Page 269 - Petroleum Geology

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The Bomu field, for example, in the Niger delta produces waxy crude oil
from a sand at about 2200 m, the oil being of 34.4"API (s.g., 0.853) with
5.1% wax (Frank1 and Cordry, 1967, pp. 204-206). There is a small shallower
accumulation, but all the other numerous sands are wet. Taking Ekweozor
and Okoye's (1980) figure of 3375 m as the depth of main oil generation
(determined in Cawthorne Channel, some 30 km to the WSW from Bomu),
the question is, how did oil generated at this depth in Bomu, which is in the
Agbada Formation, bypass all the intervening sands, including those adjacent
to the source, to reach that at 2200 m?
Evamy et al. (1978, p. 23) reported that crude oils that have acharacter
attributed to biodegradation are separated from lighter, paraffinic crudes con-
sistently by a temperature of 65"--8O0C. If these crude oils migrated up faults
into reservoirs, -and if they cannot be biodegraded once they accumulate,
then they must have been biodegraded during migration up the fault (unless,
as we noted earlier, sufficient bacteria are trapped in the connate water to
effect the changes and leave no cell walls).
The two opposing hypotheses, which appear to be mutually exclusive,
may be summarized thus:
- In favour of deeper origin and migration up faults in the Niger delta is
the chemical evidence of maturity and the coincidence of oil/water contacts
with the intersection of faults and the upper surface of many reservoirs,
- In favour of the interbedded sources is the apparent haphazard distribu-
tion of crude oils and oil-bearing sands amongst the wet sands, the simplicity
of migration and the possibility of biodegradation. Of lesser weight, perhaps,
are: the more likely source beds for waxy crudes are in the paralic environ-
ment. (as Reed, 1969, found), and the undercompacted, overpressured mud-
stones have yet to expel much of their water (but they may have expelled
the oil). In connexion with the latter point, Weber, in the panel discussion
mentioned above, said that few commercial accumulations had been found
with pore-pressure gradients of more than 0.5 psi/ft (11 kPa/m) from the
surface.
There are similar conflicts of views in the U.S. Gulf Coast, the North Sea,
and elsewhere, and it is probably true to say that the chemical view of matur-
ity prevails, and the consensus of opinion now favours deep sources in over-
pressured mudstones, with migration up faults. There are serious hydrodyn-
amic difficulties with sources removed stratigraphically from the accumula-
tions (as discussed on p. 245ff), and the matter of faults as conduits for migra-
tion will be taken up below.
The importance of resolving this problem is obvious: if the source is in
fact deep, variability in the crudes must be created either during migration or
within the reservoir, and the light thrown on this aspect would be of great
interest in other contexts. If in fact the source is interbedded, then the con-
cept of maturity - and so many of the geochemical concepts of petroleum
generation -may be wrong, or seriously misleading. These concepts are widely