Page 89 - Petroleum Geology

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able reserves and the siting of development wells that will produce these re-
serves as efficiently as possible.
The exploration geologist is concerned with regional geology deduced
from surface outcrop, geophysical surveys, and the results of any boreholes
drilled in the area. In spite of the enormous advances in geophysical tech-
niques, the stratigraphy of the area may only be determinable in a rather
general sense. The development geologist, on the other hand, is concerned
with the detailed stratigraphic sequence, and its structure, over a relatively
small area. Yet this detailed stratigraphic sequence is not obtained from a
study of the rocks themselves, but rather from the electrical and other geo-
physical responses obtained in the boreholes.
It is commonly said that the petroleum geologist has the advantage of
working without financial restraint. This is rarely, if ever, true. It is true
that many areas are investigated using several disciplines, some of which
(geophysics, drilling) are very expensive to apply, but all will be operating
under some financial restraint. It is not so often said, but is nevertheless
true, that most petroleum geological work has a time constraint (which is
also a financial constraint) and so the conclusions may be based on inade-
quate data. Much of this work is carried out in parts of the world that would
otherwise have waited decades for investigation, and it is carried out with
time limitations. The danger of false inference is always present. When such
work finds its way into print, the conclusions are likely to be accepted by
geologists with no local knowledge. There are also competitive restraints. It
would be invidious to mention specific published articles, but we have all
experienced the frustration of papers that give formation names but no
lithologies, the stratigraphy of the petroleum-bearing part of the sequence
but not of those overlying and underlying it.
The great advances in geological knowledge, thought and understanding
during the 19th Century are attributable largely to the construction of coal
mines, railways and canals. A study of petroleum geology suggests that
the construction of boreholes in the 20th Century has not contributed as
much to modern geological thought as it could, and that some of the diffi-
culties we encounter are due to the extension of concepts developed from
surface and near-surface geology to the subsurface, and a failure to revise
these concepts in the light of the borehole data. We shall return to this topic
in later chapters.

GEOLOGICAL INFERENCE
Geological inference is a process of argument based on observations and
inductions. The nature of the argument may be logical, as when determining
the relative ages of superimposed strata, or by analogy with areas where the
evidence is more conclusive. Conclusive geological arguments are those in