Rock-Eval pyrolysis and organic petrographic analysis of the Maastrichtian coals and shales at Gombe, Gongola Basin, Northeastern Nigeria

The Gongola Basin forms one of the inland Cretaceous to tertiary sedimentary basins in Nigeria with relatively unknown petroleum system. In this study, the investigated source rock intervals of the Maastrichtian Gombe Formation are located at the Maiganga coal mine, near Gombe, Gongola Basin, Nigeria. The exposed part of the mine consists of about 35-m thick coarsening upward section with the basal part made up of coal and shale interbeds while the upper part consists of siltstone and sandstone and probably deposited in fluvio-deltaic environment. The coal and interbedded shale appear most prospective source rock facies in the formation and were evaluated with respect to their hydrocarbon source rock potential. The organic geochemical results showed high TOC for the coals (37.71–65.29 wt%) and moderate organic carbon concentration (1.19–4.81 wt%) for the shales. S2 values ranges from 57.96 to 103.21 mgHC/g rock and 0.51–6.22 mgHC/g rock for the coal and shales, respectively. The HI is less than 200 mgHC/gTOC in all the coal and shale samples suggesting Type III kerogen and predominant plant contributions from terrestrial sources. Consequently, gaseous hydrocarbon potential is exhibited in the source beds. The organic petrography shows vitrinite as dominant maceral followed by inertinites and liptinites with inorganic minerals like pyrite and kaolinite. The Tmax is less than 435 °C in all the samples indicating pre-oil window stage; this is corroborated by the Romax value ranging from 0.45 to 0.55 %. Plot of PI against Tmax indicates low level organic matter conversion while the vitrinite reflectance values suggest a sub-bituminous coal. The study suggests that the investigated coal and shale samples constitute good source rock and have potential for gas. The generally low HI in the coals and the associated shales indicate more allochthonous and hydrogen poor organic matters in the source beds. Maceral composition of the coals suggests wet swamp environment and predominance of arborescent vegetation type. At present level, the organic matters are immature to marginally mature deeper level equivalents might have generated gas.