Page 306 - Petroleum Geology

280

that overlies the old topography. As the water deepens, fine-grained material
accumulates diachronously over its coarser equivalent; and if the finer-grained
material also accumulates petroleum source material, it comes to act both as
source rock and cap rock to the carrier bed -the underlying coarser unit. As
the transgression proceeds, islands are formed from the hills, and the amount
of terrigenous material available decreases; but once an island becomes sub-
merged below some baselevel of energy, the coarse sediment can no longer
be generated, and fine-grained sediment from elsewhere (or carbonates, if the
environment is right) accumulate over the immersed island. From this point
on, the trap is closed. The Augila field is such a trap.
Much of the world experienced a late Cretaceous transgression, an event
of great geological significance that is not yet properly understood, and one
of great importance to the petroleum reserves of the world. Suess (1888;
1906, p. 290ff) observed that the Cretaceous System has a worldwide trans-
gressive tendency and discussed “the Cenomanian transgression” at some
length. This is still a topic of great interest because it suggests a world-wide
eustatic event. Detailed study (e.g., Matsumoto, 1977, 1980) has shown that
“the Cretaceous transgression” was not a synchronous event around the
world. In Libya (and some other parts of the world) it continued well into
the Tertiary. In the Sirte basin (see Fig. 12-11) it began in Albian-Cenoma-
nian times and transgressed southwards over land comprising granitic and
other igneous rocks, and Palaeozoic sediments, with a topographic relief of
some hundreds of metres at least. The Sirte basin was also subsiding, and
subsiding irregularly due to contemporaneous movement of faults, so that
considerable thicknesses of Upper Cretaceous and Tertiary sediments (mainly
carbonates) accumulated (Fig. 13-1). This basin was to become one of the
major oil provinces of the world, with estimated proven recoverable reserves
of the order of 25 X lo9 bbl(4 X lo9 m3) of oil in 1975, only 15 years after
the first discovery. Production in the same year averaged 1.47 X lo6 bbl/day
(235,000 m3/day), mainly from carbonate reservoirs.
The Augila field (Fig. 13-2) was discovered in 1966 from seismic reflec-
tion surveys in an area in which previous work, including drilling and the dis-
covery of the Amal field (see Roberts, 1970), had indicated a large basement
high that had probably existed from the early Palaeozoic (Williams, 1972).
The first test found some oil. The second (Dl-102) was drilled on another
prospect nearly 20 km to the west and tested 2350 m3/day (14,800 bbl/day)
of 36” API oil from a porous limestone 2598-2612 m below the surface.
D2 found no limestone on top of the basement, but tested 1213 m3/day
(7627 bbl/day ) from devitrified rhyolite and fractured, weathered granophyre.
Of the first 11 wells drilled to the D prospect, only two failed to find com-
mercial production (one being wet). The basement was productive in three
wells in addition to D2: D5 tested 2248 m3/day (14,140 bbl/day) from two
intervals, 12 m of granite and 18 m of carbonate; D8 tested 2860 m3/day
(18,000 bbl/day) from an open-hole completion in basement and 11 m of