--> Abstract: Sequence Stratigraphy of the Lower Cretaceous Lake-Sediments of the West African Rift - a Tool for Hydrocarbon Exploration, by R. H. Bate; #90933 (1998).

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Abstract: Sequence Stratigraphy of the Lower Cretaceous Lake-Sediments of the West African Rift - a Tool for Hydrocarbon Exploration

Bate, Raymond H. - Lacustrine Basin Research Limited

Introduction

Lake sediments can be rich sources of hydrocarbons and consequently of considerable economic importance to the oil industry. As lake-basins may also contain significant clastic and carbonate reservoirs, it is essential, when investigating a basin's potential, to be able to determine past depositional environments and associated source and reservoir potential. Consequently an analytical technique that not only presents a local interpretation of deposition, environment and age, but also allows for an extrapolation into regions not yet explored (for example the offshore deep-water area of West Africa) has to be seriously considered.

The Technique

Ostracods being small aquatic crustaceans are readily extractable from well ditch cuttings, and as such are invaluable both for dating and for interpreting depositional environments. Studies of both freshwater and euryhaline ostracods have shown that changes in shell ornamentation can be brought about as a direct response to changes in water chemistry. Species diversity is also affected by changes in lake-water level. It is possible, therefore, to apply shell morphology and diversity to the recognition of lake-systems tracts.

Application

Two cycles of lake development: each commencing with a transgressive systems tract and terminating in a saline lowstand systems tract are recognizable for the Pre-Salt of West Africa. The first lake cycle was initiated in the Neocomian as a direct response to crustal expansion and rifting that established the Eastern Hinge Margin. Lowstand conditions brought this first cycle to a close. The second lake cycle commenced in the early Barremian, following the second phase of crustal expansion that led to the establishment of the Atlantic Hinge Margin. This period of lake-expansion resulted in the coalescence of many smaller lakes and the accumulation of the organic rich Melania and Organic Bucomazi in the late Barremian. The final lowstand phase of lake-development came in the Aptian when saline lake lowstand conditions brought syn-rift lake deposition to an end.

Conclusions

An understanding of the Pre-Salt ostracod faunas has not only established a detailed chronostratigraphy, but has made possible the identification of the environmental conditions as well as the associated highstand, lowstand and transgressive systems tracts of these Lower Cretaceous lake-sediments.

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