Paleoclimatic Controls on the Distribution and Quality of Lacustrine Source Rocks in Western Africa's Early Cretaceous Rift Lakes

Karen L. Bice, George T. Moore, Eric J. Barron

By the earliest Cretaceous, a 5000 km-long, meridionally-oriented rift system had begun splitting northern Gondwana into the continents of South America and Africa. The rifting created a series of parallel, elongate depressions which were the deposition sites of Neocomian-Barremian lacustrine sediments. These sediments are typically described as organic-rich black shales deposited in anoxic bottom-water lakes. Total organic carbon (TOC) values for these sediments are >2% although the African basins' Bucomazi Formation and its equivalents commonly have significantly higher TOC values. More than 10 billion barrels of oil and oil equivalent are believed to be sourced from the rift lake sediments with a majority of those reserves found in the South American margin basins. hese source rocks and their generated oils have been shown through geochemistry and biomarker studies to change character northward from the Rio Grande Rise-Walvis Ridge complex. The southern rift lake basins that evolved into the Santos, Campos, and Espirito-Santo basins on the South American margin and the Angola, coastal Congo, and Cabinda basins on the African margin generated oils from sediments deposited in brackish to saline water anoxic-bottom lakes. In the Sergipe-Alagoas, Reconcavo (South America) and Gabon (Africa) basins the sediments were deposited in freshwater lakes that were dysaerobic to anoxic. These relationships imply net evaporative conditions in the south and a net positive water balance in the north. A global general circulation model simulation with boundary condi ions approximating those of the Neocomian produced temperature, wind and moisture distribution patterns which show good agreement with the observed distribution and quality of organic-rich rift lake sediments. The model simulates paleoclimatology conducive to the development of a stable, stratified lake structure with anoxic bottom water, a condition which would have increased the potential for TOC values > 2% in the preserved sediments. In addition, when the rift lakes are plotted on paleoclimate maps derived from the model, the southern brackish to saline lakes are in an arid region. The freshwater lakes are in a region affected by a massive, seasonal system of monsoonal and trade wind-dominated precipitation that covers most of the northern portion of the continent.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995