Geology Reference
In-Depth Information
Fig. 8.13.
Structure contour map illus-
trating fault cutoff line in-
ferred from throw.
Heavy solid
line:
known cutoff line;
heavy
dashed line:
inferred cutoff
line on opposite side of fault;
T:
throw at known points,
arrows
point directly up the
fault dip
8.3.2.3
Restored Vertical Separation
Contouring based on restored vertical separation (restored tops) is appropriate where
the geometry of the hangingwall and footwall beds is unchanged by faulting except for
the vertical separation on the fault (Jones et al. 1986; Tearpock and Bischke 2003). This
method is used to find the shape of a surface before faulting and its configuration after
a displacement amount equal to the vertical separation of the fault (Fig. 8.14). Begin
the interpretation by finding the vertical separation from the stratigraphic separation
at a fault cut (well 4, Fig. 8.14a). To calculate the vertical separation (Eq. 7.3) it is nec-
essary to know or assume the dip of bedding at the fault cut. For dips below about 25°,
the stratigraphic separation is similar in magnitude to the vertical separation and can
be used as an approximation. The stratigraphic separation is always less than the ver-
tical separation, but below a marker dip of 25° the difference is under about 10%. Next,
remove the vertical separation by adding it to all marker elevations on the downthrown
side of the fault (wells 1-3, Fig. 8.14a). Then contour the marker horizon as if no fault
were present to obtain the restored top (Fig. 8.14a). Then, subtract the vertical sepa-
ration from all contour elevations on the downthrown side of the fault (Fig. 8.14b). Re-
contour the data while maintaining the shapes of the structure contours from the re-
stored-top map. Break the map at the fault by intersecting the contoured fault surface
with the marker horizon on both the upthrown and downthrown sides of the fault.
This method ensures that the vertical separation on the fault is included on the final
map and that the marker geometry is related across the fault.
The strength of this method is that it guarantees that the marker horizon shape is
related across the fault. The weakness in the method is that in many structural styles
the hangingwall and footwall geometries should not have the same shape (Fig. 8.10a,b).
If there is any strike-slip component to the fault displacement, it must be removed
before the points are contoured in order to correctly relate the hangingwall and foot-
wall geometry.
