Geology Reference
In-Depth Information
surface mapping based on wells, and in constructing maps from two-dimensional
seismic data. Correlating faults in an area of multiple faults is one of the most chal-
lenging problems in structural interpretation. A number of fault properties can
be used to establish the correlation between fault cuts. Correlation criteria include
the fault trend, the sense of throw across the fault, the smoothness of the fault surface,
the amount of separation, and the growth history. The rules for correlation that are
presented below may have exceptions and should be used in combination to obtain
the best result.
7.7.1
Trend and Sense of Throw
The consistency of the fault trend and the consistency of the sense of throw are
usually the first two criteria applied in the correlation of fault cuts. The fault trend
can sometimes be established at the observation points by direct measurement
in outcrop, by physically tracing the fault, or by SCAT analysis of outcrop traverses
or dipmeter logs (Chap. 9). If, for example, the fault trend in the map area of
Fig. 7.29 can be shown to be east-west, then observation points 2 and 3 are more
likely to be on the same fault than are points 1 and 3 or 1 and 2 (Fig. 7.30a), and the
fault through point 1 is also likely to trend east-west. A preliminary structure con-
tour map on a horizon for which there is good control may give a clear indication
of the fault trends (Fig. 7.30b). Faults that are directly related to the formation of the
map-scale folds are commonly parallel to the strike of bedding, especially in regions
where the contours are unusually closely spaced. Such zones may represent unrecog-
nized faults.
The high and low areas on a structure contour map of a marker horizon should be
explained by the proposed faults. The upthrown and downthrown sides of a fault will
commonly be the same along strike. For example, the proposed fault linking observa-
tion points 2 and 3 in Fig. 7.30b is downthrown on the north at both locations. A con-
stant sense of vertical separation is a common and reasonable pattern, but not the only
possibility on a correctly interpreted fault.
Faults that offset folds may produce complex patterns of horizontal and vertical
separations. Vertical displacement on a fault that strikes at a high angle to fold axes
Fig. 7.30.
Maps showing possible corre-
lations between fault cuts at
points 1-3. a Correlations
( dashed lines ) along an east-
west trend. b Preliminary
structure contour map on a
marker horizon. Fault cor-
relations ( heavy solid lines
marked F) along structure con-
tour trends are supported by
the consistent sense of throw
across the faults
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