Oil Exploration in Pre-Salt and Post-Salt Sequence of West Africa--Results in Cabinda: ABSTRACT

Abstract

The Aptian evaporites separate two different oil habitats in the offshore of Cabinda: the pre-salt and the post-salt sedimentary sequence, with three-quarters and one-quarter of the oil production, respectively. In the pre-salt sequence, three oil fields are being produced: Malongo North, Malongo West, and the Block 121 trap. In the post-salt, three shallow accumulations are present, in the Malongo North area. The pre-salt sequence is extremely variable in thickness, and its sedimentary environment is strictly connected with the initial rifting between Africa and South America. In the earliest Cretaceous, a basement peneplane was flooded by a few fluviatile streams along which the thick Lucula sands were accumulated. Production from Malongo North and part of Malongo West fields comes from two of these independent sandstone bodies. Subsequent subsidence of the basin resulted in the deposition of the Bucomazi black shales in a lacustrine environment. These shales may grade laterally into the Toca carbonate rocks, mainly as a function of the water depth of the lake, every shallowing being accompanied by an increase in carbonate deposition. The Toca carbonate rocks around the Malongo West basement high are the second important reservoir of the field. They are largely dolomitized rocks derived from algal biostromes or pelecypod coquinas. After the Aptian salt deposition and the definitive opening of the Atlantic Ocean, the basin was tilted to the west, its original trough shape being modified into a monocline. During the Albian, the eastern part of Cabinda offshore was covered by continental red sandstones of the Vermelha Formation. Seaward of this, a definite shallow marine environment existed where the Pinda carbonate beds were deposited, frequently substituted by sandstones and siltstones. These are the reservoir rock of GCO and Mibale fields in Zaire. After a brief marine transgression during the Cenomanian, the heterogeneous Senonian sediments of the Iabe Formation form the reservoir rock of the shallow Malongo fields. It seems that the proximity of a growth fault may enhance the quality of the post-salt reservoirs: silt and clay accumulated along the downthrown sides of the faults, while salt flowage created localized highs where clean carbonate rocks were deposited in a high-energy environment. End_of_Article - Last_Page 427------------