Changes

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]]).