Page 93 - Petroleum Geology

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and used as an example, detailed study led Haeberle (1951) to conclude that
variations of facies were intimately associated with the variations of oil den-
sity. This association in its turn suggests that the source of the oil is strati-
graphically close to the reservoirs. However, the environmental influence is
unlikely to be the only influence because temperature demonstrably affects
the diagenesis of organic matter to crude oil.
The goal of petroleum geology and the industry cannot be to drill every
anticline (one is reminded of the definition of an engineer as one who can
do for $1 what any fool can do for $2). The goal of petroleum geology must
be to understand the processes of oil and gas generation, migration, and en-
trapment so well that the speculative element in the industry is significantly
reduced and finally eliminated. Even if this goal is unattainable, it is worth
seeking; and some measure of success must be achieved if the remaining un-
discovered reserves are to be discovered economically. Petroleum geology is
not just a technically based variety of geology. It contibutes significant ideas
as well as important data to the geological understanding of the Earth, and
will certainly continue to do so.

PETROLEUM GEOLOGY

The study of petroleum geology centres logically around the three main
processes - petroleum generation, migration, and accumulation. However,
the emphasis is placed in the reverse order. Entrapment is the heart of the
industry. It is observable, definable and measurable.
The experience of the industry is that petroleum occurs more commonly
and in larger quantities in sedimentary basins than in areas of thin and in-
complete sedimentary sequences; more commonly and in rather larger quan-
tities in rocks of Mesozoic and Tertiary ages than in older or younger rocks;
and more commonly in anticlinal traps than in other types of trap. Expe-
rience also indicates that petroleum is found more cheaply in sedimentary
basins where petroleum has been found before, and in areas where petroleum
has been found before.

Pe troleu m

The chemistry of petroleum is a subject in itself, and some knowledge of
it is essential for petroleum geologists because of the importance of geo-
chemistry in petroleum exploration and the urgent need to solve the prob-
lems of petroleum genesis and alteration in nature. The books of Tissot and
Welte (1978) and Hunt (1979) are recommended for serious study.
Petroleum is a natural substance that occurs in the Earth as semi-solids,
liquids, or gases - or mutual solutions of these. There are two types of
petroleum liquids: crude oil, which leaves a residue of compounds of high