Petroleum source rock evaluation of organic black shales in the Paleogene N'kapa Formation, Douala Basin, Cameroon

Abstract

Black shales outcropping in the Douala Basin were studied using organic petrography and Rock-Eval pyrolysis. The goal of this study was to use organic petrography and bulk geochemical analysis to evaluate N'kapa Formation shales in terms of organic matter (macerals) composition and its relationship to oil and gas productivity potentials. The quantity of organic matter, kerogen type and thermal maturity were determined, and their relationship to oil and gas generation was evaluated. The results of this investigation indicate that the nine samples analyzed had average random vitrinite reflectance values (VRo random) between 0.48 and 0.57% and are indicative of immature source rocks. Calculated maximum reflectance values (VRo maximum) ranged from 0.50 to 0.59%, showing a maximum volatile C bituminous rank and Liptinite macerals were found to fluoresce in the yellow-green to the yellow range, which is consistent with the indicated rank. The total organic carbon (TOC) of the samples ranges from 0.55 to 2.23 wt% (average of 1.18 wt%), implying fair to good concentration of organic matter, while the S1 values range from 0.04 to 0.10 mg HC/g rock (average of 0.07 mg HC/g rock) and S2 values range from 0.10 to 0.91 mg HC/g rock (average of 0.38 mg HC/g rock) and are described as showing poor source potential. Based on Genetic Potential (GP = S1+S2) values (0.18-1.01 mg HC/g rock), the studied N'kapa Formation shales are dominated by type III gas-prone kerogen with a minor association of type IV kerogen. The hydrogen index (HI<50mg HC/g TOC) and oxygen index (OI<75mg CO2/g TOC) values are too low for the shales to have generated hydrocarbons. The low values HI and OI and the absence of type II and mixed type II/III may be due to some degree of weathering of the samples.