--> Rock-Eval 6/TOC Pyrolysis Results From the Lower Karoo Group Mudstones and Coal Source Rocks, Maamba Basin, Southern Zambia

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Rock-Eval 6/TOC Pyrolysis Results From the Lower Karoo Group Mudstones and Coal Source Rocks, Maamba Basin, Southern Zambia

Abstract

The Permian aged Lower Karoo Group is host to prospective mudstone and coal source rocks in the African Karoo Rift basins. Hydrocarbon research studies conducted in the Zambia's Karoo Rift basins include, temperature time index (TTI) and thermal alteration index (TAI), and have shown that the source rocks are within the oil window, yet no commercial production of hydrocarbons has been found. This paper attempts to examine the source rock parameters, including quantity, quality and thermal maturity using Rock-Eval 6/TOC pyrolysis method. A total of 35 core samples were obtained from borehole wells drilled in the Maamba Basin of southern Zambia by Maamba Collieries Limited (MCL) as part of coal exploration and delineation programme. Quantity (richness) in terms of total organic carbon (TOC), range from 0.48 – 59.1 wt. % reflecting the poor to very good source rock generative potential. This wide variation in TOC may be a result of facies change, matrix effects, surficial weathering and erosion. Quality measured by cross-plot of hydrogen index (HI) vs. pyrolysis temperature (Tmax) suggests the predominance in mostly type III and /or mixed type II/III kerogens. Using the cross plot of pyrolysable hydrocarbons (S2) vs. TOC, this type of kerogen is consistent with continental non-marine origin. The range of thermal maturity measurements based on Tmax (440 – 485°C) and calculated vitrinite reflectance (Ro) (0.76 – 1.57%) suggests mature to post mature stages. However, production index (PI) values are less than 0.1, suggesting immature stage. The possible explanations to the causes of the contrasting thermal maturity readings include: hydrocarbon expulsion, errors resulting from low S2, organic matter reworking and lean materials. The cross-plot of genetic potential (S1+S2) vs. TOC shows a strong correlation (R2 = 0.87) between source rock continuum, and that the organic material is capable of yielding mostly gas and/or oil. The southern margin of Maamba Basin is sparsely drilled and mostly unexplored. The existence of similar deeply buried source rocks with potential for gas and/or oil as shown by Rock-Eval pyrolysis results may unravel the undiscovered hydrocarbon resource.