Coal petrography, mineralogy and geochemistry of lignite samples from the Ogwashi–Asaba Formation, Nigeria

Abstract Organic sediments picked up randomly from seven small outcrops within the Ogwashi–Asaba Formation, southern Nigeria, are examined and evaluated by means of coal petrology and chemical and mineralogical analyses in order to determine the palaeoenvironmental conditions and the factors controlling their formation. Six samples proved to be low-rank coals C to B (lignite), one carbonaceous shale. The lignite samples display low ash yield, low telohuminite and high detrohuminite and liptinite contents; they contain small amounts of clastic minerals, mainly quartz and clays, which point to the topogenous character of the depositional palaeoenvironment. The palaeomires formed in a continental basin crossed by the mid-Tertiary palaeo-Niger River; the latter, as well as the tropical rainfall supplied the mires with water. The dense vegetation cover on the mire surface and the surroundings and/or the low relief energy of the broad area restricted the inorganic influx resulting in high-grade coal formation. As the outcrops are distributed over a distance of 60 km, the expected reserves of good quality lignite constitute a very promising exploration target.