Petrology, Rank And Geochemical Evaluation Of Maastrichtian Coals From Se Nigeria: Implication For Petroleum Generation.

Abstract

The Maastrichtian Mamu and Nsukka Formations 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 these formations are distributed in a north-south trending belt from Enugu-Onyeama to Okaba in the north of the basin. Coals from these formations have been characterized by petrological and geochemical techniques. The aims of this study were to assess the quality of its organic matter, evaluate its thermal evolution and highlight its potential as a source rock. The total organic carbon (TOC) (67.21wt%) of the coal constitutes that of a good source rock with gas-prone kerogen indicated by Rock-Eval S2/S3 (21.0). The high oxygen index (OI) (15.72 mgCO2g -1 TOC) and pr/ph (7.93) suggest deposition in a shallow marine environment. 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, the n-alkane distribution dominated by n-C24 - n-C31, 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 0.59% and Tmax values between 414 and 432 o 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 183.09 to 344.53 mg HC/g TOC and S1+S2 yields range from 120.5 to 242.04 mg HC/g rock, suggesting that the coals have gas and oil-generating potential. This thermal evolution is thought to account for its current hydrogen index (250.24 mgHCg -1 TOC). Generated petroleum may not have reached the threshold for hydrocarbon expulsion but a review of petroleum system elements in the basin will stimulate high prospects in the Anambra basin.