Changes

366 bytes added , 19:24, 19 January 2022

m

added Category:Methods in Exploration 10 using HotCat

Line 11: Line 11:

| pdf = http://archives.datapages.com/data/specpubs/methodo1/images/a095/a0950001/0300/03310.pdf

}}

−

Evaluation of a structurally complex reservoir requires integration of geological and geophysical data to generate maps and cross ~~sections~~ that show the attitude, geometry, and thickness of key reservoir beds; the locations of crestal highs and synclinal troughs; the position, dip, and character of faults; and the location and orientation of the cutoffs of key beds on both sides of the faults. Also, fault blocks must be accurately delineated since they can effectively compartmentalize a reservoir. Maps and sections must be integrated so that they agree with each other, and they should be tested for viability and admissibility by balancing or backstripping.

+

Evaluation of a structurally complex reservoir requires integration of geological and geophysical data to generate maps and [[cross section]]s that show the attitude, geometry, and thickness of key reservoir beds; the locations of crestal highs and [[Syncline|synclinal]] troughs; the position, [[dip]], and character of faults; and the location and orientation of the cutoffs of key beds on both sides of the faults. Also, fault blocks must be accurately delineated since they can effectively compartmentalize a reservoir. Maps and sections must be integrated so that they agree with each other, and they should be tested for viability and admissibility by balancing or [[backstripping]].

==Section construction==

−

+

−

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, 1982~~{{citation needed}}~~.)

+

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 [[fold]]s 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′; likewise, 2′ + 4 is transferred to 4′ 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′; likewise, 2′ + 4 is transferred to 4′ 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_fig5.png|'''Figure 5.''' Example of a balanced section through a complex thrust ramp structure showing both the deformed and undeformed sections.<ref name=Mitra_1986>Mitra, S., 1986, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0070/0009/1050/1087.htm Duplex structures and imbricate thrust systems-Geometry, structural position, and hydrocarbon potential]: AAPG Bulletin, v. 70, p. 1087-1112.</ref>

</gallery>

===Well and seismic constraints===

−

Cross ~~sections~~ and maps are usually constructed simultaneously and need to be continually checked against each other (see [[Geological cross sections]] and [[Subsurface maps]]). The first well in a field provides depth, velocity, dipmeter, and hydrocarbon distribution information that can improve the accuracy of predrill seismic and geological interpretations. Time pick corrections and seismic reprocessing generate improved seismic sections. Ideally, seismic sections should be time migrated and displayed with no vertical exaggeration so that true scale cross sections can be constructed for section restoration and balancing.<ref name=Dahlstrom_1970>Dahlstrom, C. D. A., 1970, Structural geology in the eastern margin of the Canadian Rocky Mountains: Canadian Society of Petroleum Geologists Bulletin, v. 18, p. 332-406.</ref>

+

[[Cross section]]s and maps are usually constructed simultaneously and need to be continually checked against each other (see [[Geological cross sections]] and [[Subsurface maps]]). The first well in a field provides depth, velocity, [[dipmeter]], and hydrocarbon distribution information that can improve the accuracy of predrill seismic and geological interpretations. Time pick corrections and seismic reprocessing generate improved seismic sections. Ideally, seismic sections should be time migrated and displayed with no vertical exaggeration so that true scale cross sections can be constructed for section restoration and balancing.<ref name=Dahlstrom_1970>Dahlstrom, C. D. A., 1970, Structural geology in the eastern margin of the Canadian Rocky Mountains: Canadian Society of Petroleum Geologists Bulletin, v. 18, p. 332-406.</ref>

Production wells may lie off the lines of seismic and geological section so that data will have to be projected carefully onto the lines using seismic cross lines or down-plunge projection.<ref name=DePaor_1988>DePaor, D., 1988, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0072/0001/0050/0073.htm Balanced section in thrust belts, Part 1--Construction]: AAPG Bulletin, v. 72, p. 73-90.</ref>

===Orientation===

−

A balanced section can only be constructed in the direction of the regional displacement direction of the faults or in the direction of flexural slip on the fold limbs.<ref name=Dahlstrom_1970 /> The appropriate direction can be chosen by an analysis of regional structure maps using the bow string rule, lateral ramps, or drawing the section normal to the trend of the regional compressional or extensional folds.<ref name=Woodward_etal_1985>Woodward, N. B., S. E. Boyer, and J. Suppe, 1985, An outline of balanced cross sections, 2nd ed.: University of Tennessee Department of Geological Sciences, Studies in Geology 11.</ref>

+

A balanced section can only be constructed in the direction of the regional displacement direction of the faults or in the direction of flexural slip on the fold limbs.<ref name=Dahlstrom_1970 /> The appropriate direction can be chosen by an analysis of regional structure maps using the bow string rule, [[lateral]] ramps, or drawing the section normal to the trend of the regional compressional or extensional folds.<ref name=Woodward_etal_1985>Woodward, N. B., S. E. Boyer, and J. Suppe, 1985, An outline of balanced cross sections, 2nd ed.: University of Tennessee Department of Geological Sciences, Studies in Geology 11.</ref>

===Structural style===

Line 39: Line 39:

===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===

Line 45: Line 45:

===Balanced cross sections===

−

Balanced cross sections are used to test the viability or admissibility of a cross section. The deformed cross section is redrawn on a template in the undeformed state so that the beds are unfolded and the ~~offsets~~ on the faults removed ([[:Image:Evaluating-structurally-complex-reservoirs_fig5.png|Figure 5]]). Section balancing requires reference pin-lines and loose lines at opposite ends of the section from which measurements of bed lengths are made. Bed thicknesses and bed lengths are generally retained so that the deformed and undeformed cross sections have the same area. For an ideal restoration, there should be no gaps or overlaps between adjacent fault blocks.<ref name=Woodward_etal_1985 />

+

Balanced cross sections are used to test the viability or admissibility of a cross section. The deformed cross section is redrawn on a template in the undeformed state so that the beds are unfolded and the [[offset]]s on the faults removed ([[:Image:Evaluating-structurally-complex-reservoirs_fig5.png|Figure 5]]). Section balancing requires reference pin-lines and loose lines at opposite ends of the section from which measurements of bed lengths are made. Bed thicknesses and bed lengths are generally retained so that the deformed and undeformed cross sections have the same area. For an ideal restoration, there should be no gaps or overlaps between adjacent fault blocks.<ref name=Woodward_etal_1985 />

Balanced sections were first constructed for thrust belts, but Gibbs,<ref name=Gibbs_1983>Gibbs, A. D., 1983, [http://www.sciencedirect.com/science/article/pii/0191814183900408 Balanced cross section construction from seismic sections in areas of extensional tectonics]: Journal of Structural Geology, v. 5, p. 153-160.</ref> Groshong,<ref name=Groshong_1989a /> and Rowan and Kligfield<ref name=Rowan_etal_1989>Rowan, M. G., and R. Kligfield, 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0008/0950/0955.htm Cross section restoration and balancing as aid to seismic interpretation in extensional terranes]: AAPG Bulletin, v. 73, p. 955-966.</ref> have successfully applied the method to extensional and salt-related structures. Extensional section balancing is more difficult than compressional balancing because of the bed thickness changes that occur across faults. The balancing template has to show these thickness changes accurately. Generally, computer-aided methods are essential because they can sequentially backstrip the section to remove tectonic as well as compaction strains. Examples of these are described by Rowan and Kligfield,<ref name=Rowan_etal_1989 /> Worrall and Snelson,<ref name=Worrall_etal_1989>Worrall D. M., and S. Snelson, 1989, Evolution of the northern Gulf of Mexico with emphasis on Cenozoic growth faulting and the role of salt, in A. W. Bally and A. R. Palmer, The Geology of North America-An Overview: Geological Society of America, v. A, p. 97-138.</ref> and Shultz-Ela and Duncan.<ref>Schultz-Ela, D., and Duncan, K., 1990, Users manual and software for Restore, version 2.0: The Univ. of Texas Bureau of Economic Geology, 75 p.</ref>

==Map construction==

−

+

−

~~[[File:~~Evaluating-structurally-complex-reservoirs_fig6.png~~|thumbnail~~|'''Figure 6.''' a) Structure map and b) restored structure map showing fault gaps removed. Remaining gaps and overlaps in the restored faults represent geometric incompatibilities in the interpretation. (From Galloway and Hobday.<ref name=Galloway and Hobday_1983>Galloway, W. E., and D. K. Hobday, 1983, Terrigenous clastic depositional systems applications to petroleum, coal, and uranium exploration: New York, Springer Verlag, 423 p.</ref>)]]

+

Evaluating-structurally-complex-reservoirs_fig6.png|'''Figure 6.''' a) Structure map and b) restored structure map showing fault gaps removed. Remaining gaps and overlaps in the restored faults represent geometric incompatibilities in the interpretation. (From Galloway and Hobday.<ref name=Galloway and Hobday_1983>Galloway, W. E., and D. K. Hobday, 1983, Terrigenous clastic depositional systems applications to petroleum, coal, and uranium exploration: New York, Springer Verlag, 423 p.</ref>)

+

Evaluating-structurally-complex-reservoirs_fig7.png|'''Figure 7.''' Fault plane section and structure map of a model field to show the effects of Syncline|synclinal and cross fault spilling. (a) simple anticlinal closure cut by an extensional fault with two stacked reservoirs on both the downthrown and upthrown sides. Positions of cross fault spill points and synclinal spill points shown. (b) fault plane section illustrating the synclinal and cross fault spill points. Reservoir beds are shown hatchured, whereas seal horizons are shown white. Note the effect of thick seal trapping across the fault. (From Allan.<ref name=Allan_1989 />)

+

</gallery>

===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]]).

−

[[File:Evaluating-structurally-complex-reservoirs_fig7.png|thumb|left|'''Figure 7.''' Fault plane section and structure map of a model field to show the effects of synclinal and cross fault spilling. (a) simple anticlinal closure cut by an extensional fault with two stacked reservoirs on both the downthrown and upthrown sides. Positions of cross fault spill points and synclinal spill points shown. (b) fault plane section illustrating the synclinal and cross fault spill points. Reservoir beds are shown hatchured, whereas seal horizons are shown white. Note the effect of thick seal trapping across the fault. (From Allan.<ref name=Allan_1989 />)]]

Bed contours and fault contours have to be combined in a series of overlays to generate the structure map. Initially, individual fault blocks bounded on all sides by faults have to be contoured separately.<ref name=Dickinson_1954>Dickinson, G., 1954, [http://archives.datapages.com/data/bulletns/1953-56/data/pg/0038/0005/0850/0854.htm Subsurface interpretation of intersecting faults and their effect upon stratigraphic horizons]: AAPG Bulletin, v. 38, n. 5, p. 854-877.</ref> <ref name=Brown_1986></ref> The intersections between the bed and the fault contours of equivalent elevation value have to be identified to define the line of intersection of the bed and the fault. These lines are the fault cutoffs of the beds. There are two on each fault, one in the hanging wall and the other in the footwall. For extensional faults, there is a gap between the cutoffs where the key reference bed is omitted, and the gap in map view defines the heave across the fault.

Line 86: Line 86:

[[Category:Geological methods]] [[Category:Test content]]

+

[[Category:Methods in Exploration 10]]

Cwhitehurst

Bureaucrats, Interface administrators, Administrators

10,323

edits

Changes

Structurally complex reservoirs: evaluation (view source)

Revision as of 19:24, 19 January 2022

Navigation menu

Search