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Geological cross sections are graphical representations of vertical slices through the earth used to clarify or interpret geological relationships with or without accompanying maps. As with other tools applied to petroleum development, cross sections are used to portray geological information in a visual form so that reservoir characteristics can be readily interpreted. For example, a thorough understanding of regional structural and stratigraphic relationships may lead to better characterization of reservoir flow units (see [[Flow units for reservoir characterization]]).
Geological cross sections are graphical representations of vertical slices through the earth used to clarify or interpret geological relationships with or without accompanying maps. As with other tools applied to [[petroleum]] development, cross sections are used to portray geological information in a visual form so that reservoir characteristics can be readily interpreted. For example, a thorough understanding of regional structural and stratigraphic relationships may lead to better characterization of reservoir flow units (see [[Flow units for reservoir characterization]]).
There are two major classes of cross sections used in understanding petroleum reservoirs.
There are two major classes of cross sections used in understanding petroleum reservoirs.
* ''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]]).
* ''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]]).
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==
[[file:geological-cross-sections_fig1.png|thumb|300px|{{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>)]]
[[file:geological-cross-sections_fig1.png|thumb|300px|{{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., S. Phillips, J. M. Boak, and M. B. Lagoe, 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 [[fold]]s or the thickening of the section across [[growth fault]]s. 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 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 1b]] 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 1b]] 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|300px|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 [[Basic open hole tools#Spontaneous potential|SP]] or [[Basic open hole tools#Gamma ray|gamma ray]] trace and the right track is a resistivity trace. (From Slatt et al.<ref name=pt06r122 />)]]
[[file:geological-cross-sections_fig2.png|300px|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 [[Basic open hole tools#Spontaneous potential|SP]] or [[Basic open hole tools#Gamma ray|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, [http://www.merriam-webster.com/dictionary/outcrop outcrops] and [[Seismic data|seismic reflection data]] can be used to constrain interpretations.
Stratigraphic sections should be oriented perpendicular to depositional strike (dip or transverse section) to show facies changes toward or away from the basin margin. Strike sections parallel to the basin margin should be drawn to show lateral variations of particular beds or sequences. In the tectonic context of a basin, these axes are also structural axes. Determining the orientation of a stratigraphic section is also complicated by the fact that stratigraphic trends may be at any angle to subsequent structural trends.
Stratigraphic sections should be oriented perpendicular to depositional strike ([[dip]] or transverse section) to show facies changes toward or away from the basin margin. Strike sections parallel to the basin margin should be drawn to show [[lateral]] variations of particular beds or sequences. In the tectonic context of a basin, these axes are also structural axes. Determining the orientation of a stratigraphic section is also complicated by the fact that stratigraphic trends may be at any angle to subsequent structural trends.
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.
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 [[Basic open hole tools#Spontaneous potential|SP]] or [[Basic open hole tools#Gamma ray|gamma ray]] log and one [[Basic open hole tools#Resistivity|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 [[Basic open hole tools#Spontaneous potential|SP]] or [[Basic open hole tools#Gamma ray|gamma ray]] log and one [[Basic open hole tools#Resistivity|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 [[: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]].
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., and D. Morrill, 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>
* δ = true dip
* δ = true dip
* ''V'' = vertical exaggeration, or
* ''V'' = vertical exaggeration, or
:: <math>\frac{I_v}{I_h}</math>, the ratio of vertical scale (''I''<sub>''v''</sub>) to horizontal scale (''I''<sub>''h''</sub>)
As a result of this relationship, low dips are exaggerated and appear larger, whereas higher dips all appear close to vertical. The effect is illustrated in Table 1, where selected values of true and apparent dip are shown for vertical exaggerations of five and ten times. Note that two horizons differing in dip by only 3° appear to differ by 14° and 22°, respectively, for the two values of vertical exaggeration. Any attempt to render structural form on a stratigraphic cross section is schematic but should take into account this effect. It is also important to remember that the image one creates with a stratigraphic cross section is a distortion of reality.
As a result of this relationship, low dips are exaggerated and appear larger, whereas higher dips all appear close to vertical. The effect is illustrated in Table 1, where selected values of true and apparent dip are shown for vertical exaggerations of five and ten times. Note that two horizons differing in dip by only 3° appear to differ by 14° and 22°, respectively, for the two values of vertical exaggeration. Any attempt to render structural form on a stratigraphic cross section is schematic but should take into account this effect. It is also important to remember that the image one creates with a stratigraphic cross section is a distortion of reality.
Linear cross sections are preferably oriented perpendicular to the major structural trends (dip or transverse sections). Bends in the section can be introduced to accommodate variable structural trends or to show different features. In a straight section, much of the data will usually be projected into the plane of section. Accomplishing this projection requires detailed knowledge of the strike direction. If the structural trend is variable so that the cross section is not everywhere perpendicular to strike, data should be projected along strike onto the section. To fully represent the structure, several transverse sections may be linked by a longitudinal or strike section running parallel to the strike. Strike sections may also be important in showing the plunge of a structure, culminations in a fold, or the importance of secondary structures (for example, normal faults across a fold axis).
Linear cross sections are preferably oriented perpendicular to the major structural trends (dip or transverse sections). Bends in the section can be introduced to accommodate variable structural trends or to show different features. In a straight section, much of the data will usually be projected into the plane of section. Accomplishing this projection requires detailed knowledge of the strike direction. If the structural trend is variable so that the cross section is not everywhere perpendicular to strike, data should be projected along strike onto the section. To fully represent the structure, several transverse sections may be linked by a longitudinal or strike section running parallel to the strike. Strike sections may also be important in showing the plunge of a structure, culminations in a fold, or the importance of secondary structures (for example, normal faults across a fold axis).
Some structures plunge steeply (>30°), producing distortion of the geometry in a vertical cross section, so that it may be preferable to construct a profile section in which the plane of section is perpendicular to the plunge of the structure rather than being vertical. This section will be important for understanding geological history and of less importance for understanding the relationship of fluids in the associated reservoirs. However, one type of nonvertical section may be crucial to understanding the filling of reservoirs. This is the ''fault plane section'',<ref name=pt06r3>Allan, V. S. 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0007/0800/0803.htm Model for hydrocarbon migration and entrapment within faulted structures]: AAPG Bulletin, v. 73, p. 803–811.</ref> which is constructed from well or seismic data to represent the surface of a fault with the trace of units that intersect the fault on either side.
Some structures plunge steeply (>30°), producing distortion of the geometry in a vertical cross section, so that it may be preferable to construct a profile section in which the plane of section is perpendicular to the plunge of the structure rather than being vertical. This section will be important for understanding geological history and of less importance for understanding the relationship of fluids in the associated reservoirs. However, one type of nonvertical section may be crucial to understanding the filling of reservoirs. This is the ''fault plane section'',<ref name=pt06r3>Allan, V. S., 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0007/0800/0803.htm Model for hydrocarbon migration and entrapment within faulted structures]: AAPG Bulletin, v. 73, p. 803–811.</ref> which is constructed from well or seismic data to represent the surface of a fault with the trace of units that intersect the fault on either side.
===Selection of data===
===Selection of data===
For understanding the geometry of structures (folds and faults), an undistorted view of the shapes of geological units is important. Logs can be reduced in size with only the major units represented ([[:file:geological-cross-sections_fig1.png|Figure la]]). Where well control is dense and computers are available, it may be best to construct structural cross sections by using gridded and contoured stratigraphic surfaces and drawing each horizon as one would a topographic profile.
For understanding the geometry of structures (folds and faults), an undistorted view of the shapes of geological units is important. Logs can be reduced in size with only the major units represented ([[:file:geological-cross-sections_fig1.png|Figure la]]). Where well control is dense and computers are available, it may be best to construct structural cross sections by using gridded and contoured stratigraphic surfaces and drawing each horizon as one would a topographic profile.
If it is important to demonstrate the control of structure on fluid contacts, it may be vital to show the primary log data from which these are interpreted (see [[Fluid contacts]]). Other data, such as dips from a dipmeter log, can be schematically represented (see [[Dipmeters]]).
If it is important to demonstrate the control of structure on fluid contacts, it may be vital to show the primary log data from which these are interpreted (see [[Fluid contacts]]). Other data, such as dips from a [[dipmeter]] log, can be schematically represented.
===Vertical and horizontal scale===
===Vertical and horizontal scale===
==Cross sections in three dimensions==
==Cross sections in three dimensions==
<gallery mode=packed heights=200px widths=200px>
geological-cross-sections_fig3.png|{{figure number|3}}An example of a fence diagram.
geological-cross-sections_fig4.png|{{figure number|4}}An example of a block diagram.
</gallery>
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]]).
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]]).
The second way computers function to streamline the preparation of cross sections is to allow the geologist to enter and manipulate one's own interpretations. This capability is especially useful when dealing with discontinuities such as faults that are not handled well by mathematical algorithms. However, a pitfall here is the indiscriminate application of personal interpretation or opinion. If one's interpretation disagrees with that made by the computer, it is important to discover why the two interpretations differ.
The second way computers function to streamline the preparation of cross sections is to allow the geologist to enter and manipulate one's own interpretations. This capability is especially useful when dealing with discontinuities such as faults that are not handled well by mathematical algorithms. However, a pitfall here is the indiscriminate application of personal interpretation or opinion. If one's interpretation disagrees with that made by the computer, it is important to discover why the two interpretations differ.
==Further reading==
==Further reading==
* [[Conversion of well log data to subsurface stratigraphic and structural information]]
* [[Conversion of well log data to subsurface stratigraphic and structural information]]
* [[Evaluating stratigraphically complex fields]]
* [[Evaluating stratigraphically complex fields]]
* [[Structural maps and cross sections]]
* [[ Regional maps and cross sections]]
==References==
==References==
[[Category:Geological methods]]
[[Category:Geological methods]]
[[Category:Methods in Exploration 10]]