Geology Reference
In-Depth Information
Fig. 8.18.
Fault cutoff map (horizontal projection) of throw on the coal seams for the fault zone indi-
cated by the
heavy solid lines
in Fig. 8.17. The section is along azimuth 321°.
Dashed
cutoff lines belong
to the footwall (
FW
) and
solid
cutoff lines belong to the hanging-wall (
HW
)
The cutoff map (Fig. 8.18) shows that the fault separation dies out to the southeast
and has a maximum at about 6 000 ft on the profile. Both seams have similar throws,
with the maximum decreasing downward from 90 ft in the America seam to 80 ft
in the Mary Lee seam. The dip separation on the 60° dipping fault is 104 ft in the
America seam and 92 ft in the Mary Lee seam (obtained from
L
=
T
/sin
φ
, where
L
= dip separation,
T
=throw, and
= fault dip). Assuming that the point of maxi-
mum dip separation is close to the center of the fault, the length/displacement ratio
is about 8 000 / 98 or 82 to 1.
φ
8.4.2
Determination of Fluid Migration Pathways
Fault cutoff maps are extremely useful in determining the possible routes of fluid
migration in fault zones where the fault plane itself is not a barrier to the migration
(Allan 1989). A fault trap is a closure in a permeable bed that is sealed by imperme-
able units across the fault. Multiple porous beds in the section can lead to very com-
plex migration paths (Fig. 8.19) in which the migrating fluid crosses back and forth
across the fault. Figure 8.19 shows eight fold closures against the fault, only three of
which are sealed for the up-dip migration of a light fluid like oil or gas. The spill
point at the base of each closure is located where permeable beds are in contact across
the fault. Fluids that are heavier than water, such as man-made contaminants, may
spiral downward across a fault into synclinal closures at some distance from the origi-
nal contamination site.
The sequential migration of hydrocarbons through multiple traps like those in
Fig. 8.19 can lead to a reversal in the expected positions of the oil and gas. The ex-
pected sequence in the filling of a trap in place is illustrated in Fig. 8.20. Thermal
maturation of the hydrocarbon source leads first to the formation of oil (Fig. 8.20a)
which is less dense than water and displaces the water in the trap. Gas forms later in
the source bed or in the trap itself and, being less dense than the oil, displaces the oil
to form a gas cap on the reservoir (Fig. 8.20b). The formation of a large volume of
hydrocarbons can fill the trap to the spill point where the closure is no longer complete
and allow displaced hydrocarbons to be forced out at the base of the accumulation to
continue migrating up dip. The process of spilling from the base of the reservoir causes
the most dense hydrocarbons to continue migrating up dip into new traps (Gussow
