ABSTRACT: Pre-Salt Subbasins and Their Source Potential in the Zairian Onshore of the West African Aptian Salt Basin

SWIRYDCZUK, KRYSTYNA, Conoco Inc., Houston, TX, DIEMU TSHIBAND, Cellule Technique Petrole, Kinshasa, Zaire, and MBAKU NYIMI, IHETA MUSOMBO, and KALUBI MBALAMBALA, Petro Zaire, Kinshasa, Zaire

The Congo-Cabinda Basin extends into onshore Zaire. Production exists offshore and in the immediate onshore. Study of the basin just east of onshore production indicates the presence of three pre-salt subbasins: Lemba, Sintu, and Tshikoko.

Thick Barremian lacustrine claystones of the Bucomazi Formation form the main source horizon in all three subbasins. Upper Bucomazi claystones from four wells average 4% and reach 12% TOC. Lower Bucomazi claystones average 2% TOC (high 6%). A mixed type I/type II algal oil-prone kerogen predominates. Up to 1% TOC is present in claystones in the underlying Lucula clastics. Dry pyrolysis of samples shows significant differences in kerogen kinetics from subbasin to subbasin. Lateral variation in organofacies within subbasins indicates direction of sediment input.

Ro and Tmax from five pre-salt wells were used to model heat flow through time. Ages are provided by biostratigraphic analyses and radiometric dating of thin volcanics within the Lucula and Bucomazi formations. Apatite fission track analyses provide control on uplift history. Pseudowells generated from seismic mapping were used in maturation modeling to predict source rock maturity in the three subbasins. The Upper Bucomazi was found to be immature except in the deeper parts of the Sintu and Tshikoko subbasins. The Lower Bucomazi and Upper Lucula are mature in all subbasins and in the deepest subbasins are overmature. Oil generation occurred shortly after deposition of the Loeme Salt. Analyses of Lindu oil support an early migration phase.

Estimates of oil that may have been generated in the Lemba subbasin suggest that the extensive Mavuma tar sands, which have been typed to a lacustrine source, could have been sourced from this subbasin. Migration of oil may also have occurred along the Chela sandstone from the offshore subbasins.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)