Regional structure of the Central Kalahari sub-basin and the geometry and effect of the dolerite sills in the area
The potential for Coal Bed Methane (CBM) has been assessed in the Karoo-aged coal seams in Eastern Botswana. This region is underlain by the Central Kalahari sub-basin of the Karoo Sequence and hosts a major thermal coal producer in the Morupule Mine. Coal bearing strata at depth in the sub-basin that are thought to be a source of CBM, occurs over an area bounded by the Okavango Dyke Swarm to the north, the Zoetfontein Fault to the south and the Kalahari Line to the west. The basin terminates to the east as a pinch out, with Karoo strata outcropping at the surface and the coal seams removed by erosion. Coal occurs in the Serowe and Morupule Formations of the lower Ecca Group and is concentrated in four principle seams, namely the UMH, Z3, Z2 and Z1 seams. The sub-surface distribution of these seams has been investigated with the use of geophysical techniques, including aeromagnetics and wireline logging of exploration boreholes. Magmatic activity dated ~180 Ma heralded the breakup of Gondwanaland and caused the Karoo basins to be intruded by dolerite sills, which had a profound effect on the coals therein. The first major effect was devolatilisation or burning of the coal as the dolerite intruded. The second major effect involved an increase in coal rank from sub-bituminous to bituminous or anthracite, as the temperature and perhaps pressure increased, due to the intrusion. Both these changes occurred on a relatively small scale, mainly affecting coal close to the intrusions. The third and last effect due to sill emplacement was metasomatism, which was caused by the elevated temperatures promoting fluid flows in the country rock on a much larger scale, as the fluids were able to migrate beyond the zone of conductive heat transfer. An important component of the metasomatic activity was the introduction of minerals such as calcite that precipitated in fractures and cleat systems in the coal, as the fluids moved through them. These minerals act as cement, thereby further reducing the permeability in an already tight reservoir. Overall, the effects of the dolerite sills that have intruded in the area have been a negative one, affecting the coals adversely. The coals have become devolatilized, heat affected, or metasomatised, and their permeability reduced. These factors decrease the quality of the reservoir, and impact negatively on the possibility of a Coalbed Methane (CBM) project in zones containing igneous intrusions. It was noted that the distribution of dolerite intrusions at depth could only be partly determined from magnetics, because of the masking effect of the overlying volcanics, and it was necessary to make use of wireline logs and coal properties to assess the CBM potential.