Abstract: Organic Petrography and Palynology of the Lower Cretaceous Pre-Salt Formations, Congo Basin, West Africa

White, Timothy S.; Gareth D. Mitchell and Nicholas B. Harris - The Pennsylvania State University

The organic petrography and palynology of the Lower Cretaceous Marnes Noire Fm and other "pre-salt" strata, Viodo Area, Congo Basin, were evaluated from cores and cuttings. Organic matter (OM) in the upper pre-salt is largely algal in origin. Conditions may have evolved from arid to more humid during deposition of upper pre-salt formations.

The Barremian Marnes Noires is a carbonate- and organic-rich lacustrine marl unconformably overlying Djeno Fm and conformably underlying Argilles Vertes Fm. Within the Marnes Noires, two lake lowstand intervals alternate with three highstands. Most OM in the Marnes Noires is Type I kerogen (lamalginite) and characterized by thin layers or shreds of thin-walled algae. Less abundant kerogen include Type I telalginite (mainly Botryococcus) and bituminite, Type II sporinite, and Type III vitrinite, semifu sinite and sclerotinite.

Samples are dominated by variously degraded amorphous, fibrous, and relict-mat structured OM. The oldest unit (Berriasian Vandji Fm) contain tubular elements and tracheids (terrestrial debris) consistent with Type III kerogen. The Djeno Fm is dominated by amorphous, OM, with common Corollina sp. pollen suggesting and conditions. Corollina sp. and significant terrestrial debris occur in the overlying Lower Marnes Noires Fm. The overall abundance of terrestrial debris diminishes upsecton in the Marnes Noires, suggesting a maximum lake-level highstand in the Upper Marnes Noires; terrestrial inputs and bituminite abundance increased during lowstands. The overlying Argilles Vertes Fm contains amorphous OM with relict-mat structures and algal spores; bisaccate pollen of Alisporites sp. affinity suggest more humid conditions.

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