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tion. These three formations are recognized over most of the delta, but of
course their ages vary with position, being younger in the directions of pro-
gradation. The Akata Formation, a thick mudstone, is generally undercom-
pacted and overpressured.
Frank1 and Cordry (1967) and Short and Stauble (1967) observed that the
crude oils vary from field to field, and also within fields, and concluded that
the source rocks are stratigraphically associated with their reservoirs, and that
migration has been short. Weber and Daukoru (1975) noted that the faults
appeared to be the spill point of many reservoirs, and concluded that the
main growth faults had acted as conduits for the migration of oil, and that
the main source beds were the deeply buried Akata mudstones lying below
the productive Agbada Formation. Evamy et al. (1978), in a detailed study
of the delta, came to the conclusion that the interbedded mudstones of the
Agbada Formation were the sources in the western delta, but that the main
source beds in the east were within the continuous mudstone section of the
Akata Formation. This conclusion was based on evidence for a threshold
temperature of 115°C (240°F) for main oil generation. Recently, Ekweozor
and Okoye (1980), in a preliminary report on a geochemical study, concluded
that the main source beds are in the “deeply buried shales” at about 3375 m
(11,000 ft) in the onshore delta, and 2900 m (9500 ft) in the offshore delta,
below the productive sands, at a threshold temperature of 95°C (203°F) for
intense oil generation.
We see that in the Niger delta, geochemical argument has advocated a source
to the petroleum that is much deeper than the accumulations, and conse-
quential hypotheses have had to be proposed for the migration of the oil.
Weber and Daukoru’s paper, read at the 9th World Petroleum Congress, did
not get unqualified support during the panel discussion that followed (Proc.
9th World Petrol. Congress, 2: 264-265). Welte queried the assumption of
the source’s lying in the overpressured Akata Formation mudstones because
of the immature nature of the shallower crude oils. Ovanesov said that the
consensus of opinion in the USSR favoured interbedded source rocks, and
Magnier thought that the Mahakam delta source rocks (East Kalimantan,
Indonesia) were probably adjacent to the reservoirs because of the haphazard
distribution of oil and gas, and the absence of faults.
We must, of course, accept that it is possible that the Niger delta may have
characteristics that differ from those of other deltas, but the fundamental
questions are these:
(1) Is it possible to generate from a single source bed crude oils that migrate
up faults and accumulate as distinctive crudes in some of the sands but not
all of them?
(2) Is our chemical concept of maturity sufficiently well established that
identification of source rocks on this basis is more reliable than thegeological
reasoning for interbedded source rocks when these are chemically “immature”?
(3) Can petroleum migrate up faults?