Geology Reference
In-Depth Information
On an isopach map a thickness anomaly will follow the trace of the fault (Fig. 7.8).
A fault with normal separation will cause the unit to be thinner and a reverse separa-
tion will cause the unit to be thicker. The greater the stratigraphic separation of the
fault, the greater the thickness change from the regional value. The maximum thinning
is to zero thickness but the maximum thickening can be any amount, depending on the
number of repetitions that occur within the boundaries of the formation. The example
shows an east-west trending fault that is losing displacement to the west while an
intersecting fault to the north increases in displacement to the west. See Sect. 8.5 for
the quantitative interpretation of fault separation from isopach maps.
7.2.5
Discontinuity in Stratigraphic Sequence
The recognition of a fault at a point such as in an outcrop or a well log is commonly based
on a break in the continuity of the normal stratigraphic sequence (Fig. 7.9). Except for
the special case of fault slip exactly parallel to the stratigraphic boundaries (Redmond
1972), some amount of the stratigraphic section is always missing or repeated at the
fault contact. The measure of the fault magnitude that can be obtained from the infor-
mation available at a point on a fault is the stratigraphic separation, equal to the thick-
ness of the missing or repeated section (Bates and Jackson 1987). Only the stratigraphic
separation can be determined at a point on the fault, not the slip. Determination of the
slip requires additional information. The true slip on the faults in Fig. 7.9 could be
oblique or even strike slip if the dip of bedding is oblique to the dip of the fault.
The method for determining the position of the fault cut and the amount of the
stratigraphic separation is the same in the field as in the subsurface. A well log is the
same as a section measured across a fault. The faulted section must be correlated to a
reference section. Correlate upward in the footwall (Fig. 7.10) until the sections no
longer match. Then correlate downward in the hangingwall until the sections no longer
match. The mismatch will occur at the same place in the faulted section if a single fault
is present. A mismatch in upward and downward correlation that fails to occur at the
same place in the faulted section could be caused by the presence of more than one
fault or by a fault zone of finite thickness. In the former situation, the process should
be repeated on a finer scale until all the faults are located.
Fig. 7.9.
Cross sections of faults recognized from a stratigraphic discontinuity where the well cuts a
fault.
a
Normal separation. Part of the normal stratigraphic sequence is missing in a vertical well.
b
Reverse separation. Part of the normal stratigraphic sequence is repeated in a vertical well
