Inorganic geochemistry and petroleum source evaluation of organic black shale in the Mamfe Basin (West Africa)

Abstract

The shales of the Mamfe basin, West Africa, were studied using inorganic geochemical analysis to investigate the origin of the organic matter, environment of its deposition, paleoproductivity and organic matter preservation, organic matter maturity and petroleum potential. Shale samples from sites 1-7 have V/Ni ratios of: 2.9, 2.36, 3.14, 2.94, 3.04, 1.76 and 2.23 respectively. The V/Ni values of samples from sites 1,2,4, and 7 indicate a mixed terrestrial and marine organic matter origin, while shales from the other three sites have values close to those that represent mixed marine-terrestrial organic matter. Ni/Co, V/Cr and U/Th ratios of the black shales are <3, <2, and <0.75 respectively, indicating organic matter deposition in an oxic environment. The Ba/Al ratios of the studied shales average 64 while the P/Ti ratio averages 1.84 indicating a moderate to high paleoproductivity. The moderate to high paleoproductivity and rapid sedimentation influence the preservation of organic matter in the Mamfe shales. The shales have K/Rb ratios, ranging between 425 and 640, enrichment of LREE over HREE, MREE over HREE, positive Gd, Eu and negative Ce anomalies. These results indicate that the organic matter found in the shales is not yet at the level of maturity to produce petroleum, but may became good petroleum source rocks in the near future.