Page 90 - Petroleum Geology

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which several independent lines of argument lead to the same conclusions.
They are unfortunately rare. We are also guided by principles that have
come to be accepted by common experience and consent, but even the Prin-
ciple of Superposition applies strictly only in a vertical sequence in one
place; if applied carelessly to diachronous units, it leads to erroneous conclu-
sions.
It has been noted by previous writers that there is more disagreement
than agreement amongst geologists, and that consequently many of us are
in error much of the time. The major cause of disagreement is lack of under-
standing of all the disciplines involved. One ordinary person cannot know
enough, and ignorance of one facet of a problem means that judgment is
incomplete. Conflicting evidence is also a cause of disagreement, but that too
is the result of ignorance.
Take for example the movement of oil through permeable rock. Gravity
segregation of gas, oil and water, in that order of increasing density with
depth in a single reservoir, is demonstrable in a great many petroleum reser-
voirs. When a well is put onto production, oil moves through the rock to the
well. There is, therefore, no difficulty at all in accepting the migration of
oil through permeable rocks. Yet those familiar with capillary forces insist
that oil disseminated as a separate phase in water cannot easily be moved
through the pore spaces. When we produce oil from a reservoir, we can
produce only about 113 of the oil in place, and significant quantities of oil
are left in the pore spaces of a reservoir that is being depleted. We know that
it is very difficult to displace this residual oil, and impossible to displace all
of it artificially with water. Evidently, physical continuity of the oil phase is
necessary for oil movement; but even then, not all the oil moves. Most newly
discovered oil reservoirs show not the slightest trace of oil below the oil/water
interface (and those that do can usually be explained by production from

Depth
Temperature
Water pressure
Water salinity
Water density
Oil density
Clay density

Fig. 4-1. Qualitative correlation half-matrix of common associations with depth. Brackets
indicate a tendency.