Tazerka Oil Field

J. Gueneau

The Tazerka field was discovered in 1979 on a heavily faulted structure 60 km off the coast of Tunisia in water depths of 150-250 m. It is situated to the north of two similar oil discoveries found in 1977-1978 (Birsa, Oudna) and is located on the same structural trend.

The accumulation consists of three sand reservoirs within the Miocene Birsa Formation at a depth of some 1,300 m subsea. The sands form regionally extensive shallow marine sand sheets whose precise relationship to a paleocoastline has yet to be determined. The sand sheets vary in thickness. "Thicks" in the sheets are interpreted as ridges on the sea floor, the crests of which were more frequently subjected to wave-generated currents than the flanks and hence comprise coarser grained and cleaner sands. This model suggests the cleaner sands are likely to form elongated lenticular bodies of limited lateral extent.

The generation of the hydrocarbon from the Cretaceous Fahdene Formation source rocks occurred in post-Miocene time and was synchronous with the trap formation. The underfilling of the Tazerka structure is the result of both limited hydrocarbon charge and default in retention.

The discovery well tested over 5,000 bbl/day of 30° API oil from each of the lower two intervals. After a second well had revealed the presence of a gas cap, a third well confirmed the structural picture and a high well potential (5,000 bbl/day from the three combined intervals). Recoverable reserves were then estimated at 8 to 20 million bbl with expected reserves of 12 million bbl.

A fourth well, designed to test the oil-water contact, found that the oil properties were deteriorating with depth (from 31° API in TZK-1 to 20° API in TZK-4). The oil in TZK-4 proved too viscous to sustain production. The subsequent development wells (7 wells) necessitated relocation to an updip position. The modest reserves and the shallow reservoir depth dictated subsea completion. Development started in 1982. Five subsea completed wells are now producing through a single anchored-leg system into a floating production storage unit. Production is declining from 10,500 bbl/day in 1982 to 3,500 bbl/day at present due to gas breakthrough. Cumulative production reached 13 million bbl in November 1987.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.