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==Stratigraphic cross sections==

==Stratigraphic cross sections==

[[file:geological-cross-sections_fig1.png|left|thumb|{{figure number|1}}(a) Stratigraphic and (b) structural cross sections of the Ranger Formation in the Long Beach unit of the Wilmington field, California. Sections are projected onto a north-south plane. (From <ref name=pt06r122>Slatt, R. M., Phillips, S., Boak, J. M., Lagoe, M. B., 1993, Scales of geological heterogeneity of a deep-water sand giant oil field, Long Beach unit, Wilmington field, California, in Rhodes, E. G., Moslow, T. F., eds., Marine Clastic Reservoirs—Examples and Analogs: New York, Springer-Verlag.</ref>.)]]

[[file:geological-cross-sections_fig1.png|left|thumb|{{figure number|1}}(a) Stratigraphic and (b) structural cross sections of the Ranger Formation in the Long Beach unit of the Wilmington field, California. Sections are projected onto a north-south plane. (From Slatt et al.<ref name=pt06r122>Slatt, R. M., Phillips, S., Boak, J. M., Lagoe, M. B., 1993, Scales of geological heterogeneity of a deep-water sand giant oil field, Long Beach unit, Wilmington field, California, in Rhodes, E. G., Moslow, T. F., eds., Marine Clastic Reservoirs—Examples and Analogs: New York, Springer-Verlag.</ref>)]]

Stratigraphic cross sections show characteristics of correlatable stratigraphic units, such as reservoir sandstones or sealing shales. They may also be vital in understanding the timing of deformation by showing the drape of sediment over developing folds or the thickening of the section across growth faults. The following elements of cross section design are presented as if they were a sequence. In practice, however, each choice affects and is affected by the others.

Stratigraphic cross sections show characteristics of correlatable stratigraphic units, such as reservoir sandstones or sealing shales. They may also be vital in understanding the timing of deformation by showing the drape of sediment over developing folds or the thickening of the section across growth faults. The following elements of cross section design are presented as if they were a sequence. In practice, however, each choice affects and is affected by the others.

===Choice of datum===

===Choice of datum===

The ''datum'' is the level or reference horizon from which elevations and depths are measured in the cross section. In a stratigraphic cross section, the geologist takes advantage of the principle of original horizontality to produce an interpretation of what the chosen slice of the earth might have looked like at some time in the past. By “hanging” all the available vertical information on a stratigraphic horizon or datum that can be correlated along the full length of the cross section, the data are transformed to reflect a different horizontal plane, one that existed at an earlier time (see Figure 1a). The assumption that this surface was horizontal when deposited assumes no original depositional slope.

The ''datum'' is the level or reference horizon from which elevations and depths are measured in the cross section. In a stratigraphic cross section, the geologist takes advantage of the principle of original horizontality to produce an interpretation of what the chosen slice of the earth might have looked like at some time in the past. By “hanging” all the available vertical information on a stratigraphic horizon or datum that can be correlated along the full length of the cross section, the data are transformed to reflect a different horizontal plane, one that existed at an earlier time (see [[:file:geological-cross-sections_fig1.png|Figure 1a]]). The assumption that this surface was horizontal when deposited assumes no original depositional slope.

The purpose of the cross section is to determine which horizon can serve as the datum. Because it is shown as horizontal, the thickness variations of the units directly above and below the datum are most simply interpretable on the cross section. The cross section in Figure lb uses the horizon labeled F as a datum because this has been interpreted as the top of a chronostratigraphic sequence<ref name=pt06r122 />.

The purpose of the cross section is to determine which horizon can serve as the datum. Because it is shown as horizontal, the thickness variations of the units directly above and below the datum are most simply interpretable on the cross section. The cross section in [[:file:geological-cross-sections_fig1.png|Figure lb]] uses the horizon labeled F as a datum because this has been interpreted as the top of a chronostratigraphic sequence.<ref name=pt06r122 />

An unconformity is commonly used as a datum. In many circumstances, unconformities represent relatively uniform and geologically important time horizons and are therefore useful features on which to hang cross sections. However, caution must be used since the sedimentary layers may reflect paleotopographic relief.

An unconformity is commonly used as a datum. In many circumstances, unconformities represent relatively uniform and geologically important time horizons and are therefore useful features on which to hang cross sections. However, caution must be used since the sedimentary layers may reflect paleotopographic relief.

===Orientation and layout of the cross section===

===Orientation and layout of the cross section===

[[file:geological-cross-sections_fig2.png|thumb|{{figure number|2}}Schematic stratigraphic cross section along part of the north flank of the Wilmington anticline in the Long Beach unit showing log displays. Distance scale is irregular to make the cross section more compact. The left track of each log is an SP or gamma ray trace and the right track is a resistivity trace. (From <ref name=pt06r122 />.)]]

[[file:geological-cross-sections_fig2.png|thumb|{{figure number|2}}Schematic stratigraphic cross section along part of the north flank of the Wilmington anticline in the Long Beach unit showing log displays. Distance scale is irregular to make the cross section more compact. The left track of each log is an SP or gamma ray trace and the right track is a resistivity trace. (From Slatt et al.<ref name=pt06r122 />)]]

The orientation of a cross section must be chosen to balance the need for a clear representation of the features of interest with the availability of appropriate information. In development geology, this information comes largely from well data (geophysical logs, mudlogs, and cores), but in some places, outcrops and seismic reflection data can be used to constrain interpretations (see [[Wellsite methods]], [[Wireline methods]], [[Laboratory methods]], and [[Geophysical methods]]).

The orientation of a cross section must be chosen to balance the need for a clear representation of the features of interest with the availability of appropriate information. In development geology, this information comes largely from well data (geophysical logs, mudlogs, and cores), but in some places, outcrops and seismic reflection data can be used to constrain interpretations (see [[Wellsite methods]], [[Wireline methods]], [[Laboratory methods]], and [[Geophysical methods]]).

When the main source of data is well logs, it is traditional to lay out cross sections to connect wells, which may result in a zigzag path in map view. The cross section is built simply by connecting selected horizons with straight lines and avoids the errors introduced by inexact projection of the data onto a single plane of section. This type of layout results in a distorted view of structural forms if one also constructs a structural cross section of the same wells, as apparent dips will vary along such a section, making a smooth structure appear irregular in form. In horizons with rapidly varying thicknesses, this approach can also create apparent irregularities in thickness.

When the main source of data is well logs, it is traditional to lay out cross sections to connect wells, which may result in a zigzag path in map view. The cross section is built simply by connecting selected horizons with straight lines and avoids the errors introduced by inexact projection of the data onto a single plane of section. This type of layout results in a distorted view of structural forms if one also constructs a structural cross section of the same wells, as apparent dips will vary along such a section, making a smooth structure appear irregular in form. In horizons with rapidly varying thicknesses, this approach can also create apparent irregularities in thickness.

For the purpose of stratigraphic correlation and interpretation, the precise rendering of structural form may be of lesser importance. For example, [[:file:geological-cross-sections_fig2.png|Figure 2]] shows a stratigraphic cross section in which horizontal scale is entirely schematic because stratigraphic and well log variations across a number of fault blocks are the main features of interest and the details of lateral variations are of lesser importance. The path of a cross section that has bends in it (Figure 1), whether to accommodate well location or for other reasons, should always be shown on an index map.

For the purpose of stratigraphic correlation and interpretation, the precise rendering of structural form may be of lesser importance. For example, [[:file:geological-cross-sections_fig2.png|Figure 2]] shows a stratigraphic cross section in which horizontal scale is entirely schematic because stratigraphic and well log variations across a number of fault blocks are the main features of interest and the details of lateral variations are of lesser importance. The path of a cross section that has bends in it ([[:file:geological-cross-sections_fig1.png|Figure 1]]), whether to accommodate well location or for other reasons, should always be shown on an index map.

The preference for sections that connect well locations may be conditioned by the computational burden of projecting well log data onto a single vertical plane. For stratigraphic cross sections, this approach is generally sufficiently exact even when wells are moderately deviated because the vertical scale is exaggerated and differences from the vertical are minimized (see [[:file:geological-cross-sections_fig1.png|Figure 1a]] versus [[:file:geological-cross-sections_fig1.png|1b]]). But the increasing importance of directional drilling means that this approximation is no longer sufficient.

The preference for sections that connect well locations may be conditioned by the computational burden of projecting well log data onto a single vertical plane. For stratigraphic cross sections, this approach is generally sufficiently exact even when wells are moderately deviated because the vertical scale is exaggerated and differences from the vertical are minimized (see [[:file:geological-cross-sections_fig1.png|Figure 1a]] versus [[:file:geological-cross-sections_fig1.png|1b]]). But the increasing importance of directional drilling means that this approximation is no longer sufficient.

Typically the SP or gamma ray log and one resistivity log are displayed ([[:file:geological-cross-sections_fig2.png|Figure 2]]). [[Porosity]] logs may also be important, and if seismic data are part of the cross section, the sonic log is a critical tool to demonstrate the velocity structure, and consistency of conversion of time to depth.

Typically the SP or gamma ray log and one resistivity log are displayed ([[:file:geological-cross-sections_fig2.png|Figure 2]]). [[Porosity]] logs may also be important, and if seismic data are part of the cross section, the sonic log is a critical tool to demonstrate the velocity structure, and consistency of conversion of time to depth.

Lines connecting correlative formation or zone tops between wells will show the lateral variation in thickness of these units. If it is important for the display to show exact correlations on logs, these lines should be drawn horizontally across the log display and angled between the edges of adjacent well displays, such as shown in Figure 2. Straight lines connecting the centers of the well displays may be more appropriate to provide a better representation of the thickness variations of units between wells. If thickness variations or the geometry of units is paramount in importance, then the logs can be reduced in scale so as to form a background or overlay to the formation data. Alternatively, they can be omitted entirely, and well courses can be represented as line segments, as shown in Figure 1(b).

Lines connecting correlative formation or zone tops between wells will show the lateral variation in thickness of these units. If it is important for the display to show exact correlations on logs, these lines should be drawn horizontally across the log display and angled between the edges of adjacent well displays, such as shown in [[:file:geological-cross-sections_fig2.png|Figure 2]]. Straight lines connecting the centers of the well displays may be more appropriate to provide a better representation of the thickness variations of units between wells. If thickness variations or the geometry of units is paramount in importance, then the logs can be reduced in scale so as to form a background or overlay to the formation data. Alternatively, they can be omitted entirely, and well courses can be represented as line segments, as shown in [[:file:geological-cross-sections_fig1.png|Figure 1b]].

If lithological data from core and/or cuttings are available, these can be displayed in columnar form between or alongside log tracks and hung on appropriate well log horizons. Other data that may form an important part of the cross section include hydrocarbon shows, productive horizons, and geochemical data (such as vitrinite reflectance). The same procedures can be applied to constructing outcrop cross sections.

If lithological data from core and/or cuttings are available, these can be displayed in columnar form between or alongside log tracks and hung on appropriate well log horizons. Other data that may form an important part of the cross section include hydrocarbon shows, productive horizons, and geochemical data (such as vitrinite reflectance). The same procedures can be applied to constructing outcrop cross sections.

===Vertical and horizontal scale===

===Vertical and horizontal scale===

To show significant details of stratigraphic variation, it is usually necessary to exaggerate the vertical scale with respect to the horizontal scale on a stratigraphic cross section. It is important to realize the effect that this distortion has on reservoir geometry and angular relationships of geological surfaces. The small angular differences between stratigraphic horizons that account for thickness variations are strongly exaggerated in such a section. The apparent dip of a bed in a vertically exaggerated cross section is related to true dip by the following equation<ref name=pt06r72>Langstaff, C. S., Morrill, D. 1981, Geologic Cross Sections: Boston, MA, IHRDC, 108 p.</ref>:

To show significant details of stratigraphic variation, it is usually necessary to exaggerate the vertical scale with respect to the horizontal scale on a stratigraphic cross section. It is important to realize the effect that this distortion has on reservoir geometry and angular relationships of geological surfaces. The small angular differences between stratigraphic horizons that account for thickness variations are strongly exaggerated in such a section. The apparent dip of a bed in a vertically exaggerated cross section is related to true dip by the following equation:<ref name=pt06r72>Langstaff, C. S., Morrill, D. 1981, Geologic Cross Sections: Boston, MA, IHRDC, 108 p.</ref>

:<math>\tan \delta_{\rm E} = V \tan \delta</math>

:<math>\tan \delta_{\rm E} = V \tan \delta</math>