Geology Reference
In-Depth Information
Fig. 8.14.
Cross sections in the fault dip
direction, showing the method
of restored vertical separation
to control position of the
marker horizon. The dip
shown at the fault cutoff is
used to determine the vertical
separation.
a
Fault separation
is added to all marker eleva-
tions on the downthrown side
of the fault and elevations are
contoured to give the restored
top.
b
Vertical separation is
subtracted from the down-
thrown side and the marker
horizon re-contoured to give
the inferred geometry
8.4
Fault Cutoff Maps and Allan Diagrams
A fault cutoff map is a map of the traces of beds where they are truncated against a
fault (Fig. 8.15). Maps can be made of either or both hangingwall and footwall cutoffs.
Superimposed hangingwall and footwall cutoff maps are known as Allan diagrams
from their use by Allan (1989) in the prediction of fluid migration pathways and traps
for hydrocarbons migrating in the vicinity of a fault zone. The cutoff lines are also
termed Allan lines. Fault cutoff maps provide quality control on the bed geometries at
the fault, are useful in predicting traps and spill points of hydrocarbon reservoirs formed
against faults, and potentially provide a relatively unambiguous method for determin-
ing the slip on a fault without piercing points.
8.4.1
Construction
The first step in producing a fault cutoff map is to project the faulted horizons to the
fault surface and find the lines of intersection between the horizons and the fault, as
has been discussed in Sect. 8.3.2.1. Projections of the fault cutoffs onto either vertical
or horizontal planes are convenient when the section is made directly from a com-
pleted structure contour map. A vertical map plane is suitable where the fault is steeply
dipping (Fig. 8.16) and will show the throw on the fault. If the fault is curved in plan
view, a vertical projection foreshortens the length of the fault. A horizontal map plane
is suitable where the fault is gently dipping and will show the heave on the fault
