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|{{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|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==

===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 Parts 3, 4, 5, and 7).

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]]).

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.

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 Figure la versus lb). 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]]).

===Selection of data===

===Selection of data===

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, 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, 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 (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 (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 “Huid 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 (see [[Dipmeters]]).

===Vertical and horizontal scale===

===Vertical and horizontal scale===

{| class = "wikitable"

{| class = "wikitable"

|-

|-

|+ {{table number|Table 1}}True dip versus apparent dip for common vertical exaggerations (Horizontal Scale/Vertical Scale)

|+ {{table number|Table 1}}True dip versus apparent dip for common vertical exaggerations (horizontal scale/vertical scale)

|-

|-

! Vertical Exaggeration

! Vertical Exaggeration

==Computer generation of cross sections==

==Computer generation of cross sections==

Cross sections are now routinely and rapidly constructed by computers. Computers process input data to construct cross sections in two ways, each of which has a pitfall. The more sophisticated method involves the use of program algorithms to interpolate and extrapolate from the limited available data to a complete cross section, such as the connecting of stratigraphic tops with straight lines. This process can go wrong when a well does not have a full set of tops picked such that false correlations are created. Thus, one pitfall to be aware of is the indiscriminant application of software to computer generation of cross sections.

Cross sections are now routinely and rapidly constructed by computers. Computers process input data to construct cross sections in two ways, each of which has a pitfall. The more sophisticated method involves the use of program algorithms to interpolate and extrapolate from the limited available data to a complete cross section, such as the connecting of stratigraphic tops with straight lines. This process can go wrong when a well does not have a full set of tops picked such that false correlations are created. Thus, one pitfall to be aware of is the indiscriminate application of software to computer generation of cross sections.

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 indiscriminant 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.

==See also==

==See also==