High Resolution Stratigraphic Analysis of
Pre-Salt Source Rocks in West Africa

Bernd Mycke Fina Exploration and Production, Zone Industrielle C, B-7181 Seneffe (Feluy), Belgium


Ralph Burwood Fina Exploration and Production, Zone Industrielle C, B-7181 Seneffe (Feluy), Belgium

Lacustrine carbonaceous sediments of the Lower Cretaceous represent prime candidates for sourcing the significant oil reserves along the West African Atlantic margin. Deposition of these sediments is an early facet in the complex sequential
evolution of a rifted passive margin. A thick Aptian-age evaporite interval essentially differentiates lacustrine pre-salt, from marine drift phase post-salt, events.

The causes for stratigraphic variability within the Barremian (pre-salt) Bucomazi Fm. source rock sediments have been studied in detail. Sediments were investigated at high sampling frequency in the Malongo-Liawenda Trough in coastal Cabinda. Three study wells, characterising marginal to deep trough depositional environments, yielded in total over four hundred densely spaced Rock Eval data sets. These revealed significant variability in the amount and composition of the organo-detrital content of these rapidly deposited sediments. Concurrently, more than one hundred biomarker data sets were acquired and exploited for organofacies description. Rapid subsidence and burial of organic matter, due to the immense sedimentation rate (estimated at 0.6 to 0.85 m/Ka), resulted in exceptional preservation of biogenic signals that could be traced over extended distances from well to well.

The repetitive fluctuations observed, for instance in the organic carbon content, were statistically evaluated and analysed by Fourier Transform Analysis (FTA) to identify their cyclic nature. The frequencies found are related to Milankovich style
periodicities and were obviously climate controlled. A simple model for the deposition of organic matter in a lacustrine basin was then developed to describe the relation to carbon isotopic enrichment/depletion observations. The lacustrine depositional model so established is able to explain the variability in organic carbon, hydrogen index and isotopic data by the change in surface water productivity and organic matter preservation during settling through the anoxic lake water column. Accumulation of organic-rich layers at the rift-lake bottom is in such an environment directly related to surface water productivity which in turn appears to be climate controlled.

It could be speculated that similar conditions might be pre-requisites for the formation of organic-rich source rocks in passive margin rift basins.