Abstract: Organic Matter Accumulation in a Thick, Lacustrine, Lower Cretaceous Sedimentary Sequence in Gabon: Facies and Maturity Variations

Ralf Littke, Heinz Wilkes

A total of about forty core samples representing an almost two kilometer thick, lacustrine sedimentary sequence of Neocomian age in the south Gabon basin was analysed for its hydrocarbon generation potential, maturity and further organic matter characteristics. Organic carbon concentrations are variable and not particularly high (0.4-5%), but the organic matter is hydrogen-rich. This hydrogen-richness finds its expression in high hydrogen indices (about 600 to 700 mg hc/g TOC) which decrease with increasing maturation. According to pyrolysis experiments, hydrocarbon generation from the immature sediments is predicted to begin only at temperatures greater than 100° degrees C and reaches a maximum only at temperatures greater than 150° degrees C, because the organ c material possesses a very high thermal stability. Such a high thermal stability was already established for lacustrine organic matter from some other deposits (e.g. Green River oil shales) and is certainly an important factor for the evaluation of hydrocarbon potentials in lacustrine basins.

The maturity of the organic matter changes from immature to mature with increasing depth. Peak oil generation stage was almost reached by the deepest samples as indicated by a variety of optical and geochemical parameters. Generated petroleum should be wax-rich and rather poor in gas, except if oil to gas cracking occurs within the source rocks. With respect to molecular geochemistry, several interesting peculiarities were found. As an example, variable distributions of several unknown tetracyclic terpanes (molecular formula C₂₄H₄₂) were detected in the samples of lower maturity. Two series of terpane pseudohomologues occur in the more mature samples, one of which is assumed to consist of diahopanes, the other yet remaining unknown. These compounds seem to be wid ly distributed in lacustrine sediments of higher maturity, thus possibly representing maturity and/or facies indicators.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France