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Evaluating-structurally-complex-reservoirs_fig1.png|'''Figure 1.''' SCAT plots used to define the complex structure seen in the discovery well of the Rail Road Gap oil field, California. The five plot types are (from left to right) [[azimuth]] versus depth (A plot), dip versus depth (D plot), dip versus depth in the direction of greatest curvature (T plot), dip versus depth in the direction of least curvature (L plot), and dip versus azimuth (DVA plot). (From Bengtsen.<ref name=Bengtson_1982 />)

Evaluating-structurally-complex-reservoirs_fig1.png|'''Figure 1.''' SCAT plots used to define the complex structure seen in the discovery well of the Rail Road Gap oil field, California. The five plot types are (from left to right) [[azimuth]] versus depth (A plot), dip versus depth (D plot), dip versus depth in the direction of greatest curvature (T plot), dip versus depth in the direction of least curvature (L plot), and dip versus azimuth (DVA plot). (From Bengtsen.<ref name=Bengtson_1982 />)

Evaluating-structurally-complex-reservoirs_fig2.png|'''Figure 2.''' Predicted transverse and longitudinal cross sections and contour map derived from SCAT plots. Depths are subsea depths. (From Bengtsen.<ref name=Bengtson_1982 />)

Evaluating-structurally-complex-reservoirs_fig2.png|'''Figure 2.''' Predicted transverse and longitudinal cross sections and [[contour]] map derived from SCAT plots. Depths are subsea depths. (From Bengtsen.<ref name=Bengtson_1982 />)

Evaluating-structurally-complex-reservoirs_fig3.png|'''Figure 3.''' Cross section through an asymmetrical ramp anticline, Whitney Canyon field, Wyoming, with SCAT and isogen data superimposed. Uncomformities, axial planes, and inflection surfaces have been identified from the diameter data and projected away from the well bore. Isogens are contours of equal dip<ref name=Ramsay_1967></ref> and can constrain the shapes of folds in section. (From Lammerson.<ref name=Lammerson1982>Lammerson, P. R., 1982, The Fossil basin and its relationship to the Absaroka thrust system, Wyoming & Utah, in R. B. Powers, ed., Geological Studies of the Cordilleran Thrust Belt: Rocky Mountain Association of Geologists, p. 279-340.</ref>

Evaluating-structurally-complex-reservoirs_fig3.png|'''Figure 3.''' Cross section through an asymmetrical ramp anticline, Whitney Canyon field, Wyoming, with SCAT and isogen data superimposed. Uncomformities, axial planes, and inflection surfaces have been identified from the diameter data and projected away from the well bore. Isogens are contours of equal dip<ref name=Ramsay_1967></ref> and can constrain the shapes of folds in section. (From Lammerson.<ref name=Lammerson1982>Lammerson, P. R., 1982, The Fossil basin and its relationship to the Absaroka thrust system, Wyoming & Utah, in R. B. Powers, ed., Geological Studies of the Cordilleran Thrust Belt: Rocky Mountain Association of Geologists, p. 279-340.</ref>

Evaluating-structurally-complex-reservoirs_fig4.png|'''Figure 4.''' Modeling extensional fault shapes from the rollover geometry. (a) the Groshong<ref name=Groshong_1989b>Groshong, R. H., 1989b, Structural style and balanced cross sections in extensional terranes: Houston Geological Society Short Course Notes, Feb. 24-25, 128 p.</ref> method uses oblique simple shear with a reference grid constructed with a spacing equal to the fault heave. Distance 2 from the rollover up to regional elevation of the same reference bed is transferred to 2&prime;; likewise, 2&prime; + 4 is transferred to 4&prime; and so on to complete the fault trajectory. Interpolation between these points is carried out using a half grid spacing. (b) fault trajectory reconstruction by the Groshong<ref name=Groshong_1989b /> method uses simultaneous modeling of three horizons. Dashed trajectories are individual solutions; solid lines are the preferred solution. (From Hossack, unpublished data, 1988.)

Evaluating-structurally-complex-reservoirs_fig4.png|'''Figure 4.''' Modeling extensional fault shapes from the rollover geometry. (a) the Groshong<ref name=Groshong_1989b>Groshong, R. H., 1989b, Structural style and balanced cross sections in extensional terranes: Houston Geological Society Short Course Notes, Feb. 24-25, 128 p.</ref> method uses oblique simple shear with a reference grid constructed with a spacing equal to the fault heave. Distance 2 from the rollover up to regional elevation of the same reference bed is transferred to 2&prime;; likewise, 2&prime; + 4 is transferred to 4&prime; and so on to complete the fault trajectory. Interpolation between these points is carried out using a half grid spacing. (b) fault trajectory reconstruction by the Groshong<ref name=Groshong_1989b /> method uses simultaneous modeling of three horizons. Dashed trajectories are individual solutions; solid lines are the preferred solution. (From Hossack, unpublished data, 1988.)

===Dip isogons===

===Dip isogons===

''Dip isogons,'' or contours of equal dip in the plane of the section ([[:Image:Evaluating-structurally-complex-reservoirs_fig3.png|Figure 3]]),<ref name=Ramsay_1967>Ramsay, J. G., 1967, Folding and fracturing of rocks: New York and London, McGraw-Hill, 568 p.</ref> can be used to fill in the geological section. The isogons can be located from projected dipmeter data and projected or correlated between the wellbores following the rules described by Ramsay and Huber.<ref name=Ramsay_etal_1987>Ramsay, J. G., and M. I. Huber, 1987, The techniques of modern structural geology__Volume 2, in Folds and Fractures: Orlando, FL, Academic Press, 700 p.</ref> A series of dip segments along the various isogons helps the geologist sketch the fold profiles. Interpolation between the isogons can also be carried out using the dip domain methods previously described or by cubic spline interpolation.<ref name=McCoss_1987>McCoss, A. M., 1987, [http://www.sciencedirect.com/science/article/pii/0191814187900599 Practical section drawing through folded layers using sequentially rotated cubic interpolators]: Journal of Structural Geology, v. 9, p. 365-370.</ref>

''Dip isogons,'' or [[contour]]s of equal dip in the plane of the section ([[:Image:Evaluating-structurally-complex-reservoirs_fig3.png|Figure 3]]),<ref name=Ramsay_1967>Ramsay, J. G., 1967, Folding and fracturing of rocks: New York and London, McGraw-Hill, 568 p.</ref> can be used to fill in the geological section. The isogons can be located from projected dipmeter data and projected or correlated between the wellbores following the rules described by Ramsay and Huber.<ref name=Ramsay_etal_1987>Ramsay, J. G., and M. I. Huber, 1987, The techniques of modern structural geology__Volume 2, in Folds and Fractures: Orlando, FL, Academic Press, 700 p.</ref> A series of dip segments along the various isogons helps the geologist sketch the fold profiles. Interpolation between the isogons can also be carried out using the dip domain methods previously described or by cubic spline interpolation.<ref name=McCoss_1987>McCoss, A. M., 1987, [http://www.sciencedirect.com/science/article/pii/0191814187900599 Practical section drawing through folded layers using sequentially rotated cubic interpolators]: Journal of Structural Geology, v. 9, p. 365-370.</ref>

===Relationship of folding and faulting===

===Relationship of folding and faulting===

===Structure contour maps===

===Structure contour maps===

The geometry of the field is defined by a series of structure contour maps of key reservoir horizons ([[:Image:Evaluating-structurally-complex-reservoirs_fig6.png|Figure 6a]]). The maps, showing several levels through the prospect or reservoir, are generated from well elevations of reference beds or depth-converted seismic sections (see [[Subsurface maps]]). Workstations for three-dimensional [[seismic interpretation]] considerably aid the process because the shapes of the structure contours and the faults are readily observable on horizontal seiscrop sections generated by the workstation.<ref name=Brown_1986>Brown, A. R., 1986, Interpretation of three-dimensional seismic data: [http://store.aapg.org/detail.aspx?id=1025 AAPG Memoir 42], 194 p.</ref> Contour maps can be quickly generated from stacked seiscrop sections.

The geometry of the field is defined by a series of structure [[contour]] maps of key reservoir horizons ([[:Image:Evaluating-structurally-complex-reservoirs_fig6.png|Figure 6a]]). The maps, showing several levels through the prospect or reservoir, are generated from well elevations of reference beds or depth-converted seismic sections (see [[Subsurface maps]]). Workstations for three-dimensional [[seismic interpretation]] considerably aid the process because the shapes of the structure contours and the faults are readily observable on horizontal seiscrop sections generated by the workstation.<ref name=Brown_1986>Brown, A. R., 1986, Interpretation of three-dimensional seismic data: [http://store.aapg.org/detail.aspx?id=1025 AAPG Memoir 42], 194 p.</ref> Contour maps can be quickly generated from stacked seiscrop sections.

Faults must be located in wellbores by omission (extension fault) or repetition (reverse fault) of stratigraphic section. These are defined on the electric logs by repetition or omission of parts of the SP and gamma ray signatures compared to a reference well that is believed to show an unfaulted section. Fault map trends and dip direction can also be defined by SCAT dipmeter analysis or on the stacked three-dimensional seiscrop sections. Generally, fault cuts have to be correlated from well to well to define the dip and curvature of the fault. Once these are estimated, fault contour maps can be generated by contouring the subsurface elevations of the fault cuts or, more directly, on the seismic workstation by stacking the seiscrop sections.<ref name=Brown_1986></ref> The faults will offset the reference beds, and the amount of offset in section and map view must be estimated. Once the separation is known, a separatin surface can be projected along the fault retaining the same trend, but adjusted in value by an amount appropriate for the offset on the fault (see [[:Image:Evaluating-structurally-complex-reservoirs_fig6.png|Figure 6a]]).

Faults must be located in wellbores by omission (extension fault) or repetition (reverse fault) of stratigraphic section. These are defined on the electric logs by repetition or omission of parts of the SP and gamma ray signatures compared to a reference well that is believed to show an unfaulted section. Fault map trends and dip direction can also be defined by SCAT dipmeter analysis or on the stacked three-dimensional seiscrop sections. Generally, fault cuts have to be correlated from well to well to define the dip and curvature of the fault. Once these are estimated, fault contour maps can be generated by contouring the subsurface elevations of the fault cuts or, more directly, on the seismic workstation by stacking the seiscrop sections.<ref name=Brown_1986></ref> The faults will offset the reference beds, and the amount of offset in section and map view must be estimated. Once the separation is known, a separatin surface can be projected along the fault retaining the same trend, but adjusted in value by an amount appropriate for the offset on the fault (see [[:Image:Evaluating-structurally-complex-reservoirs_fig6.png|Figure 6a]]).