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past several potential reservoir sands to accumulate in a select few. The co-
incidence of oil/water contacts with fault intersections at the top of reservoirs,
to make a spill point, could perhaps be due to migration through a fault
when in juxtaposition with another sand, the oil column exceeding its critical
vertical dimension.
Lambert-Aikhionbare (1982) studied the relationship between pore-fluid
chemistry and diagenesis in some of the oil-bearing sands of the Agbada For-
mation, and found that the diagenesis of the oil-bearing part of the sands
differed from that of the water-bearing part of the same sands. The sands are
dominantly quartz-rich, fine to medium grained, with less than 10% feldspars,
clays and other minerals. Both authigenic and detrital clay minerals are pre-
sent, of which kaolinite is dominant among the authigenic clays. Within the
oil-bearing part of a sand, kaolinite only is found, whereas in the water-bear-
ing part the authigenic minerals include kaolinite, siderite, pyrite and calcite,
as well as minor amounts of smectite and illite. He concluded, as others have
done in other areas, that early entry of the crude oil into the sands inhibited
diagenesis. Most of the pore water was displaced, and the connate water that
remained could not sustain production of kaolinite, let alone other authigenic
minerals.
On present evidence, therefore, the geochemical concepts that led to the
postulate of a deep source in the Niger delta may well be incomplete or in-
correct - and, by analogy, the same may apply to other areas in which the
same concepts have been used with the same result. If this is found to be the
case, there will be no need to postulate faults as conduits for petroleum migra-
tion.
The issue is of immeasurable importance to petroleum geology, and must
not be prejudged. Areas in which this conflict of evidence appears to exist
must be scrutinized for evidence that will discriminate between two alter-
natives. It seems impossible that both can be right, and much will be learnt
by finding out which is right and which is wrong.

REFERENCES

Byrd, W.D., 1975. Geology of the Ekofisk field, offshore Norway. In: A.W. Woodland
(Editor), Petroleum and the continental shelf of north-west Europe, Vol. 1, Geol-
ogy. Institute of Petroleum, London, pp. 439-444.
Campbell, I.R. and Smith, D.N., 1982. Gorgon 1 - southernmost Rankin Platform gas
discovery. J. Aust. Petrol. Explor. Ass., 22: 102-111.
Deroo, G., Powell, T.G., Tissot, B. and McCrossan, R.G., 1977. The origin and migration
of petroleum in the Western Canada sedimentary basin, Alberta. A geochemical and
thermal maturation study. Bull. Geol. Surv. Can., 262, 136 pp.
Ekweozor, C.M. and Okoye, N.V., 1980. Petroleum source-bed evaluation of Tertiary
Niger delta. Bull. Am. Ass. Petrol. Geol., 64: 1251-1259.