Changes

* ''Structural cross sections'', which show the present day geometry of an area

* ''Structural cross sections'', which show the present day geometry of an area

* ''Stratigraphic cross sections'', which show prior geometric relationships by adjusting the elevation of geological units to some chosen geological horizon (Figure 1).

* ''Stratigraphic cross sections'', which show prior geometric relationships by adjusting the elevation of geological units to some chosen geological horizon ([[:file:geological-cross-sections_fig1.png|Figure 1]]).

 

[[file:geological-cross-sections_fig1.png|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>.)]]

A third type of cross section called a ''balanced cross section'' is a combination of these two. This type attempts to portray the form of geological units prior to some episode of deformation (see [[Evaluating structurally complex reservoirs]]). It can provide important conclusions about present day geometry and past stratigraphic relationships.

A third type of cross section called a ''balanced cross section'' is a combination of these two. This type attempts to portray the form of geological units prior to some episode of deformation (see [[Evaluating structurally complex reservoirs]]). It can provide important conclusions about present day geometry and past stratigraphic relationships.

==Stratigraphic cross sections==

==Stratigraphic cross sections==

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.

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

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

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, 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 (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 Figure la versus lb). 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.

In a substantially deviated well, it is important to correct for the deviation of the wellbore to give a proper representation of the stratigraphic thickness of units. In many areas, this can be accomplished by using a true stratigraphic thickness (TST) log (see [[Conversion of well log data to subsurface stratigraphic and structural information]]).

In a substantially deviated well, it is important to correct for the deviation of the wellbore to give a proper representation of the stratigraphic thickness of units. In many areas, this can be accomplished by using a true stratigraphic thickness (TST) log (see [[Conversion of well log data to subsurface stratigraphic and structural information]]).

If the object of the cross section is to show lateral and vertical details of the stratigraphy, log properties are of utmost importance.

If the object of the cross section is to show lateral and vertical details of the stratigraphy, log properties are of utmost importance.

Typically the SP or gamma ray log and one resistivity log are displayed (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 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).

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==Cross sections in three dimensions==

==Cross sections in three dimensions==

When the full three-dimensional aspect of a field must be shown, a single cross section or even a suite of cross sections may not be sufficient. The display of numerous wells in a three-dimensional array can be accomplished by a ''fence diagram'', in which the datum horizon is represented by the plane of the map. Well plots are displayed vertically, with the datum at the well location on the map plane (Figure 3). The wells are the “fence posts,” and the lines connecting formation tops are the “rails” that give this diagram its name. Geological relationships can also be portrayed on a ''block diagram'', in which the sides and top of a schematic block cut into the earth at the location of interest are shown in a three-dimensional representation (Figure 4).

[[file:geological-cross-sections_fig4.png|thumb|{{figure number|4}}An example of a block diagram.]]

[[file:geological-cross-sections_fig3.png|thumb|{{figure number|3}}An example of a fence diagram]]

When the full three-dimensional aspect of a field must be shown, a single cross section or even a suite of cross sections may not be sufficient. The display of numerous wells in a three-dimensional array can be accomplished by a ''fence diagram'', in which the datum horizon is represented by the plane of the map. Well plots are displayed vertically, with the datum at the well location on the map plane ([[:file:geological-cross-sections_fig3.png|Figure 3]]). The wells are the “fence posts,” and the lines connecting formation tops are the “rails” that give this diagram its name. Geological relationships can also be portrayed on a ''block diagram'', in which the sides and top of a schematic block cut into the earth at the location of interest are shown in a three-dimensional representation ([[:file:geological-cross-sections_fig4.png|Figure 4]]).

 

[[file:geological-cross-sections_fig4.png|thumb|{{figure number|4}}An example of a block diagram.]]

==Computer generation of cross sections==

==Computer generation of cross sections==