Journal of Petroleum Geology, Vol.9, No.4, pp. 439-450, 1986

©Copyright 2000 Scientific Press, Ltd.

THE EXPULSION CRITERION IN THE EVALUATION OF THE PETROLEUM SOURCE BEDS OF THE TERTIARY NIGER DELTA

J. E. Ejedawe*

* Department of Geology, University of Benin, Benin City, Nigeria. Present address: Shell Pet. Dev. Co. Nigeria Ltd, EPDE EXPE, PO Box 263, Port Harcourt, Nigeria.

Abstract

Semilog plots of hydrocarbon ratios with depth for four wells in the offshore Niger Delta show that hydrocarbon expulsion has occurred at an interval shallower than the geochemically-accepted top of the oil-generative window (OGW). This shallow expulsion zone, which spans the maturation range 0.3-0.6% Ro, is related to the transformation of resinite- and sporinite-rich organic matter to hydrocarbons. The deeper generation/expulsion zone (conventional OGW) has not released its generated hydrocarbons, presumably because of overpressure development.