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Abstract: Exploration for Lacustrine Carbonate Reservoirs: Insights from West Africa

A. J. Lomando

Production from lacustrine carbonate reservoirs, some of which are giant fields, makes them an important exploration target in many nonmarine rift basins. Chevron and its partners produce significant quantities of oil from pre-salt lacustrine carbonate reservoirs, offshore Cabinda, Angola. The TOCA carbonates are highly productive in two of the four fields discovered and developed to date. These fields are distributed among three subbasins which have varied from connected to isolated at different times.

Kambala Field, in the Kambala subbasin, is productive from two zones in the TOCA. The lower zone is a dolomitized coated-grain grainstone/packstone shoal system with primary and secondary porosity. The upper zone is an algal biolithite which has been hydrothermally dolomitized, resulting in secondary vuggy and intercrystalline porosity. Malongo West Field, in the Malongo subbasin, also produces from the TOCA, but here reservoir pore systems range from chalky microporosity- dominated to vuggy karst related types. This field has produced over 240 MMBO from these lacustrine carbonates and is still producing today.

Both of these fields are trapped in different structural configurations of the rift basin fault blocks. Kambala Field is located on the bounding fault side of a tilted fault block. This structural configuration produced a perched platform during deposition which facilitated the accumulation of a thick carbonate sequence. Malongo West Field is located on the "ramp" side of a major basement controlled high where synsedimentary faulting is less important. These reservoirs share many similarities with their counterparts in the Campos Basin of Brazil.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela