Page 20 - Origin and Prediction of Abnormal Formation Pressures

INTRODUCTION TO ABNORMALLY PRESSURED FORMATIONS 3

pore fluids. These AHFPs can only exist if the formation is separated by impermeable
barriers that contain the pressure in the reservoir. The origins of these barriers may be
physical, chemical, or a combination of both (Louden, 1972). There are a multitude
of origins for AHFPs among which are (1) compaction, (2) tectonic compression, (3)
faulting, (4) diapirism, (5) unusually high geothermal temperature gradients, (6) phase
changes of minerals, (7) hydrocarbon (oil and gas) generation, (8) upward migration of
hydrocarbon gases along faults, and (9) osmosis. Formation of a fluid seal (caprock) in
the subsurface and development of the zone of abnormally high pore pressure is a highly
complex mechanism. All of the mechanisms listed above, in any combination, with the
passage of geologic time work together to cause the changes in the physicochemical
environment (Fertl, 1976).

ORIGIN OF VERTICAL BARRIERS RESULTING IN ABNORMAL FORMATION PRESSURES

Fig. l-1 shows the approximate average subsurface pressure gradient. The rate of sed-
imentation and compaction and the density of the rock determine the overburden pres-
sure gradient. As indicated in Fig. 1-1, the hydrostatic pressure gradient is 10.5 kPa/m
(0.454 psi/ft), and at the other extreme, the lithostatic gradient is about 22.6 kPa/m (1.0

Hydrostatic gradient
10.5 kPa/m (0.454 psi/ft)
\
9 Lithostatic gradient
~
22.6 kPa/m (I.0 psi/ft)
A 6 a
!
o ~,~ Geopressured zone
m 3
x v "~~0.3 kPa/m (0.9 psi/ft}
F-
LU
UJ 9
IJ. \
-r"
I--
Q.
LU -.
"12
%
, \
\ 9
" \
\
16 \ %
- 0 20 40 60 80 I00 120
PRESSURE (mPo)
I I I I I I i I I I
0 2 4 6 8 I0 12 14 16 18
PRESSURE (PSI x 10 -3 )
Fig. 1-l. Approximate average subsurface pressure gradient in a geopressured zone.