Page 223 - Petroleum Geology

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fluid potential gradient. The oil moves upwards, and displaces the water
downwards until hydraulic continuity is lost and the water saturation becomes
irreducible. At this stage, we infer, the oil comes into contact with the solid
surfaces (or a very thin layer of adsorbed water not more than about 1 nm
thick) and isolates the pendular rings.
The reasons for this are not clear. Just prior to the acquisition of irreduc-
ible water saturation, the effective permeability to water is very small indeed,
while the effective permeability to oil approaches the intrinsic permeability.
It is possible, therefore, that the steadily increasing reservoir pressure, con-
comitantly reducing effective stress and increasing porosity (Terzaghi’s rela-
tionship) proceed at a rate that cannot be matched by water movement. The
inferred size of pendular rings suggests small capillary pressures and so early
acquisition of irreducible water saturation. Whatever the cause, a significant
proportion of the pore space remains filled with water that is apparently im-
mobile. This water is the original water at the time of petroleum accumula-
tion, and may therefore differ in quality from the water subsequently found
below the oillwater contact.
Petroleum accumulation may therefore affect the diagenesis of the reser-
voir rock within the accumulation. A most interesting study of the Gifhorn
Trough in Germany (Phillip et al., 1963) revealed that the reservoir sands
contained little cement within the accumulations but were well-cemented
outside. The authors drew the conclusion that the accumulation of oil inhibit-
ed the deposition of cement, and that therefore the oil accumulated before
the processes of cementation had proceeded very far. Similar observations
led to similar conclusions for some Nigerian accumulations (Lambert-Aik-
hionbare, 1982), and in Triassic gas sands in the North West Shelf of Australia
(Campbell and Smith, 1982).
It must therefore be noted that if porosity, as determined from the sonic
or other log, is found to increase upwards across a water contact, this may
well be a real effect and not one induced by the pore fluid’s influence on the
velocity of sound (or the parameter being logged).

ORIGIN AND MIGRATION OF PETROLEUM IN CARBONATE ROCKS

While most commercial accumulations originate from organic matter in
fine-grained argillaceous source rocks, some accumulations seem to require
a source in mark or limestones. There are no particular difficulties with this.
We assume that the physico-chemical requirements for a carbonate source
rock are the same as for argillaceous source rocks - the accumulation of
organic matter with sediment in an anoxic environment, and its preservation
until buried deep enough for the processes of petroleum generation to begin.
There would be few field geologists who have not found dark, dense lime-
stones that give off a bituminous smell on fracture.