Geology Reference
In-Depth Information
Fig. 7.28.
Expansion index for a fault that begins to move, reaches a growth maximum and then stops.
a
Cross section.
b
Expansion index vs. stratigraphic interval graph. (After Thorsen 1963)
tory. If the relative offset is large and if shale units are being compared at less than
3 000 ft of burial, it might be important to correct
E
for compaction. At greater depths,
the relative change in thickness will be small and have little effect on
E
.
The expansion index plot characterizes the growth history of a fault and might be
correlated to other time-dependent features such as the sand/shale ratio or the time
of hydrocarbon migration. The use of
E
eliminates the effect of absolute interval thick-
ness, allowing the growth rates of a generally thin interval to be directly compared to
that of a generally thick interval. The use of the similarity between expansion index
plots at different fault cuts as an aid to fault correlation will be illustrated in Sect. 7.7.4.
7.7
Fault-Cut Correlation Criteria
Faults are commonly mapped on the basis of observations made at a number of sepa-
rate locations, called fault cuts. If more than one fault may be present, a very signifi-
cant problem is to establish which observations belong to the same fault (Fig. 7.29).
This problem arises in surface mapping where the outcrop is discontinuous, in sub-
Fig. 7.29.
Map showing three locations
where faults have been ob-
served. Which, if any, points
are on the same fault?
Num-
bered circles
are observation
points,
dashed lines
are some
possible fault correlations
