Page 264 - Petroleum Geology

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Paleocene mudstones off-structure, showed both rearranged and unrearranged
steranes. This relationship, according to Seifert and Moldowan (1978) is char-
acteristic of relatively immature organic matter. Van den Bark and Thomas
drew attention to the Jurassic (Kimmeridge) mudstones encountered in Eko-
fisk wells, in which the vitrinite reflectance values range from R, = 0.93 to
1.16%, indicating the peak range of thermal maturity for hydrocarbon genera-
tion. They also produced the results of other analyses that support the conclu-
sion that the accumulated crude oil has a Jurassic source, showing that Juras-
sic extracted oil has several features in common with the accumulated oil,
while these features are not shared with Paleocene extracted oils. The manner
of migration of the Jurassic hydrocarbons “is not yet clear”, but they suggested
that faults in other parts of Ekofisk have acted as conduits. They attributed
the oil in the Paleocene mudstone cap rock to “oil migrating through the sec-
tion” and concluded that it is not indigenous.
This is a good example of a conclusion that was based on published data
being later retracted for reasons that are not published in detail. We are not
told of the difficulties in accounting for oil migration from the Jurassic source
to the accumulation, but a consequential hypothesis - faults acting as con-
duits - is introduced to overcome the difficulties. Nor are we told why the
“definite evidence” of dissimilarities between accumulated oil and oil in the
Tertiary mudstones supports the other consequential hypothesis that the
latter oil is migrating through the section.
The physical reasons for postulating the overlying Paleocene source origi-
nally have not been refuted, and are worth further examination. The facts
appear to be these:
- There is a downward energy gradient from the overlying mudstones to
the top of the reservoir,
- There is oil in these mudstones that has “the same general characteristics”
as the oil in the accumulation,
- The pressures in the reservoir are well above normal hydrostatic.
If these are indeed facts, they seem to require the following conclusions:,
- Crude oil from the reservoir cannot enter the cap rock because it has in-
sufficient energy. This was the conclusion of Byrd (1975, p. 443) and, curious-
ly, also the conclusion of Van den Bark and Thomas (“such a downward pres-
sure drop serves to strengthen the shale caprock . . .”: Van den Bark and
Thomas, 1981, p. 2357).
Theref ore :
- The oil in the Paleocene mudstone cannot be oil migrating upwards
through the section;
- The downward energy gradient suggests that the Paleocene mudstone is
the source of at least some of the accumulated oil, and that this oil could be
in primary migration to the accumulation;
- The abnormally high pore pressures in the chalk reservoir indicate that
lateral permeability through the chalk unit as a whole is poor, and that there-