Changes

===Structural style===

===Structural style===

Appropriate seismic lines, close to the line of section, define the structural style of the folds and faults, so this style should be incorporated directly into the section (Dahlstrom, 1970)<ref name=Dahlstrom_1970 />. Dip domain construction methods are popular guides to section drawing (Suppe, 1983; Groshong, 1989a) in both compressional and extensional systems.

Appropriate seismic lines, close to the line of section, define the structural style of the folds and faults, so this style should be incorporated directly into the section (Dahlstrom, 1970<ref name=Dahlstrom_1970 />). Dip domain construction methods are popular guides to section drawing (Suppe, 1983; Groshong, 1989a<ref name=Groshong_1989a />) in both compressional and extensional systems.

===Use of dipmeter===

===Use of dipmeter===

===Relationship of folding and faulting===

===Relationship of folding and faulting===

Modern theories of structural geology generally relate the formation of folds to accommodation on irregular fault surfaces (Hamblin, 1965<ref name=Hamblin_1965>Hamblin, W. K., 1965, Origin of "reverse drag" on the downthrown side of normal faults: Geological Society of America Bulletin, v. 76, p. 1145-1164.</ref>; Dahlstrom, 1970<ref name=Dahlstrom_1970 />). Generally, the folds are more obvious on seismic sections than faults, but fortunately there are geometric rules that allow us to predict one shape from the other (Suppe, 1983; Verrall, 1982; Gibbs, 1983; Williams and Vann, 1987; Groshong, 1989a) in both extensional and compressional examples. An example of a cross section solution explaining the relationship between extensional rollover and listric faults is shown in Figure 4.  

Modern theories of structural geology generally relate the formation of folds to accommodation on irregular fault surfaces (Hamblin, 1965<ref name=Hamblin_1965>Hamblin, W. K., 1965, Origin of "reverse drag" on the downthrown side of normal faults: Geological Society of America Bulletin, v. 76, p. 1145-1164.</ref>; Dahlstrom, 1970<ref name=Dahlstrom_1970 />). Generally, the folds are more obvious on seismic sections than faults, but fortunately there are geometric rules that allow us to predict one shape from the other (Suppe, 1983<ref name=Suppe_1983>Suppe, J., 1983, Geometry and kinematics of fault-bend folding: American Journal of Science, v. 283, p. 684-721.</ref>; Verrall, 1982<ref name=Verrall_1982>Verrall, P., 1982, Structural interpretation with applications to North Sea problems: Geological Society of London.Course Notes No 3, JAPEC (UK).</ref>; Gibbs, 1983<ref name=Gibbs_1983 />; Williams and Vann, 1987<ref name=Williams_etal_1987>Williams, G., and I. Vann, 1987, The geometry of listric normal faults and deformation in their hanging walls: Journal of Structural Geology, v. 9, p. 789-795.</ref>; Groshong, 1989a<ref name=Groshong_1989a />) in both extensional and compressional examples. An example of a cross section solution explaining the relationship between extensional rollover and listric faults is shown in Figure 4.  

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_1.png|thumb|{{figure_number|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).. ]]

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_1.png|thumb|{{figure_number|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, 1982)<ref>Bengtson, C. A., 1982, Structural and stratigraphic uses of dip profiles, in M. T Halbouty, ed., Deliberate Search for the Subtle Trap: AAPG Memoir 32, p. 31-45.</ref>. ]]

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_2.png|thumb|{{figure_number|2}}Predicted transverse and longitudinal cross sections and contour map derived from scat plots. Depths are subsea depths.. ]]  

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_2.png|thumb|{{figure_number|2}}Predicted transverse and longitudinal cross sections and contour map derived from scat plots. Depths are subsea depths. (From Bengtsen, 1982)<ref>Bengtson, C. A., 1982, Structural and stratigraphic uses of dip profiles, in M. T Halbouty, ed., Deliberate Search for the Subtle Trap: AAPG Memoir 32, p. 31-45.</ref>. ]]  

===Balanced cross sections===

===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 (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 (Dahlstrom, 1970; Woodward et al., 1985)<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 offsets on the faults removed (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 (Dahlstrom, 1970; Woodward et al., 1985<ref name=Woodward_etal_1985 />).

Balanced sections were first constructed for thrust belts, but Gibbs (1983)<ref name=Gibbs_1983>Gibbs, A. D., 1983, Balanced cross section construction from seismic sections in areas of extensional tectonics: Journal of Structural Geology, v. 5, p. 153-160.</ref>, Groshong (1989a), and Rowan and Kligfield (1989)<ref name=Rowan_etal_1989>Rowan, M. G., and R. Kligfield, 1989, 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 (1989)<ref name=Rowan_etal_1989 />, Worrall and Snelson (1989)<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 .

Balanced sections were first constructed for thrust belts, but Gibbs (1983)<ref name=Gibbs_1983>Gibbs, A. D., 1983, Balanced cross section construction from seismic sections in areas of extensional tectonics: Journal of Structural Geology, v. 5, p. 153-160.</ref>, Groshong (1989a)<ref name=Groshong_1989a>Groshong, R. H., 1989a, Half graben structures--balanced models of extensional fault bend folds: Geological Society of America Bulletin, v. 101, p. 96-105.</ref>, and Rowan and Kligfield (1989)<ref name=Rowan_etal_1989>Rowan, M. G., and R. Kligfield, 1989, 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 (1989)<ref name=Rowan_etal_1989 />, Worrall and Snelson (1989)<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 (1990)<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==

==Map construction==

===Structure contour maps===

===Structure contour maps===

The geometry of the field is defined by a series of structure contour maps of key reservoir horizons (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 (see Part 8). 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 (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 (Brown, 1986) (see Part 8). 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.

Faults must be located in wellbores by omission (extension fault) or repetition (reverse fault) of stratigraphic section.

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_3.png|thumb|{{figure_number|3}}Cross section through an asymmetric ramp anticline, whitney canyon field, Wyoming, with scat and isogon data superimposed. Unconformities, axial planes and inflection surfaces have been identified from the dipmeter data and projected away from the wellbore. Isogons are contours of equal dip (see Ramsay, 1967)<ref name=Ramsay_1967 /> and can constrain the shapes of folds in section. (From Lammerson, 1982)<ref name=Lammerson_1982>Lammerson, P. R., 1982, The Fossil basin and its relationship to the Absaroka thrust system, Wyoming and Utah, in R. B. Powers, ed., Geological Studies of the Cordilleran Thrust Belt: Rocky Mountain Association of Geologists, p. 279-340.</ref>. ]]

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_3.png|thumb|{{figure_number|3}}Cross section through an asymmetric ramp anticline, whitney canyon field, Wyoming, with scat and isogon data superimposed. Unconformities, axial planes and inflection surfaces have been identified from the dipmeter data and projected away from the wellbore. Isogons are contours of equal dip (see Ramsay, 1967)<ref name=Ramsay_1967 /> and can constrain the shapes of folds in section. (From Lammerson, 1982)<ref name=Lammerson_1982>Lammerson, P. R., 1982, The Fossil basin and its relationship to the Absaroka thrust system, Wyoming and Utah, in R. B. Powers, ed., Geological Studies of the Cordilleran Thrust Belt: Rocky Mountain Association of Geologists, p. 279-340.</ref>. ]]

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_4.png|thumb|{{figure_number|4}}Modeling extensional fault shapes from the rollover geometry. (a) the groshong (1989b) 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 (1989b) method uses simultaneous modeling of three horizons. Dashed trajectories are individual solutions; solid lines are the preferred solution. (From Hossack, unpubl. Data, 1988). ]]  

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_4.png|thumb|{{figure_number|4}}Modeling extensional fault shapes from the rollover geometry. (a) the groshong (1989b)<ref>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 (1989b)<ref>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 simultaneous modeling of three horizons. Dashed trajectories are individual solutions; solid lines are the preferred solution. (From Hossack, unpubl. Data, 1988). ]]  

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_5.png|thumb|{{figure_number|5}}Example of a balanced section through a complex thrust ramp structure showing both the deformed and undeformed sections. (From Mitra, 1986)<ref name=Mitra_1986>Mitra, S., 1986, Duplex structures and imbricate thrust systems--geometry, structural position, and hydrocarbon potential: AAPG Bulletin, v. 70, p. 1087-1112.</ref>. ]]  

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_5.png|thumb|{{figure_number|5}}Example of a balanced section through a complex thrust ramp structure showing both the deformed and undeformed sections. (From Mitra, 1986)<ref name=Mitra_1986>Mitra, S., 1986, Duplex structures and imbricate thrust systems--geometry, structural position, and hydrocarbon potential: AAPG Bulletin, v. 70, p. 1087-1112.</ref>. ]]  

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. 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 Figure 6a).

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 (Brown, 1986). 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 Figure 6a).

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 (Dickinson, 1954<ref name=Dickinson_1954>Dickinson, G., 1954, Subsurface interpretation of intersecting faults and their effect upon stratigraphic horizons. AAPG Bulletin, v. 38, n. 5, p. 854-877.</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.

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 (Dickinson, 1954<ref name=Dickinson_1954>Dickinson, G., 1954, Subsurface interpretation of intersecting faults and their effect upon stratigraphic horizons. AAPG Bulletin, v. 38, n. 5, p. 854-877.</ref>; Brown, 1986). 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.

===Map restorations===

===Map restorations===

===Fault sealing characteristics===

===Fault sealing characteristics===

The map patterns of oil-water and gas-oil contacts are important features that define field geometry. Common or separate oil-water contacts and gas-oil contacts in separate fault blocks will define sealing and nonsealing faults. The sealing characteristics of faults can also be gauged from the mud weights used during drilling or the production data from the well after completion. These data can be plotted on the maps and sections. A special type of section is the fault plane section of , drawn within the plane of a single fault showing the positions of key reservoir and seal beds on either side of the fault and their contacts against one another across the fault (Figure 7). This type of section allows the interpreter to perceive top seal and cross fault seal potential and pill points and to identify undrilled prospective fault blocks. Fault plane sections may have to be drawn in many different sections, particularly where faults cross-cut or splay off one another (Allan, 1989; Downey, 1984<ref name=Downey_1984>Downey, M. W., 1984, Evaluating seals for hydrocarbon accumulations: AAPG Bulletin, v. 68, p. 1752-1763.</ref>; Smith, 1966<ref name=Smith_1966>Smith, D. A., 1966, Theoretical considerations of sealing and nonsealing faults: AAPG Bulletin, v. 50, p. 363-374.</ref>, 1980).  

The map patterns of oil-water and gas-oil contacts are important features that define field geometry. Common or separate oil-water contacts and gas-oil contacts in separate fault blocks will define sealing and nonsealing faults. The sealing characteristics of faults can also be gauged from the mud weights used during drilling or the production data from the well after completion. These data can be plotted on the maps and sections. A special type of section is the fault plane section of Allan (1989), drawn within the plane of a single fault showing the positions of key reservoir and seal beds on either side of the fault and their contacts against one another across the fault (Figure 7). This type of section allows the interpreter to perceive top seal and cross fault seal potential and pill points and to identify undrilled prospective fault blocks. Fault plane sections may have to be drawn in many different sections, particularly where faults cross-cut or splay off one another (Allan, 1989; Downey, 1984<ref name=Downey_1984>Downey, M. W., 1984, Evaluating seals for hydrocarbon accumulations: AAPG Bulletin, v. 68, p. 1752-1763.</ref>; Smith, 1966<ref name=Smith_1966>Smith, D. A., 1966, Theoretical considerations of sealing and nonsealing faults: AAPG Bulletin, v. 50, p. 363-374.</ref>; Smith, 1980<ref name=Smith_1980>Smith, D. A., 1980, Sealing and nonsealing faults in Louisiana Gulf Coast salt basin: AAPG Bulletin, v. 64, p. 145-172.</ref>).  

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_6.png|thumb|{{figure_number|6}}(a) structure map and (b) (next page) restored structure map showing fault gaps removed. Remaining gaps and overlaps in the restored faults represent geometric incompatibilities in the interpretation. (From Galloway et al. , 1983). ]]  

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_6.png|thumb|{{figure_number|6}}(a) structure map and (b) (next page) restored structure map showing fault gaps removed. Remaining gaps and overlaps in the restored faults represent geometric incompatibilities in the interpretation. (From Galloway et al., 1983)<ref name=Galloway_etal_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>. ]]  

Fig. 6. Continued.  

Fig. 6. Continued.  

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_7.png|thumb|{{figure_number|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.. ]]  

[[File:Hossack_etal__evaluating-structurally-complex-reservoirs__Fig_7.png|thumb|{{figure_number|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, 1989). ]]  

==See also==

==See also==