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dustry at large to note that geochemical hypotheses concerning the source
rocks of some oil fields and provinces are at variance with geological and log-
ical arguments. Our purpose in this chapter is to concentrate on the discrep-
ancies because it is from these that true relationships and processes will most
convincingly be demonstrated eventually. We cannot be satisfied until a co-
herent geological, geochemical and hydrogeological synthesis for each field
emerges. We start on a positive note. -
Probably the most convincing case in which geological argument concern-
ing the source, migration and accumulation of significant quantities of oil
and gas is not contradicted by geochemical or hydrogeological arguments lies
in the North American Silurian reef province of the Michigan basin, and the
Devonian reef province of the Western Canada basin.
There are two reasons for stating that the source rock of reef accumula-
tions does not lie in the reef itself:
(1) The contemporary reef environment is one of high energy, rich in
oxygen, with much of the organic matter in the food chain. All these 9.re
considered inimicable to the preservation of large quantities of organic matter.
(2) In groups of reefs, some contain gas, some contain oil and gas, some
contain oil and some only water. Groups of reefs probably shared closely
comparable environments while they grew, and have since had almost identi-
cal geological histories, so it seems inconceivable that the variety of petroleum
content could arise from an internal source.
GUSSOW’S
(1954) hypothesis of differential entrapment, which arose from
his observations in the Western Canada basin, has since been found to apply
convincingly to the Michigan basin (Gill, 1979) and to other areas (e.g. West
Irian: Trend Exploration Technical Staff, 1973). This hypothesis requires
for those areas where differential entrapment has occurred that the source is
down-dip from the deepest, gas-filled reef, and that the secondary migration
path to the shallowest, oil-bearing reef extends at least to the deepest. The
progression from lighter to heavier crudes from the deepest to the shallowest
oil-bearing reefs may also be a function of the length of the secondary migra-
tion path (not necessarily exclusively), with the removal of the more water-
soluble components during migration, as well as any effect of de-gassing.
These geological observations require for those areas with differential en-
trapment that the source of the petroleum is stratigraphically closely related
to the accumulations: the stratigraphy suggests that the source rock is the
fine-grained mudstones or mark that ouerlie the permeable carrier bed -the
permeable carbonate “platform” on which the reefs grew (Fig. 11-1). These
source rocks accumulated while the reefs grew, but they accumulated in the
still, deep water at some distance from the reefs. Generation, migration and
accumulation could have begun as soon as the reefs were drowned and closed.
In any case, generation and migration probably took place over a consider-
able span of time as subsidence gradually took the source rocks to tempera-
tures at which generation could begin.