Heat flow in the Main Karoo Basin, South Africa

Abstract

The Main Karoo Basin, and its Late Carboniferous to Early Jurassic stratified fill, the Karoo Supergroup, is the largest geological entity in South Africa and one of the economically most strategic because of its coal resources and potential shale gas resources. It also has a potential future in renewable energy in the form of geothermal energy. Although the basin has not received sufficient dedicated attention from a thermal perspective, numerous heat flow studies have resulted in the gradual accumulation of a substantial data base of the thermal conductivity of the constituent rocks, geothermal gradients and heat flow data. The main purpose of this paper is to collate this information, as well as new observations and thus generate a reference for future geothermal investigations. Approximately 900 conductivity measurements have resulted in well-established average conductivities for most stratified rock types, which vary by an order of magnitude from less than 0.3 W m-1 K-1 for coal to more than 5.0 W m-1 K-1 for sandstone. The maximum recorded temperature in the deepest parts of the basin (greater than 5 km) is 160°C and the overall average thermal gradient is approximately 29 K/km, which is greater than most other geological environments in southern Africa. The heat flow at 74 localities varies in the range 41 to 83 mW m-2 and the average for the central and southern parts of the basin is 62 ± 11 mW m-2 (34 values); this is approximately equal to the average heat flow for the mid-Proterozoic Namaqua tectonic province that underlies the southern half of the basin. The heat flow decreases to less than 50 mW m-2 in the northern and north-western parts of the basin, which values are typical of the underlying Archaean Kaapvaal Craton. Simple models illustrate the application of the heat flow and thermal conductivity data for calculating crustal temperature in the Karoo Basin.