Changes

  | isbn    = 0891816607

  | isbn    = 0891816607

}}

}}

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.

[[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>)]]

[[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 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.

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 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.

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]].

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===

[[Category:Geological methods]]

[[Category:Geological methods]]