Changes

==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 [[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.

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"

|-

|-

| [[Basic open hole tools#Density|Density]] log

| [[Basic open hole tools#Density|Density]] log

| Lithology (coal), [[porosity]] [[Quick-look lithology from logs#Neutron and density logs combined|(with neutron)]]

| Lithology ([[coal]]), [[porosity]] [[Quick-look lithology from logs#Neutron and density logs combined|(with neutron)]]

|-

|-

| [[Wireline formation testers|Repeat formation test (RFT]])

| [[Wireline formation testers|Repeat formation test (RFT]])

===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., R. W. Tillman, 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.

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===

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]]).

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]]).

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.

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 [[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]]).

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]]).

[[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 />]]

[[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 />]]

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., 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 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.

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.

'''Major clastic depositional environments'''

'''Major clastic depositional environments'''

* Alluvial sediments

* [[Alluvial]] sediments

** Alluvial fans

** Alluvial fans

** Fan deltas

** Fan deltas

===Alluvial fan deposits===

===Alluvial fan deposits===

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.

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.

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.

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.

===Braided and meandering fluvial deposits===

===Braided and meandering fluvial deposits===

[[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.

[[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 [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.

''[[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.

[[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.

[[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. [[Directional permeability|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 ([[: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.

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.<ref name=pt06r2>Ahlbrandt, T. S., Fryberger, S. G., 1982, [http://archives.datapages.com/data/specpubs/sandsto2/data/a058/a058/0001/0000/0011.htm Introduction to eolian deposits], in Scholle, P. A., Spearing, D. eds., Sandstone Depositional Environments: [http://store.aapg.org/detail.aspx?id=627 AAPG 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]]  (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 [[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., Ahlbrandt, T. S., 1979, Sediments of the ancient eolian environment—reservoir inhomogeneity, in McKee, E., D., 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>

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: [https://members.sepm.org/iMISpublic/Core/Orders/Default.aspx? 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===

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:

* ''River- or [[fluvially dominated deltas]]'' (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 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., Sneider et al.<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>).  For more information on deltas, see also [[Deltaic environments]],  [[Delta plain, upper]], [[Delta plain, lower]], and [[Delta plain, subaqueous]].

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 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.

[[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.

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., Paynter, D. D., 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.

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.

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.

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 [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.

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===

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>

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. [[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., 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-stratification|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 [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.

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 [[ 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.

[[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==