Abstract
Nigeria has significant coal deposits that are not well documented. This study considers the petrography of coal confined in the Benue Trough, Nigeria. Petrographic analyses, supplemented by mineralogy, and geochemistry, with a view to infer the depositional conditions that controlled the formation. The Benue Trough is rift-rift-rift system (triple junction) that trends NE-SW directions from south of Chad Basin to north of Gulf of Guinea.
Twenty-nine (29) grab samples acquired in nineteen (19) coal areas from Lower Benue Trough (LBT), Middle Benue Trough (MBT), and Upper Benue Trough (UBT). The proximate data of the coals shows high volatile matter yield, and fixed carbon, low ash, and high calorific values (24.82 MJ/kg areal average). Sulphur values are usually low (areal average of 0.72%).
Vitrinite reflectance values show the coal rank from low to medium in the categories of lignite to subbituminous and bituminous coal. Coal displayed predominantly vitrinite, with variable content of inertinite and liptinite macerals composition. Microlithotype composition showed variation in the three sub-regions with MBT samples being predominantly vitrite and carbominerite (carbargillites/clays, carbosilicates /clays, with limited carbopyrite).
Coal facies studies can be used to decode paleoenvironmental settings under which organic matter is accumulated. Indices that are usually considered in paleoenvironmental studies includes the following: gelification index (GI), tissue preservation index (TPI), ground water index (GWI), vegetation index (VI), and wood index (WI). MBT samples generally reported a high gelification index which is indicative of a wet environment. Relating the displayed facies models applied in this study, MBT differ from UBT and LBT, in agreement with coal quality differences. UBT and LBT samples were clustered in upper deltaic to drier piedmont plain setting, while MBT coals in lower deltaic marsh to wet forest swamp setting. All the samples plot was cluttered in ombrotrophic paleomire condition. An assessment of the equations visa-vice model facies used shows that, interpreting depositional environments from just a single model is not good enough for a conclusive decision.
The analysis plots show depletion in the major elements, trace, and rare earth elements, which could be linked to the source of the sediments from weathered basement rocks transported into the Benue Trough with mineral composition dominated by kaolinite, quartz, pyrite, and compounds like germanium. Dominant oxides in the coal samples are: Al2O3,
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SiO2, Fe2O3 with variations in others (BaO, CaO, Cr2O3, K2O, MgO, MnO, Na2O, NiO, P2O5, SO3, TiO2, & V2O5), with carriers being quartz, clay minerals and pyrite. Major oxides and trace elements studies are used to interpret paleo-setting and paleo redox conditions of peat swamp. V/Ni, U/Th, V/Cr, Sr/Ba, Ni/Co ratios reveal a new (fresh) aquatic setting of predominantly humid climate.
Gross calorific value (GCV) and combustion profiles show that the coals could be subjugated for power generation. Other applications such as energy in cement manufacture, block manufacturing works, foundries, washings, battery manufacture, and fuelwood in local cookery (smokeless coal briquettes) are also possible.