Geothermal Modeling of the Karoo Basin, South Africa, using Rock Magnetic Methods

Monday, 15 December 2014
Leonie Maré1,2, Michiel de Kock2, Bruce Cairncross2, Hassina Mouri2, Eric C. Ferre3 and Michael J Jackson4, (1)Council for Geoscience, Pretoria, South Africa, (2)University of Johannesburg, Geology, Auckland Park, South Africa, (3)Southern Illinois University Carbondale, Carbondale, IL, United States, (4)University of Minnesota, Institute for Rock Magnetism, Minneapolis, MN, United States
The viability of using magnetic fabric analyses as geothermometers in sedimentary basins was investigated and new constraints on regional-scale geotherms for the Karoo sedimentary Basin were identified. A comparative study of the variation in magnetic properties in eight boreholes located along a southwest to northeast traverse across the Karoo Basin was performed. The thermal impact of the intrusive Karoo Dolerite Suite on the surrounding sediments was determined using several magnetic experiments. The methods employed during the investigation include low field anisotropy of magnetic susceptibility, variation in magneto-stratigraphy using the classic baked contact test, the magnetic Alteration Index (A40) as well as looking at the possibility to use magnetite-pyrrhotite geothermometry.

The experiments indicated the heating effect of the dolerite intrusions to be limited to short distances within the contact aureole. Boreholes that intersected dolerite sills indicated that the heating effect occurred no wider than half the sill thicknesses. However, due to the extensive network of sills and dykes in the Karoo Basin, an overall elevation in the geomagnetic temperatures of the stratigraphic sequence to temperatures above 150 °C was observed. This knowledge could have major implications for any potential shale-gas industry in South Africa.

Temperatures calculated by the A40 method (minimum observed values) indicated a general increase from southwest to northeast in the thermal effect of intrusions on the Karoo sediments. This correlates with reported increased coal maturity from west to east. Several hypotheses exist for this geothermal variation including the influence that distance to magma source might have had. The most probable hypothesis however, relates to the different environmental settings that prevailed between the western and eastern parts of the basin during magma intrusion and the associated differences in thermal conductivity of low permeable marine shale in the west compared to the porous lacustrine sandstone and siltstone towards the east.