Geology and potential hydrocarbon play system of Lower Karoo Group in the Maamba Coalfield Basin, southern Zambia

Abstract

This study attempts to augment geology and potential hydrocarbon play system database not only in the Maamba Coalfield basin of southern Zambia but in other similar continental non-marine Karoo rift basins in the region as well. Geological analyses were conducted through extensive outcrops and exposures and subsurface boreholes. Six (6) major lithofacies (diamictites, conglomerates, sandstones, siltstones, coal and mudstones) represents Lower Karoo Group sequence. Four (4) mudstone core samples were prepared for thin section petrography. In addition, six (6) samples of sandstones obtained from outcrops, exposures and cores were impregnated with blue epoxy before thin sectioning in order to facilitate easy recognition of porosity. Quantification of framework grain composition and porosity was achieved by point counting a total of 300 points per thin section. The identification of diagenetic constituents and pore types was made possible by the use of scanning electron microscopy (SEM). Rock-Eval pyrolysis analyses utilised 35 core samples of mudstones and coal.According to results of the analyses, three (3) deposition settings which include; alluvial, fluvial-lacustrine and lacustrine setting are envisaged. . Fluvial-lacustrine deposits are host to mudstones and coal source rocks and sandstone reservoir rocks. Mudstones and coal source rocks gave the total organic carbon (TOC) that is well above the recommended thresholds of 0.5 wt % and 2.5 wt % of gas and oil generation respectively. The hydrogen index (HI) values are mostly below 200 mg HC/g TOC, indicating fair quantities of type III kerogen. The thermal maturity readings measured by temperature Tmax range from 440 to 485 °C in agreement with calculated vitrinite reflectance (Rocalc) range of 0.76–1.57% indicating mature to post mature stages. This maturation is attributed to the burial temperatures and near-surface heat flows by faults. Production Index (PI) values are less than 0.1 suggesting some hydrocarbon expulsion possibly gases and gas condensates. Sandstones are moderate to poor sorted sublitharenites. Total porosity in sandstones range between 0 and 6.3%, averaging 2.5%, and is mainly secondary porosity resulting from dissolution of labile minerals and fracture. Deposition and diagenetic related controls influenced much of the reservoir quality in sandstones. Lacustrine mudstones are the potential seal rocks. The major Structural traps are faults (e.g. normal and reverse faults) and the stratigraphic traps are sandstones.In this study, our area of focus is in the north and central part of Maamba Coalfield Basin where coal mining and exploration activities are currently taking place. The southernmost part of the basin is under developed such that the hydrocarbon play system potential is still unknown. However, recent coal exploration drilling has revealed the presence of source rocks at depth overlain by extensive overburden seal rocks. Based on the play concept analysis, it is possible to find more encouraging results from this area.