Determination of total organic carbon content using Passey's method in coals of the central Kalahari Karoo Basin, Botswana

Abstract

This paper focuses on determining total organic carbon (TOC) from boreholes in the Kalahari Basin, Botswana, using Passey's method. The Kalahari Karoo basin is one of several basins in southern Africa filled with Late Carboniferous to Jurassic sedimentary strata that host Permian age coal seams. Nine exploration boreholes (wells) drilled in the central Kalahari Karoo basin are used to determine the Total Organic Carbon potential. Vitrinite reflectance (R_o), proximate and ultimate analyses were conducted on cored coal intervals. Passey's ΔLogR method applied in this study employs resistivity and porosity logs to identify and quantify potential source rocks. Results of Passey's method compared with laboratory-measured carbon showed that Passey's method effectively identifies coal intervals. In terms of TOC calculations, the method works poorly in coal metamorphosed by dolerite intrusions. The heat affected coal samples had R_o from 0.77% to 5.53% and increased in maturity from primarily maceral controlled to high volatile bituminous and anthracite coal. Results from proximate analysis showed compositional changes in the coal were controlled by proximity to sill intrusion, with a decrease in Fixed Carbon and an increase in ash yield in the contact metamorphism zone (2–12 m from sill). For the unaltered coal that has undergone burial maturation displaying R_o of 0.44%–0.65%, the method works well. In unintruded boreholes, correlations between Carbon and calculated TOC indicate strong relationships. Passey's ΔLogR method proved to be a suitable method of estimating TOC on coal that has undergone burial maturation. This study has demonstrated that TOC calculated from the sonic log is more reliable in coal not affected by contact metamorphism than TOC calculated from the density log.