Organic Petrology, Thermal Maturity, Geology, and Petroleum Source Rock Potential of Lower Permian Coal, Karoo Supersystem, Zambia

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Abstract
Data concerning organic petrology and thermal maturity of Lower Karoo coal measures (Lower Permian) are of considerable importance in determining the hydrocarbon potential of sediments in the rift-valley and half-graben complexes of the Luangwa and Zambezi valleys of eastern and southern Zambia, respectively, and in the extensive sedimentary basin developed on relatively stable Precambrian basement in western Zambia, a total area in excess of 3000 km2. Samples from seven outcrop and subsurface localities situated in the northeast (northern Luangwa Valley), east (mid-Luangwa Valley), south (mid-Zambezi Valley), and the Western Province of Zambia were studied. The coal measures are from 9 to 280 m thick, but individual coal seams are generally less than 6 m. The coal macerals contain an average of 60% vitrinite and 9 % liptinite, enough to have potential to generate hydrocarbon. A few samples contain twice this amount of liptinite. Reflected-light microscopy and the thermal alteration index of spores were used to determine the thermal maturity. The organic matter in samples studied is within the oil generation zone (thermal alteration index 2— to 2 + ; %R0 max = 0.5-0.9). Considerable differences in depth of burial of the coal measures, such as might be expected in rift valleys or half-grabens, have probably resulted in varying degrees of thermal maturity elsewhere. For example, in localities with significant thickness of younger Karoo and post-Karoo rocks (approximately 3500 m in some localities) the thermal maturity at depth (no samples available) is probably higher than %R0 = 1.35 and falls within the gas generation zone. The penological and palynological data indicate that the organic matter consists of Types II (generally approximately 25% in carbonaceous shale samples), III, and IV, indicating source rock potential. Late Karoo(?) and post-Karoo fault blocks with differential vertical displacements may have produced structural traps suitable for oil and gas accumulation.