Geology And Petroleum System Of Late Carboniferous To Permian Lower Karoo Group, Maamba Basin, Southern Zambia

The Maamba Basin is situated in the southern part of Zambia within the confines ofthe Mid-Zambezi Valley Basin which straddles along the border with Zimbabwe. TheBasin corresponds to Karoo rifts sutures developed along NNE-SSW trending Pan Africanmobile belts related to Gondwana Orogeny, that affected southern Africa in the Paleozoicand beginning of Gondwana break-up in Mesozoic. The basin developed as a consequenceof the WNW-ESE extensional forces initiated by the reactivation of sinistral movementsalong the Mwembeshi Shear Zone (MSZ) and became filled with continental nonmarineLower Karoo Group (LKG) sequence of Late Carboniferous to Permian age.This study is intended to augment geology and potential hydrocarbon play systemsdatabase of the LKG in the Maamba basin as well as in other similar continentalnonmarine Karoo rift basins in the region. Geologic analyses were conducted throughextensive outcrops and exposures and drill cores.6major lithofacies (diamictites,conglomerates, sandstones/siltstones, coal and mudstones) represents LKG fining-upwardsdeposition sequences.4mudstone drill core samples were thin sectioned for petrographicanalyses. In addition,6sandstone samples were impregnated with blue epoxy before thin sectioning to facilitate easy recognition of porosity. Quantification of framework graincomposition and porosity was achieved by point counting a total of300points per thinsection. Scan electron microscopy (SEM) was performed to confirm the identification ofdiagenetic constituents and pore types. A total of35drill-core samples comprising ofmudstones and coals were analysed for Rock-Eval6/TOC pyrolysis.According to results of the analyses,3depositional settings are envisaged whichincludes alluvial floodplain, floodplain/fluvial-lacustrine and lacustrine deposits.Floodplain/fluvial-lacustrine deposits are hosts to mudstones and coal source rocks andchannel sandstone reservoir rocks. Mudstones and coal source rocks have averages of totalorganic carbon (TOC) that is above the recommended TOC of0.5wt.%. However,hydrogen index (HI) values are below200mg HC/g rock, indicating fair quantities of typeIII organic matter. The thermal maturity measured by temperature Tmaxrange from440~485oC in agreement with calculated vitrinite reflectance (Rocalc) range(0.76%~1.57%) indicating mature to post mature stages. Production Index (PI) values areless than0.1suggesting some hydrocarbon expulsion possibly gas and gas condensates.Sandstones are moderate to poorly sorted sublitharenites. Total porosity in channelsandstones range between0~6.3%, averaging2.5%, and is mainly secondary porosityresulting from dissolution of labile minerals, and fracture. The reservoir quality is leastfavourable due to deposition and diagenetic related controls. Potential seals are lacustrinemudstones whereas traps are structures (normal and reverse faults) and minor stratigraphictraps (facies deposition).This research focuses on areas in the north and part of central basin where coalmining and exploration is currently taking place. Hydrocarbon play systems are unknownin the southernmost parts of the basin. Recent coal exploration drilling has revealed the presence of source rocks at depth overlain by extensive overburden seals. Based on ourplay concept analysis, it is possible to find more encouraging results in these areas.