Determination of total organic carbon content using Passey’s ΔLogR method in coals of the Central Kalahari Karoo Basin, Botswana.
The Kalahari Karoo Basin is one of several basins in southern Africa filled with Late Carboniferous to Jurassic sediments that are primary targets for Permian aged coal. In order to determine the Coalbed Methane (CBM) potential of the Central Kalahari Karoo Basin, 9 exploration boreholes were drilled. Vitrinite reflectance (%Ro) and proximate analysis were conducted on cored coal intervals. Passey’s ΔLogR method used in this thesis employs the use of resistivity and porosity logs to identify and quantify total organic carbon (%TOC) in potential source rocks. Compared with lab measured %Fixed Carbon, the results showed that Passey’s ΔLogR method effectively identifies coal intervals as organic enriched. In terms of %TOC calculations, the method works poorly in coal metamorphosed by dolerite intrusions. These heat affected coal samples display %Ro from 0.77% to 5.53% and were increased in rank from primarily sub-bituminous to higher ranking volatile bituminous and finally to anthracitic coal. Their higher level of organic metamorphism (LOM), accompanying compositional changes and increased density associated with accelerated coal rank seem to have hindered the method in its estimations or lack thereof. Compositional changes in the coal were controlled by proximity to sill intrusion, with a decrease in fixed carbon and volatile matter, and increases in ash and moisture in the contact metamorphism zone (2-12m from sill). In heat unaltered coal that has undergone normal burial maturation characterized by %Ro of 0.44% to 0.65%, the method works very well even attaining accuracy in some samples. In unintruded boreholes CH1 and CH6, correlations between fixed carbon and generated %TOC curves indicate strong relationships with R2 from 0.70 to 0.83. Therefore, it was found that Passey’s ΔLogR method can be applied effectively on coal that has undergone normal burial maturation only.