GEOLOGICAL AND GEOPHYSICAL INVESTIGATION OF A MAASTRICHTIAN COAL SEAM AT EHA-ALUMONA-ORBA ENVIRONS OF ENUGU STATE
Anambra Basin is one of the basins in Nigeria, harbouring the largest deposit of sub-bituminous coal. Sub-bituminous coal exposures have been encountered in the Eha-Alumona/Orba environs. This study therefore aims at delineating the coal seam and estimating the coal tonnage available. Integration of geological and geophysical interpretations was used in study of a coal sequence in MaastritchianMamu Formation at. Outcrop study in the field shows that the coal seagm was deposited in a swampy environment, followed by a tidally-influenced shallow marine environment which led to the deposition of heterolitic layers of shale and siltstone, then deposition of fluvial sediments of the Ajali Formation. Core samples were obtained from two wells drilled in the area. Well 1 penetrated thick layers of Ajali Sandstone, then heterolithic beds of shale and siltstone before encountering the coal seam at an interval of 41 – 41.6 m. Thickness of coal at this depth is 0.6 m, which is just equal to the thickness of the coal seam at Iyi Coal spring 417 m southeast. Well 2 penetrated much of the Mamu Formation to a depth of 61 m without any seam. Geophysical survey was then carried out to investigate coal seam continuity. Four Vertical Electrodes Sounding (VES) were carried out. Results show that resistivity of the shale and mudstone beds masked the influence of the relatively thin coal seam. VES 1 and VES 4 showed Type KQ and KA curves. Geo-electric sections matched significantly with Well 1 core data. Coal seam occurs at the fourth layer at depths 40.7 -56.6 m. The thickness of the seam cannot be deduced. VES 2 and VES 3 showed Type H curves. Geo-electric sections from VES 2 and VES 3 were tied to Well 2 core data and no Coal seam was penetrated in Well 2. Three dipole-dipole profiles were done to further analyse the lateral variation in resistivity in order to understand the attitude of the seam laterally. Profile 1 show that the seam occurs at depth of 30 m with resistivity values ranging from 4114 – 12,986 ohm.m, while Profile 3 revealed resistivity contours equivalent to that of Profile 1 but with coal seam at depth of 24 m. Low resistivity values of Profile 2 signifies absence of coal seam. Resistivity depth slice maps of the three dipole-dipole profiles showed high resistivity (about 18,000 ohm.m) at the northwestern corner of the area which suggest area occurrence of coal seam. NW-SE resistivity profile on these slice showed unusual uniformity in resistivity values across the seam which is proves lateral homogeneity. The seam tonnage within the study area was estimated accordingly while observing the Australian Guidelines for the Estimation and Classification of Inventory Coal. An estimate of 8,719 tonnes was recorded within an area of 10,380 m2 overlain byc the Ajali Sandstone.
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