Page 301 - Petroleum Geology

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An interesting, but unexplained observation of this reef area is that markers
are found on the gamma-ray log that are independent of gross lithology yet
correlate from reef to reef.
Terry and Williams (1969) considered that the Intisar “A” reef never form-
ed a relief on the sea floor greater than 60 m, which would suggest that trans-
gression and subsidence accounted for about 300 m of the reef’s vertical thick-
ness. However, Brady et al. (1980, p. 563) concluded from detailed electrical
log correlation between the “A” and “D” reefs that the relief was approxi-
mately 240 m (800 ft) when the reefs were terminated. This figure, though
large, is not out of proportion to the size of the reefs, and certainly does not
require a similar depth tolerance of the reef-building organisms. The combined
thinning and differential compaction of the Kheir Formation has given it a
relief of about 140 m, and the very varied lithologies of the Kheir suggest
local influences. The accumulation of mudstone and marl into the stratigraphic
record does not necessarily mean that the water was muddy at shallow depth,
because most of this material would have been in the traction load (as we
saw in the discussion of growth faults on p. 28). Perhaps the gamma-ray
logs provide evidence that has not been reported.
Although some reefs in this province remain to be described, we see that
the character of these Paleocene reefs in Libya is much the same as those
we have already discussed from other continents and other ages. There is
apparently no significant deformation: they are pure stratigraphic traps on a
carbonate base.
As regards the source rocks for this oil, Terry (in the discussion that follow-
ed the reading of his paper) said that the Kheir marls probably sourced the
Intisar “A” oil from an off-reef position. Brady et al. (1980, p. 564) report
that source rocks are plentiful, and regard the Sheterat shales, underlying the
reefs and platform, as the main source rock, with some contribution from
the Kheir marls and Kakb shales. The distribution of undersaturated crudes,
lightest in the north and heaviest in the south, is reminiscent of the Western
Canada basin, and suggests a northerly source with southerly migration.
The weak water drive of the “A” and “D” accumulations only indicates
poor regional permeability on a short time scale. The development of both
required water injection for pressure maintenance, and a point of practical
interest was the drilling of “dump flood” wells in addition to normal water
injection wells. A dump-flood well connects an aquifer to the reservoir directly
so that the water drive can be augmented by enlarging the volume of water
in hydraulic continuity with the reservoir. This technique requires, of course,
that the aquifer potential is at least equal to the undepleted potential of the
reservoir, and the vertical distance between the two shall not be very large or
temperature reduction may fill the pipe with scale (if a pipe is used) and clog
the formation.