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Case Studies: Productive and Non-Productive Rifts |
1 Banks Consulting, College Farmhouse, Denton, Oxford OX44 9JJ, UK
2 Intera Information Technologies, Highlands Farm, Greys Road, Henley-on-Thames, Oxon RG9 4PS, UK
3 Hydrocarbon Unit, Ministry of Mines, PO Box 30385, Lusaka, Zambia
An integrated study of the Luangwa Valley Karoo Rift basins of NE Zambia has been carried out. Approximately 3000 km of seismic data were used in addition to gravity and aeromagnetic surveys and data from two exploration wells. The present day Luangwa Valley is underlain by two opposing Karoo half-graben separated by a central accommodation or transfer zone. The basins contain up to 8000m of Permo-Triassic, mainly clastic, sediments. The Lower Karoo consists of basal glacial deposits overlain by a sequence of interbedded coal, shale and siltstone, and capped by a very thick (up to 5000 m) lacustrine section of interbedded siltstone and mudstone. The Upper Karoo consists of interbedded coarse sandstones and siltstones thought to be of braided stream and flood plain origin. Rifting appears to have been initiated in the early Permian. The timing of subsequent rifting and inversion events has been related to strike-slip movements along the Mwembeshi Shear Zone, a major Precambrian lineament. Study of the seismic data has revealed a number of structural and stratigraphic plays in the basin. The best potential for hydrocarbon accumulation occurs within the lowest part of the Karoo section where source rocks and reservoirs are postulated in the Luwumbu Formation and the lowest part of the Madumabisa Mudstone. Since these intervals were not reached by either of the exploration wells drilled it is felt that the hydrocarbon potential of the Luangwa Graben has not yet been fully tested.
