Page 350 - Petroleum Geology

Sandstone in Canada, both of which not only have pressures below normal
hydrostatic but have waters that are fresh. The same conditions are found in
the Molasse basin of southern Germany.
Some pore pressure anomalies appear to be related to osmosis (see Fertl,
1976, pp. 33-36), but there is no evidence that the generality of abnormal
pressures is so caused, and some that it is not so caused.

Petroleum generation

Petroleum generation could affect mudstone pore pressures in two ways:
the generation of petroleum itself could involve an increase in net volume,
and the products could reduce the relative permeabilities to water and to
petroleum. We shall take the latter first.
Chapman (1972) noted that upward flow of water through the transition
zone is towards lower pressures, and concluded that any petroleum exsolu-
tion during this migration would reduce the effective permeability of the
mudstone to water and so reduce the rate of expulsion of pore fluids. Such a
process cannot eliminate permeability, but if relative permeability diagrams
determined for sandstone reservoirs (p. 168, Fig. 8-12) are generally applic-
able to mudstones, the permeability to both fluids together (i.e., the sum of
their relative permeabilities) may be reduced to about l/lOth of its value for
a single fluid.
Visser and Hermes (1962, p. 228) described the drilling of the Gesa anti-
cline in the Mamberamo delta, Irian Jaya, and the problems encountered with
abnormal pressures. Methane and some carbon dioxide were found in the mud-
stones only, and so could have contributed to these pressures by reducing
the permeability to the fluids. Gas is sometimes (but not always) detected in
the mud while drilling the transition zone (see Fertl, 1976, pp. 137-140, for
a fuller discussion).
Illing (1938, p. 227) stated that the generation of oil from organic matter
involved an increase in volume, and that this could give rise to considerable
pressures in compacted rocks. Hedberg (1974,1980) suggested that the genera-
tion of methane and other hydrocarbons of low molecular weight is an im-
portant source of energy for primary migration, abnormal pressures, mud-
stone diapirs, and mudvolcanoes. Such pressures are enhanced and retained
by the effects on effective permeabilities.
In the Lower Mississippian Bakken Shale of the Williston basin in North
America, Meissner (1978) found abnormal pressures in certain areas over cer-
tain intervals. The prevailing pressure regime in the sequence is even a little
below normal hydrostatic, and Meissner argued that the abnormality is directly
due to oil generation in a relatively well-compacted source rock.
Similarly, the Kimmeridge Clay (Jurassic) of the northern North Sea is
overpressured and considered by many to be the source rock of much of the
North Sea oil. This example is confused, but not irrelevantly to our purpose,