Changes

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]]).

[[File:Evaluating-structurally-complex-reservoirs_fig7.png|thumb|300px|'''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 />)]]

[[File:Evaluating-structurally-complex-reservoirs_fig7.png|thumb|300px|'''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 />)]]

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.

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.