir quality of tidal flat environments varies as a function of sand to mud ratio of the deposits. Reservoir quality of estuarine channel deposits also varies as a function of sand to mud ratio and degree of bioturbation.

Shallow marine clastic deposits[edit]

The marine shelf is an environment affected by storm- and tidal-driven waves and currents and sometimes by oceanic currents. Although shelf sand ridges of either storm or tidal origin formed during transgression are the best known examples (Figure 3g), sand bodies associated with the marine shelf also include reworked delta front and barrier sands, amalgamated storm sheets, and oceanic current deposits.[19]

Most marine sand bodies are upward coarsening with the best reservoir quality rocks at the top of the body. Gamma ray, SP, and resistivity logs have a corresponding upward-coarsening character. In the case of storm-deposited sheet sands either attached or detached from the shoreface, amalgamation of individual storm deposits at the top of the bodies produces the greatest permeability and porosity and the most laterally continuous units.[20][21] In the case of tidal- and storm-generated shelf sand ridges, best reservoir quality is also at the top in the form of several different types of large scale cross bedding.

Deep water marine deposits[edit]

Reservoir quality sand bodies form on both the continental slope and at the base of the slope. Slope environments include sand bodies formed within submarine canyons and gullies cut into the slope and as spillover sheets.[22] Sands can also accumulate on tectonically formed small basins within the slope itself.

Submarine fans may form at the base of slopes that have a delta-like appearance in plan view (Figure 3i). Internal facies vary from channelized sand and gravel bodies to sheet-like, thin, graded beds deposited by turbidity flows in distal parts of the fan. Vertical sequences through channelized portions of the fan typically show an upward-fining character accompanied by an upward-fining wireline log motif. Vertical sequences through more distal parts of the fan show an alternation between sandstone and mudstone beds, so that wireline logs are typically interdigitate and irregular. Reservoir quality varys accordingly. Many variations of morphologies and internal facies configurations occur in submarine fans as a function of sediment supply, sea level, type of continental margin, and local tectonic features.

See also[edit]

References[edit]

  1. ↑ Miall, A. D., 1984, Principles of Sedimentary Basin Analysis: New York, Springer-Verlag, 490 p.
  2. ↑ Brown, A. R., 1986 Interpretation of three-dimensional seismic data: AAPG Memoir 42, 194 p.
  3. ↑ 3.0 3.1 3.2 3.3 Walker, R. G., ed., 1984, Facies Models, 2nd ed.: Geoscience Canada Reprint Series 1, 317 p.
  4. ↑ Siemers, C. T., and R. W. Tillman, 1981, Recommendations for the proper handling of cores and sedimentological analysis of core sequences, in C. T. Siemers, R. W. Tillman, and C. R. Williamson, eds., Deep-Water Clastic Sediments—A Core Workshop: SEPM Core Workshop, n. 2, p. 20–44.
  5. ↑ Spearing, D. R., 1974, Summary sheets of sedimentary deposits: Geological Society of American Publication MC-8.
  6. ↑ Cant, D. J., 1984, Subsurface facies analysis, in Walker, R. G., ed., Facies Models: Geoscience Canada, Reprint Series 1, p. 297–319.
  7. ↑ Rider, M. H., 1986, Geological interpretation of well logs: New York, John Wiley, 175 p.
  8. ↑ Snedden, J. W., 1987, Validity of the use of the spontaneous potential curve shape in the interpretation of sandstone depositional environments, in White, B. R., Kier, R. eds., Transactions of the 34th annual meeting of the Gulf Coast Association of Geological Societies and 31st annual meeting of the Gulf Coast Section of SEPM, v. 34, p. 255–263.
  9. ↑ 9.0 9.1 Galloway, W. E., and D. K. Hobday, 1983, Terrigenous Clastic Depositional Systems Applications to Petroleum, Coal, and Uranium Exploration: New York, Springer Verlag, 423 p.
  10. ↑ Berg, R. R., 1986, Reservoir Sandstones: Englewood Cliffs, NJ, Prentice-Hall, 481 p.
  11. ↑ Reading, H. G., ed., 1986, Sedimentary Environments and Facies, 2nd ed.: Boston, MA, Blackwell Scientific Publications, 615 p.
  12. ↑ Beaumont, E. A., and N. H. Foster, 1987, Reservoirs II—Sandstones: AAPG Treatise of Petroleum Geology Reprint Series No. 4, 573 p.
  13. ↑ Qiu, Y., 1984, Depositional model, heterogeneous characteristics, and waterflood performance of sandstone reservoirs in a lake basin case study of oilfields, eastern China: Proceedings of the 11th World Petroleum Congress, v. 3, p. 113–125.
  14. ↑ 14.0 14.1 Ahlbrandt, T. S., and S. G. Fryberger, 1982, Introduction to eolian deposits, in P. A. Scholle, and D. Spearing, eds., Sandstone Depositional Environments: AAPG Memoir 31, p. 11–47.
  15. ↑ Lupe, R., and T. S. Ahlbrandt, 1979, Sediments of the ancient eolian environment—reservoir inhomogeneity, in E. D. McKee, ed., A Study of Global Sand Seas: U.S. Geological Survey Professional Paper 1052, p. 241–252.
  16. ↑ Weber, K. J., 1987, Computation of initial well productivities in aeolian sandstone on the basis of a geological model, Leman Gas field, UK, in R. W. Tillman, and K. J. Weber, eds., Reservoir Sedimentology: SEPM Special Publication 40, p. 335–354.
  17. ↑ 17.0 17.1 Sneider, R. M., C. N. Tinker, and L. D. Meckel, 1978, Deltaic environment reservoir types and their characteristics: Journal of Petroleum Technology, Nov., p. 1538–1546.
  18. ↑ Hartman, J. A., and D. D. Paynter, 1979, Drainage anomalies in Gulf Coast Tertiary sandstones: Journal of Petroleum Technology, Oct., p. 1313–1322.
  19. ↑ Barwis, J. H., 1989, The explorationist and shelf sand models—where do we go from here?: 7th Annual Research Conference Proceedings, Gulf Coast SEPM, p. 1–14.
  20. ↑ Atkinson, C. D., B. G. Goesten, A. Speksnijder, and W. van der Vlugt, 1986, Storm-generated sandstone in the Miocene Miri Formation, Seria Field, Brunei (NW Borneo), in R. J. Knight and J. R. McLean, eds., Shelf Sands and Sandstones: Canadian Society of Petroleum Geologists Memoir 11, p. 213–240.
  21. ↑ Gaynor, G. C., and M. H. Scheihing, 1988, Shelf depositional environments and reservoir characteristics of the Kuparuk River Formation (Lower Cretaceous), Kuparuk field, North Slope, Alaska, in A. J. Lomando, and P. M. Harris, eds., Giant oil and gas fields—A core workshop: Society of Economic Paleontologists and Mineralogists Core Workshop 12, p. 333–389.
  22. ↑ Slatt, R. M., 1986, Exploration models for submarine slope sandstones: Transactions of the 36th Annual Meeting of the Gulf Coast Association of Geological Societies, Continental Slope—Frontier of the 80's, p. 295–304.

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