APPLICATION OF MAGNETIC GEOTHERMOMETERS IN SEDIMENTARY BASINS: AN EXAMPLE FROM THE WESTERN KAROO BASIN, SOUTH AFRICA

Abstract

The reconstruction of thermal history is essential for evaluating the potential for hydrocarbon generation within sedimentary basins. Magnetic techniques provide an alternative to more traditional methods to study the geothermal history of sedimentary basins (such as illite crystallinity and vitrinite reflectance), which are often associated with significant uncertainty. In this paper the application of various magnetic geothermometers to the western Karoo Basin of South Africa are evaluated. Three magnetic experiments were conducted on samples from stratigraphic borehole G39977 to determine the thermal effect of large scale dolerite intrusions on the sedimentary strata of the Karoo Supergroup in western South Africa. Alteration index (A 40 ) data indicate maximum acquired temperatures for the sedimentary units ranging between 200°C and 650°C, with the highest temperatures restricted to short distances (less than half the sill thicknesses) within the contact aureoles. Both magnetostratigraphy and anisotropy of low field magnetic susceptibility (AMS) data confirm that re-magnetization of magnetic fabric does not exceed distances more than half the sill thicknesses. Our results indicate the general elevation of the palaeotemperatures of the organic-rich sedimentary rocks of the Ecca Group to temperatures where hydrocarbons are normally converted into gas. Importantly, it is clear from this study that the greatest thermal effects of the sill intrusions on the sedimentary strata are limited to the contact aureoles, suggesting that there is an, as yet unquantified, potential for hydrocarbon resources remaining between these intrusions.