Geology Reference
In-Depth Information
Fig. 8.19.
Allan diagram showing fluid
migration pathways through
permeable beds separated by
impermeable beds; footwall
shaded
.
Arrows
give migration
routes of fluids that are lighter
than water. Oil accumulations
are
solid black
; gas accumula-
tion is indicated by
vertical
lines
. (After Allan 1989)
Fig. 8.20.
Sequential formation of oil
(
black
) and gas (
vertical lines
and
open circles
) and filling
of a trap.
a
Burial to the tem-
perature of the formation of
oil.
b
Additional burial to the
temperature of the formation
of thermal gas. The gas dis-
places oil in the trap
1954; Allan 1989). The lightest hydrocarbons, mainly gas, remain in the deeper traps.
Thus the presence of deep gas-filled traps and shallow oil-filled traps as shown in
Fig. 8.19 can be caused by the upward migration of hydrocarbons across multiple spill
points (assuming that oil and gas are both thermally stable at the trap depths).
8.4.3
Determination of Fault Slip
The slip on a fault is normally determined by the offset of geological lines that pierce
the plane of the fault. Geological lines may be formed by original stratigraphic fea-
tures, such as linear sand bodies or paleo-shorelines; intersection lines, such as a vein-
bed intersection or a vein-vein intersection; or a structural feature like a fold hinge
line. An Allan diagram provides an excellent format for the display of the cutoff infor-
mation.
An anticline-syncline pair offset by a normal fault (Fig. 8.21a) appears at first glance
to have both right-lateral and left-lateral slip, a common map pattern where faults cut
folded beds (see also Fig. 7.17). The Allan diagram (Fig. 8.21b) makes it clear that the
fault is dip slip and shows the amount of the slip from the offset of the fold hinge lines.
