Page 265 - Petroleum Geology
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fore a source at higher energy than the accumulation is required in the area
of the permeable reservoir.
The importance of resolving this conflict of evidence is obvious, because
either the geochemical approach is right or the physical approach is right:
they cannot both be right. By far the more serious of the two possibilities of
error is the first, because exploration takes geochemistry more seriously into
account than fluid mechanics. If the geochemical concept of maturity of
source rock is in error, as seems possible from the Ekofisk data, areas may be
dismissed as unprospective when they should not be. Some questions that
need answers for Ekofisk are:
(1) If the analytical techniques that discriminated against a Paleocene source
for the accumulated oil are valid, why do they not also discriminate against
migration “through the section”? Is this migration through the section seen
as leakage from the reservoir, or by some other path?
(2) Where is the oil in the Paleocene mudstone going?
(3) If faults are not conduits for oil migration, do physical arguments similar
to those above preclude a Jurassic source? Does the postulated Jurassic source
satisfy the requirement of possessing greater energy than the oil at the oil/
water contact, with a path of continuously decreasing energy between them?
(4) Are there two sources for this oil, above and below?
The most important contemporary problem of petroleum geology is, in
my view, the reconciliation of geochemistry and fluid mechanics because,
once petroleum has been generated, its movement is governed by physical
laws within geological constraints. The most fundamental of these is that
fluids lose energy while in motion. If the source postulated for an accumula-
tion has less energy than the accumulation, then it must be established that
it could have had higher energy at the time of migration, or that there was
a source of energy. No real progress can be made in understanding petroleum
generation and migration until this central problem is understood.
Ekofisk is not an isolated example. About 400 km north of Ekofisk lies
the Frigg gas field, on the Norwegian and United Kingdom boundary. This
field, one of the largest offshore gas fields in the world, was found in what
has been interpreted as a submarine fan (Hkritier et al., 1979,1980) of Paleo-
cene age, sealed by middle Eocene mudstones. Recoverable reserves are esti-
mated at 7 Tcf (200 X lo9 m3), and the gas is 95.5% methane. There is a 10
m naphthenic oil zone that cannot be economically produced on account of
its density (23--24”API, s.g. 0.91-0.92). The pressures are normal hydrostatic.
The crude oil of the main Frigg accumulation is described as “anomalous,
suggesting biodegradation caused by bacteria” (Hkritier et al., 1979, p. 2018;
1980, p. 78), compounds with carbon numbers less than C,7 being a minor
fraction of the crude oil, and n-alkanes are almost absent. This quality is not
shared with other accumulations in the area: the East Frigg pool, for example,
also has an oil leg, but it contains a significant proportion of n-alkanes. If we
are correct in our conclusion that such alterations took place during second-