Oil and Geology in Cuanza Basin of Angola

Abstract

The Cuanza basin is in northwestern Angola on the Atlantic Coast of West Africa. This basin is about 300 km. long north-south and 170 km. wide east-west, and contains an Early Cretaceous carbonate-evaporite sequence and a Late Cretaceous and Tertiary argillaceous-arenaceous sequence. The Precambrian crystalline basement is partly covered by extrusive rocks and granite-wash type sediments. Surface and subsurface sediments of the basin consist of Lower and Upper Cretaceous, Paleocene, Eocene, and Miocene strata. Occurrences of oil and gas have been reported in almost all of the stratigraphic units in the Cuanza basin, and there is major production from the Cretaceous rocks. Study of these hydrocarbon occurrences and of the geological history of the basin shows that close relationships exist between sources, migration, and entrapment of oil, and environment of deposition controlled by the basement and salt tectonics. During Early Cretaceous time, subsidence of the central part of a restricted basin determined the regional cyclical deposition of a carbonate-evaporite sequence providing a favorable situation for genesis and entrapment of oil. Thus, the deposition during Aptian time of a very fine crystalline limestone, interbedded with argillaceous limestone and overlain by an oolitic sandy calcarenite, itself underlying evaporites, had an important influence on the subsequent extent of oil accumulations in the Binga Formation. During Aptian-Albian time, differential subsidence on the western margin of the basin caused lateral interfingering of back-reef calcarenite, argillaceous carbonate, and evaporite. This interfingering is believed to be related closely to oil accumulations in this area. Very i portant vertical development of reef deposits in the Longa area is related to lateral migration of the underlying Massive Salt, which flowed with the help of the excess of weight introduced by the growing reef. On the eastern margin, upper Albian reef buildups capped by marine shale also provided a favorable situation for generation and accumulation of oil. During Late Cretaceous and Tertiary time, a major basement flexure or fault zone appears to have been associated genetically downdip with deposits that accumulated with greater thickness than elsewhere. This flexure and the loci of maximum deposition moved eastward during Late Cretaceous and Paleocene, then westward during Eocene and Miocene. These thick formations, which are mainly argillaceous-arenaceous and which were deposited partly in deltaic and lagoonal environments, grade westward into thinner marine deposits and eastward into thinner continental deposits. During each particular epoch corresponding with a stabilization of this moving flexure, favorable conditions for genesis of hydrocarbons seem to be related to these transitional environments. Oil production is located above the Massive Salt at the crest of salt anticlines, and one small oil field has been discovered below the Massive Salt along a ridge of the Basement Complex in a pinch-out of sandstones between Precambrian mica-schist below, and salt above.