Page 310 - Petroleum Geology

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Its porosity is small due to poor sorting and clay and carbonate matrix. Its
lateral gradation into sandy limestones of the lower Rakb carbonates is, of
course, also a vertical one (Walther’s law), and the two lithologies form a single
reservoir of which the terrestrial component decreases upwards. Three distinct
environments are recognized in the carbonates of the reservoir, each relating
to its position amongst the islands prior to their submergence and the depth
of water prevailing. Porosity in the low-energy zones is due to faunal pelleting
of micrites and to solution during diagenesis. In the higher-energy zones,
porosity is due to winnowing, with some increase due to solution.
The Rachmet shale did not accumulate in the field area. It is mainly a glau-
conitic shale with sandstone stringers, with fine-grained sandstone above and
below. It is probably the source rock for the oil in areas that were deeper
and more tranquil, with primary migration downwards into basal sands and
the basement, and secondary migration through them to the traps closed by
the accumulation of the upper Rakb carbonates over the whole area. These
are argillaceous micrites and calcarous shales of low porosity and permeability.
Oil saturation suggests that these may be the source rocks for some of the
oil, or for oil that accumulated elsewhere, e.g. Amal (Roberts, 1970, p. 445).
We see from this description of the Augila field, which is based on the in-
formative and perceptive paper by Williams (1972), that the stratigraphic
evolution of this trap is entirely consistent with subsidence of an existing
subaerial surface, and marine transgression over it. This transgression, while
not strictly global at this time, was sufficiently widespread to suggest that
subsidence of the Sirte basin was not the only cause. Accepting an organic
origin of oil, we can assert that the source rock is in sediments that accumu-
lated while the surface was being inundated or, at the latest, in the immediately
overlying sediments. The trap was closed when the Rachmet shale was buried
about 100 m. The similarity between Amal, Nafoora and Augila oil suggests
that the source for all lay in sediments that accumulated in an euxinic environ-
ment - not necessarily in very deep water because Williams (1972, pp. 628
and 632) considered that the Rakb could have accumulated in water as shallow
as 35-45 m - and that this favourable environment continued through most
of late Cretaceous time. Parts of the upper Rakb are oil-stained, and pyrite is
a common filling of foraminifera.
These three fields are not identical, though we may assume that Nafoora
and Augila are very similar. Amal differs mainly in that its reservoirs are Palae-
ozoic fractured quartzose sandstones of the Amal Formation lying unconform-
ably beneath the transgressive, Cretaceous, Maragh Formation and the Rakb
Formation that oversteps the Maragh. Sandstones of the Maragh Formation
itself form reservoirs in those areas where it exists above the oil/water con-
tact. The basal Maragh is demonstrably derived from the Amal Formation in
some areas, so the principal is the same as for Augila. The source rock for the
petroleum almost certainly lies stratigraphically above the reservoirs.
On a rather broader scale, favourable conditions for petroleum source