Abstract

The Karoo basins of Southern Africa are very unique in Africa because they are the only basins which record a complete Carboniferous to Jurassic rock record and very interesting change in palaeoclimate from glacial temperature to arid desert. These basins are vital in South Africa as they host all coal deposits. Traces of oil have also been discovered in the northwest sector of the Karoo.

The size of the Karoo (700 000 sq km) indicate that no intensive exploration has been done to conclude the existence/non existence of natural gas and conventional oil in these basins.

It is believed that significant amount of natural gas can be obtained in the Karoo to boost South Africa's energy crisis.

Discussion

The Karoo basins of Southern Africa are unique as they record a complete late Carboniferous to Jurassic succession and a change in palaeoclimate from glacial through temperate to dry desert (Cadle et al., 1993). Because of the nature of the mountain-building episode in the Cape fold belt, as well as the underlying lithosphere, the Karoo sequence of rocks is highly asymmetrical. It is thickest in the south (maximum cumulative thickness of 12 km) and thins drastically to the north. The Karoo basins hold an important place in the South African geology and economy as they occupy more than 50% of the nation’s land area and also all of the country’s important black coal deposits (McLachlan, 2007).

Exploration for natural oil and gas in the Karoo basins was stimulated by the widespread presence of traces of oil in the sandstones of the Ecca and Beaufort Groups. Initial interest started as early as 1889, but exploration peaked during the sixties and seventies. However, by the eighties, explorers still had had no commercial success. This exploration raised as many questions as answers and some explorers still believe that the enormous size of this basin, 700 000 km², has not been adequately tested and that it is too early to conclude that there is no natural gas and oil potential in this basin.

Since the promulgation of the Mineral and Petroleum Resources Development Act in 2002, new interest has been sparked in onshore exploration for petroleum with more than 130 applications for exploration rights onshore having been received to date by the State-owned institution, Petroleum Agency SA. This suggests that explorers believe, on the basis of comparisons with similar basins to the South African Karoo basins, that there is still potential for oil and gas discoveries to be made.

Positive factors relating to petroleum potential include the confirmation of oil prone source rocks in the southern and northern parts of the Great Karoo Basin as well as the demonstration of a number of active petroleum systems which include the still viable Free State and Evander gas fields, natural oil in the sandstones of the Ecca and Beaufort Groups, the Smaldeel helium and methane gas field and evidence of coalbed methane resources associated with South Africa’s abundant coal reserves.

Natural oil occurs in a broad arc across the northern part of the Great Karoo Basin with a focus on the Dannhauser and Wakkerstroom-Utrecht areas. According to van Vuuren et al. (1998) oil occurs:

• In the pore space of sandstones of the Vryheid Formation and lower most part of the Beaufort Group.
• In secondary sandstone pores, fractures or cavities at the contact zones of dolerite dikes.
• In vesicles in the basal part of the lava of the Drakensberg Formation.

The porosity and permeability of sandstones in the Vryheid Formation and lower part of Beaufort Group are generally poor south of about 28°S and increase northward to good and very good in places (Rowsell and De Swardt, 1976). The reservoir quality is, however, adversely affected by extensive of dolerite intrusion. The quartzites of the Table Mountain Group in the south could have been better reservoirs however their porosity has been destroyed by diagenesis due to deep burial and formation of the Cape Fold Belt.

The shales of the Ecca Group and some of the lowermost Beaufort Group are fair to fairly good source rocks except where affected by dolerite intrusions (Rowsell and De Swardt, 1976). In the southern Karoo source rocks are the shales of the Bokkeveld, Dwyka Groups, the lower part of the Ecca Group. Some shales in the upper Ecca have high organic carbon content and could have been good source rocks for hydrocarbons, however their potential has been destroyed diagenesis and dolerite intrusions.

Upper Ecca shales have a moderate organic content of one to two percent with average vitrinite reflectance values of 1.1 percent and fixed carbon content of 66 percent in the Wakkerstroom area, and 0.93 vitrinite reflectance and 65.5 percent fixed carbon content in the Dannhauser area. Oil shales (torbanites) are also a potential hydrocarbon source in the Wakkerstroom and Dannhauser areas.

In the Ermelo Coalfield, oil is extracted from the C Upper Seam which is torbanitic over large areas. The thickness of the composite seam varies between 0.7m and 4m. Proximate analyses show the fixed carbon to be 64 percent and vitrinite reflectance to be 0.65 percent.

Current exploration interest in CBM in the Karoo basins has taken off dramatically, encouraged by the pressure of growing global environmental concerns and the need to reduce South Africa’s carbon footprint. Attention is focused on almost all of the coalfields including the central basin and Kwa-Zulu Natal coalfields. According to National Energy Council (1991) there are seven coalfields known to have CBM potential and they are ranked as follows:

• Waterberg coalfield: good depth of burial (400m) particularly in the east makes this suitable for accumulation of CBM.
• Perdekop-Amersfoort area of the Ermelo coalfield: coals are known to be gassy, but the seams are relatively shallow and inertinite rich.
• Vryheid coalfield: the deeply dissected terrane may have caused depressurization of the seams.
• Evander area of the Highveld coalfield: sustained gas flows have been recorded for many years but seams are relatively shallow and rich in inertinite.
• Pafuri coalfield: favorable rank, type and depth and adequate fracturing due to faults.
• Tuli coalfield: similar to Pafuri coalfield but less fractured.
• Nongoma coalfield: high rank and high vitrinite content, adequate fracturing due to faulting.

Anglo Operations has estimated that there is 1Tcf of recoverable gas in the Waterberg area. There is little hard data at present on which to base a sound estimate of CBM potential in other basins, but the growing interest of explorers promises more data and results which will provide the basis of future evaluations.

Another natural gas resource which is of interest is the associated helium – methane gas discovered in the Free State and Evander areas. In the Free State it occurs in natural gas which mainly consists of methane that is emitted by surface bore-holes, faults and cracks in underground workings (Hugo, 1963).

In the Evander area the gas field is ~ 400 km² and overlies the Pre-Cambrian Witwatersrand Evander Goldfield. The source of this helium is not clearly understood. It is possible that it is produced by the spontaneous radioactive decay of elements. The methane which occurs in the Witwatersrand strata is believed to have been derived from distillation of coal or carbonaceous shale by intrusion of dolerite (Hugo, 1964).

Although there is a positive and enthusiastic approach to exploration at present, it must be borne in mind that there are a number of negative factors which impact on the petroleum potential of the Karoo basins. Geological factors include the intrusion of dolerite sills and dikes into the sediments of the northern Karoo, and excessive depth of burial in the southern Karoo. Although South Africa has an abundance of coal, low gas content, shallow depth of burial and thinness of coal seams can all impact negatively on CBM development.

The economic viability of projects can also be affected by competition for energy markets by cheap coal and coal-generated electricity, and the remoteness of prospective areas from markets and transport networks. Fortunately, in the onshore environment much smaller deposits of oil and gas can be commercially viable and brought into production by better and cheaper technology, particularly if government legislation and regulation is formulated to facilitate small gas developments in the face of large, established energy suppliers.

Conclusion

The Karoo basins of South Africa are an immense frontier area and have not been thoroughly explored. Positive factors include the confirmation of a number of active petroleum systems. Past exploration for conventional oil and gas has been disappointing with no commercial accumulations having been discovered, but a number of critical factors have changed. These include a better understanding of the geology, better and cheaper technology for exploration and production of oil and gas which will allow smaller deposits to be brought into production and the need to address the problems of global environmental concerns directing focus into unconventional gas resources. The Karoo basins therefore remain exciting if challenging areas for exploration of oil and gas.

References

Cadle, A.B., B. Cairncross, A.D.M. Christie and D.L. Roberts, 1993, The Karoo Basin of South Africa: Type basin for coal-bearing deposits of Southern Africa.

Hugo, P.J., 1963, Helium in the Orange Free State Goldfield, Geological Survey of South Africa, 39, 28 pp.

Hugo, P.J., 1964, The Evander gas field, Bulletin Geological Survey of South Africa, 41, 40 pp.

McLachlan, I.R., 2007, An evaluation of the petroleum potential of the South African onshore Karoo Basins, Report, Petroleum Agency SA, 9 pp.

Rowsell, D.M., and A.M.J. De Swardt, 1976, Diagenesis in Cape and Karoo sediments, South Africa, and its bearing on their hydrocarbon potential, Transactions, Geological Society of South Africa 79 (1),.p. 81-145.

National Energy Council, 1991, Coal-bed methane resource development: Report, National Energy Council, Pretoria, 58 pp.

Rowsell, D.M., and J. Connan, 1970, Oil Generation Migration and Preservation in the Middle Ecca Sequence near Dannhauser and Wakkerstroom.

van Vuuren C.J., D.S. Broad, E.A.H. Jungslager, J. Roux and I.R. McLachlan, 1998, Oil and Gas in Mineral Resources of South Africa (M.G.C. Wilson and C.R. Anhaeusser eds).