Geology Reference
In-Depth Information
Fig. 7.10.
Location and amount of stratigraphic separation determined from the missing or repeated
section across the fault.
a
About 300 m of section is missing across a normal fault.
b
About 600 m of
section is repeated across a reverse fault
An overturned fold limb can easily be mistaken for a fault zone. The upward and
downward correlations will break down at the axial surfaces that bound the over-
turned limb. The overturned sequence will appear to be an unfamiliar unit and so
is easily interpreted as being a fault zone. Try correlating an upside-down stratigraphic
column to the possible fault zone to see if an overturned section is present. Beds
near the fault may be steeply dipping, causing a great exaggeration of the bed thick-
nesses in a vertical well, representing another factor to consider when correlating to
the type section.
The precision of the determination of stratigraphic separation depends on the level
of detail to which the stratigraphy is known. For example, if the fault cut in Fig. 7.10a
was at the top of unit e rather than near the base, the missing section would be signifi-
cantly (50-75 m) greater. The error in the amount of missing section is the sum of the
stratigraphic uncertainties in the units on both sides of the fault. Lack of stratigraphic
resolution means that small faults may not be detectable by correlation of sections.
The
fault cut
refers to the position of the fault in a well and the to the amount of
the missing or repeated section compared to the reference section (Tearpock and
Bischke 2003). The location of the fault cut, the amount of the fault cut and the ref-
erence section should be recorded for each fault cut. It is not unusual to find that after
preliminary mapping has been completed, the original stratigraphic thicknesses were
inappropriate. For example, the units in a reference well may be shown by mapping
to have a significant dip and their thicknesses therefore will have been exaggerated.
Mapping may also reveal that the reference section is faulted. Any change in thick-
ness in the reference section requires changes in the magnitudes of all the fault cuts
determined from it.
