Source rock evaluation of coals from the lower Maastrichtian Mamu Formation, SE Nigeria
Journal of Petroleum Geology
Department of Geology, University of Ilorin, Ilorin, Nigeria; Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark
The Lower Maastrichtian Mamu Formation in the Anambra Basin (SE Nigeria) consists of a cyclic succession of coals, carbonaceous shales, silty shales and siltstones interpreted as deltaic deposits. Sub-bituminous coals within this formation are distributed in a north-south trending belt from Enugu-Onyeama to Okaba in the north of the basin. Maceral analyses showed that the coals are dominated by huminite with lesser amounts of liptinite and inertinite. Despite high liptinite contents in parts of the coals, an HI versus Tmax diagram and atomic H/C ratios of 0.80-0.90 and O/C ratios of 0.11-0.17 classify the organic matter in the coals as Type III kerogen. Vitrinite reflectance values (%Rr) of 0.44 to 0.6 and Tmax values between 417 and 429°C indicate that the coals are thermally immature to marginally mature with respect to petroleum generation. Hydrogen Index (HI) values for the studied samples range from 203 to 266 mg HC/g TOC and S1 + S2 yields range from 141.12 to 199.28 mg HC/g rock, suggesting that the coals have gas and oil-generating potential. Ruthenium tetroxide catalyzed oxidation (RTCO) of two coal samples confirms the oil-generating potential as the coal matrix contains a considerable proportion of long-chain aliphatics in the range C19-35. Stepwise artificial maturation by hydrous pyrolysis from 270°C to 345°C of two coal samples (from Onyeama, HI = 247 mg HC/g TOC; and Owukpa, HI = 206 mg HC/g TOC) indicate a significant increase in the S 1 yields and Production Index with a corresponding decrease in HI during maturation. The Bitumen Index (BI) also increases, but for the Owukpa coal it appears to stabilize at a T max of 452-454°C, while for the Onyeama coal it decreases at a Tmax of 453°C. The decrease in BI suggests efficient oil expulsion at an approximate vitrinite reflectance of ∼I%Rr. The stabilization/ decrease in BI is contemporaneous with a significant change in the composition of the asphaltene-free coal extracts, which pass from a dominance of polar compounds (∼77-84%) to an increasing proportion of saturated hydrocarbons, which at >330°C constitute around 30% of the extract composition. Also, the n-alkanes change from a bimodal to light-end skewed distribution corresponding to early mature to mature terrestrially sourced oil. Based on the obtained results, it is concluded that the coals in the Mamu Formation have the capability to generate and expel liquid hydrocarbons given sufficient maturity, and may have generated a currently unknown volume of liquid hydrocarbons and gases as part of an active Cretaceous petroleum system. © 2007 Scientific Press Ltd.
Carbonaceous shale; Catalytic oxidation; Oil bearing formations; Pyrolysis; Silt; Long-chain aliphatic; Maceral analysis; Petroleum generation; Ruthenium tetroxide catalyzed oxidation; Stepwise artifical maturation; Coal; hydrocarbon generation; Maastrichtian; source rock; subbituminous coal; thermal maturity; vitrinite reflectance; Africa; Anambra Basin; Nigeria; Sub-Saharan Africa; West Africa