Page 97 - Petroleum Geology

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Crude oil is said to be saturated when no more gas can be taken into so-
lution - or unsaturated. It may also be saturated, with an associated gas cap.
Petroleum gas is usually a mixture of several hydrocarbons and exists in
the subsurface as separate accumulations, in association with oil accumpla-
tions, in solution in oil, and in solution in formation water. Gas is compres-
sible, and under certain conditions of temperature and high pressure, some
of the components may be liquid in the subsurface (their physical properties
then being similar to those of oil). It is important as a substance in its own
right as a fuel and as a feedstock for industrial chemical plants. It is also the
prime source of energy in many oil reservoirs. Methane is the dominant con-
stituent of natural gas, usually with smaller amounts of alkanes with higher
carbon numbers. A gas may be wet and contain liquid oil vapours, or dry.
Carbon dioxide is usually present as a contaminant; nitrogen is found in
some areas, notably the German North Sea. Sulphur is a common contami-
nant that must be removed before the gas (or oil) is used as a fuel. Most of
the world’s commercial sulphur comes from this source.
Solid hydrocarbons are relatively rare, and are perhaps best known from
Pitch Lake in south-west Trinidad, where the semi-solid bitumen is mined at
the surface. There are other surface occurrences, such as that of the Berm&
dez Pitch Lake in eastern Venezuela. They also occur as bitumen dykes.
Bitumen is also valuable to the geologist in its elastico-viscous properties, for
it demonstrably flows yet can be broken by a hammer.
Varieties of solid bitumen include albertite, elaterite, gilsonite, grahamite
and wurtzillite. Kerogen is a solid bituminous substance disseminated in sedi-
mentary rocks, and in “oil shales” and coals. There are several varieties, which
will be considered in some detail in Chapter 10. It consists of about 80% car-
bon, with oxygen, hydrogen, sulphur, and some nitrogen. It is a pyrobitu-
men, yielding hydrocarbons on heating (in the laboratory, to temperatures
much higher than those in petroleum reservoirs). Some varieties appear to be
primary source material for oil, others for gas; but kerogen itself is the insol-
uble residue of diagenesis of organic matter, and so may be the residue of
early petroleum genesis.

Water

Water is the most common fluid in pore spaces in sedimentary rocks in the
subsurface. It is found in many parts of the world within a few metres of the
surface; and in most parts of the world within a few tens of metres of the
surface. Water has been iound in the deepest wells drilled, and is probably
only eliminated as free water during metamorphism.
Water is important, of course, as a natural resource when it is relatively
free of dissolved solids. For the geologist, though, the role of water in the
rocks is of fundamental importance, equal to that of the solid constituents.
For the petroleum geologist, it could almost be said to be more important