Petrographic composition of coal within the Benue Trough, Nigeria and a consideration of the paleodepositional setting

The petrographic composition of Cretaceous-age coals hosted in the Benue Trough, Nigeria is presented and discussed in terms of the paleodepositional settings that influenced the coal-bearing formations. The Benue Trough is a failed arm of the triple junction of an inland sedimentary basin that extends in a NE-SW direction from the Gulf of Guinea in the south, to the Chad Basin in the north. A total of twenty-nine (29) coal samples were obtained from nineteen coal localities in the Upper (UBT), Middle (MBT), and Lower Benue Trough (LBT). The high average volatile matter yield, low average ash yield, high calorific value (24.82 MJ/kg, on average), and low sulphur values indicate good quality coal deposits. The organic matter is dominated by vitrinite, reported at an average of 59.3% by volume (mineral-matter free). Variation was noted in the inertinite content across three sub-regions. Liptinite macerals were not commonly observed in the studied samples and were absent in the MBT samples. Coal facies studies decipher the paleoenvironmental conditions under which the vegetation accumulated. Indices commonly used are the gelification index (GI), tissue preservation index (TPI), ground water index (GWI and variations), vegetation index (VI), and wood index (WI). Comparing the array of coal facies models applied, the MBT samples differ from the UBT and LBT samples, concurring with the coal quality data. The UBT and LBT coals formed in an upper deltaic to drier piedmont plane depositional environment, while the MBT coal formed in a lower deltaic marsh to wet forest swamp depositional environment. All samples indicate an ombrotrophic paleomire. In view of the modified equations and the plots used, interpreting depositional environments from just a single model is not reliable.