Abstract: The Origin of Lacustrine Petroleum Source Rocks, Congo Basin, West Africa: Preliminary Results of a Multidisciplinary Study

Harris, N. B.; Freeman, K. H.; White, T. S.; Mitchell, G. D.; Pancost, R. D. and Sandomenico, T. - The Pennsylvania State University

Critical factors in the origin of petroleum source rocks have been controversial for years, particularly with regard to ?productivity? versus ?preservation.? We address this problem in a multidisciplinary study of lacustrine sediments in the Lower Cretaceous pre-salt sequence of the Congo Basin, West Africa, with the goal of understanding the interplay of tectonic, paleoclimatic and other factors that lead to the deposition of source rocks.

Shales in the pre-salt sequence are typically good to excellent source rocks; the Barremian Marnes Noires Formation is remarkable, averaging more than 6% total organic carbon (TOC) over a thickness of 100 to 400 meters. Rockeval data (HI = 450-900; OI = 0-30) indicate the presence of abundant Type I kerogen. Other pre-salt formations contain lower TOCs (average 1.3 to 2.3% TOC) and mixtures of Types I, II and III kerogen. Organic molecular abundances and sedimentary structures in cores indicate that a II major lithologic units were deposited under strongly reducing conditions. Water depths were generally deep (below storm wave base). Shale mineralogy suggests that saline lacustrine conditions developed strongly during Marnes Noires deposition and that the lake may have been saline-stratified, enhancing bottom water anoxia. Palynology indicates that the Marnes Noires was deposited in arid conditions, which could have promoted development of a saline lake. Major algal contributions in the Marnes Noires are indicated by organic petrography; however organic molecular studies suggest a significant contribution from bacteria. Bacterial sulfate reduction resulted in formation of relatively abundant pyrite.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil