An evaluation of the coal bed methane potential of the Mid-Zambezi and North Eastern Kalahari Karoo basins

Abstract

With the growing energy demand worldwide it is very important to identify any new fossil fuel resources for future use. Coal remains the most widely used fossil fuel for electricity generation in Southern Africa but over the past two decades gas has been seen as a possible supplement and ultimate replacement for the coal. A lack of world class conventional gas accumulations in Southern Africa, unconventional gas deposits, hosted in the Karoo Supergroup, have been investigated as an alternative gas source. The primary unconventional resource focused on in north-eastern Botswana and north-western Zimbabwe to date has been coal bed methane (CBM), a natural gas generated during the coalification process and stored within internal coal structures. A major limiting factor for a regional investigation into the CBM resource potential is the lack of exploration information specifically focused on gas rather than coal. The gas saturation state of coal has a notable impact on the measureable gas content value as well as the production potential within an area. One of the assumptions of previous semi-regional assessments was that the coal is fully saturated, which has not been the case from dedicated gas exploration campaigns in the region. As part of this evaluation the coal ranks, obtained from historic borehole data over the study area, were compared to the laboratory measured maximum sorptive capacities to determine the theoretical gas content of the coal. Investigations of two regional analogous coal fields showed that the coals are unlikely to be fully saturated and for a resource evaluation based on coal rank it is imperative to use a range of saturations for the final data inputs. Schlumberger’s GeoX software was used for a probabilistic resource calculation using Monte Carlo simulations with ten thousand iterations. The resource estimation results showed a wide distribution of probable values. Even with a resource value of 22Tcf, the major basins in Canada and the US have significantly higher resource densities than that of the Study Area indicating a lower prospectivity for CBM.