Page 349 - Petroleum Geology

319

portant: it must generate a volume of water greater than that that can be dis-
sipated, so that the rate of generation exceeds the rate of dissipation.

Tectonic
Various authors have suggested uplift as a cause of abnormal pressures. A
pressure regime that is normal for one depth would, if elevated and preserved,
be abnormal for shallower depths. There may be areas in which this has oc-
curred, but the unambiguous evidence of the major regressive sequences with
abnormal pressures, such as the US. Gulf Coast, the Niger delta and several
sedimentary basins of South-east Asia, is that the abnormal pressures were
generated during subsidence and the accumulation of the regressive sequence,
and that these areas are still subsiding (p. 352).
Likewise, tectonic compression cannot be a general cause because the Niger
delta and the U.S. Gulf Coast are tectonically passive, and there is unambi-
guous evidence from growth faults that the stress field is and was one with a
component of horizontal tension.
Nevertheless, tectonic compression could cause abnormal pressures in mud-
stones, and Berry (1973) has described a zone in California 650-800 km
long and 40-130 km wide (400-500 X 25-80 miles) associated with the
San Andreas fault.
There is little doubt that on a local scale, abnormal pressures may be asso-
ciated with faulting, the pressures being due to mechanical deformation of
the mudstone and the consequent tendency to reduce porosity.

Osmosis

When a semi-permeable membrane separates two liquids of different salini-
ties, the less saline liquid moves through the membrane into the more saline
liquid. This tendency to equalize chemical potentials across the membrane
can lead to pressure differences. As a process that could generate abnormal
pressures in thick mudstones of regressive sequences, it is unconvincing.
Magara (1978, pp. 283-284) has argued that the osmotic gradient in a mud-
stone opposes the generation of abnormally high pore pressures in mudstones
by assisting the expulsion of pore water. There is laboratory evidence (McKelvey
and Milne, 1962; Von Engelhardt and Gaida, 1963) that water expelled early
from a mudstone is more saline than that expelled later. And there is field
evidence (Schmidt, 1973) that the salinity of mudstone pore water is less
than that of the adjacent sandstones when the latter are normally pressured.
When abnormally pressured, the salinity of sandstone pore water tends to be
less than that of normally pressured sandstones, and more comparable to
that of the abnormally pressured mudstone.
Hill et al. (1961) suggested that osmosis might be the cause of abnormally
low pore pressures in the Mesaverde sandstone in central U.S.A. and the Viking