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

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‐acceptedtop 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 sporiniterich organic matter to hydrocarbons. The deeper generation/expulsion zone (conventional OGW) has not released its generated hydrocarbons, presumably because of overpressure development.The critical hydrocarbon saturation at the onset of migration is 10–22 mg per gm organic carbon. Within the expulsion zone, expulsion efficiency varies considerably (10–81%; average 50%), and expulsion ceases when the hydrocarbon saturation falls 10–60% below the critical saturation level.The relative contribution of the paralic Agbada and marine, pro‐delta Akata source shales to the reservoired hydrocarbons in various areas of the delta depends on the present depth position of top supernormal pressure (Hsnp), relative to the top of the Akata Formation (Hak), and the top of the OGW (Hogw). Since the top of OGW must be above the top of the overpressure (Hogw < Hsnp) for effective hydrocarbon expulsion, three basic models are recognised in the Niger Delta:—(a) Hak Hsnp > Hogw: developed in the younger offshore and central onshore areas, with the Agbada constituting the major source rock;(b) Hogw < Hak < Hsnp: developed in the northern frame of the central Niger Delta, with both the Akata and Agbada contributing to the reservoired hydrocarbons; and(c) Hak Hogw < Hsnp; developed in the older northern fringe of the delta, with the Akata constituting the majar source rock.