Organic matter characteristics in black shales of the southwestern paleogene N'kapa formation, Douala Sub-basin Cameroon: Evidence from stable isotopes and rock-eval pyrolysis

Abstract

Shallow marine black shales from outcrops, river channels, drilled and dug wells in Dibombari-Kompina-Mian (Stage-1) and Pendanboko (Stage-2) of the Paleogene N'kapa Formation in Douala Sub-basin were studied. Paleoclimate, paleoredox and paleoproductivity was examined, and the mechanism of organic matter accumulation and enrichment in shales and mudstones of N'kapa Formation was formulated. It provides detailed analysis of organic matter richness and hydrocarbon generation potentials of black shales. The rocks were analyzed using elemental geochemistry, SEM, TOC, organic carbon isotope, sulfur isotopes (pyrite sulfur and organic bound), total sulfur and Rock-Eval pyrolysis. Paleoclimate proxies show that the rocks were deposited under humid to semi-arid conditions. Paleoredox proxies reveal that the sediments were deposited dominantly under suboxic to anoxic conditions. The parameters for paleoproductivity: biogenic barium, δ³⁴S_py, δ¹³C_org, TOC, Mo/Al, P_org/Al, Cu/Al and S/¹³C_org ratios indicate that the humid-to semi-arid climate and reducing conditions were suitable for high primary productivity and favorable for the accumulation and enrichment of organic matters. The model of organic matter accumulation reveals that high primary productivity, suboxic-to-dominantly anoxic sulfidic bottom water, bacterial sulfate reduction (BSR) and supply of nutrient materials were responsible for accumulation and preservation of organic matter. The black shales have fair to good source rock richness for gaseous hydrocarbon generation, except for samples KO6-9, KO10-1 and KO13-2 having good gas/oil source rock potentials. The rocks are mainly type III kerogen, with few mixed type II/III kerogens present. Most samples are thermally immature, with three exceptional samples at marginally early mature stage for hydrocarbon generation.