Page 298 - Petroleum Geology

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Assuming general contemporaneity around this huge atoll, the present con-
figuration (Fig. 12-10) indicates tilting to the east with a maximum regional
dip of about 5” - less on average. There are local patterns to the petroleum
distributions, but no general pattern*, so there are probably several source
rocks.

PALEOCENE REEFS OF LIBYA
The discovery of oil in a Paleocene reef province of Sirte basin, Libya
(Fig. 12-11) in 1967 once again illustrated that fossil reefs could trap signifi-
cant quantities of crude oil that is producible with relatively few wells. The
Idris “A” discovery well, subsequently renamed Intisar, tested 6916 m3/day
(43.500 bbl/day) 43.5” API oil; the Intisar “C” discovery well tested 2798
m3/day (17,600 bbl/day) [37.5”] API oil; and the Intisar “D” discovery
well tested 11,903 m3/day (74,867 bbl/day) 37.2” API oil from 223 m of
reef reservoir (Heatzing and Michel, 1968, p. 1500, table XIV). Of the sixreefs
found in the area, two (A and D) were found to be giants with 1500 and 1200
million barrels of recoverable oil (240 X lo6 and 190 X lo6 m3), respectively,
ranked 76 and 91 among the giants of the world by Halbouty et al. (1970, p.
504, table 1). Three had small accumulations: Intisar “C” was commercial,
and “B” and “E” subsequently became so, producing 50” and 34.7” oil, re-
spectively. One was dry. Their location is shown on Fig. 10-14. All were dis-
covered from seismic reflection surveys.
The Intisar “A” discovery well (Terry and Williams, 1969) penetrated the
top of the reef at 2870 m and found an oil column of 292 m in an Upper
Paleocene reef complex 364 m thick. Appraisal and development drilling
showed it to be an oval structure, 5 X 4 km, with a NW-SE orientation. Its
recoverable reserves of 240 X lo6 m3 were expected to be producible from
42 wells (with about 20 water injection wells for pressure maintenance) from
depths between 2700 m and the oil/water contact at about 3065 m below
sea level.
The stratigraphy is shown in Fig. 12-12. Terry and Williams (1969) recog
nized two stages in the growth of the bioherm: first, the development of a
foraminiferal bank, loosely bound by algae; then, a period of coral-reef growth
that was terminated by drowning or by smothering with argillaceous sediment.
The sequence is dominantly transgressive, with two transgressive pulses. The
first resulted in the accumulation of a thin calcareous shale on top of the
Heira (lower Sabil) dolomite. This shale grades upwards into the Lower Zelten
(upper Sabil) carbonates (dense biomicrite) on which formed foraminiferal

* See “Mexico Report” in Oil and Gas Journal, 77 (34), p. 73.