Changes

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.

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.

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., <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., <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., <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 [[bentonit]]e 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).

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).