99/02357 Studies on the progressive extraction and depolymerisation of coal macerals

Abstract

The rank and composition of Upper Cretaceous-Tertiary coals from five localities (Enugu, Orukpa, Okaba, Ihioma and Azagba-Ogwashi) in Southern Nigeria are described. Ranks determined by huminite reflectance measurements indicate a range from sub-bituminous C to sub-bituminous A coalification grade for the Lower Maastrichtian coals of the Mamu Formation whereas the Oligocene-Miocene coals in the Ogwashi-Asaba Formation are predominantly of lignite rank. The highest grade of sub-bituminous A was attained in the Enugu area (Onyeama mine) in the south and this decreases towards the Okaba mine where ranks are within the sub-bituminous C coalification grade.Composition was determined by maceral analyses. Results indicate that the Lower Maastrichtian sub-bituminous coals are characterized by moderate to high concentrations of huminites (humodetrinites, humocollinite and humotellinite) and varied concentrations of inertinites and liptinites. Concentration of large amounts of liptinites (especially resinite, sporinite and cutinite) at the bottom of the Orukpa seam indicate the wet conditions of a reed swamp depositional environment, whereas the large abundance of inertinites (mainly fusinite) on the top parts of the Onyeama and Okaba seams is interpreted as a shift to drier conditions, possibly forest swamp facies. The occurrence and predominance of humodetrinite (attrinite and densinite) in the Tertiary lignites of Ihioma and Azagba-Ogwashi are interpreted as indications of reed swamp depositional environments.We conclude that peat depositional environments for the Lower Maastrichtian coals was dominated by alternating high and low groundwater tables which resulted from recurrent subsidence. Forest and reed (marsh) swamps appear to have developed in delta plain environments. Lignites in the Oligocene-Miocene stages accumulated in reed swamps of a less telmatic depositional environment.