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.

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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]]).

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==