PROSPECTIVITY OF BLOCK D , BURUNDI, LAKE TANGANYIKA POSTER
D.Macgregor, J.Brown and C.Pitman, Surestream Petroleum C. Scholtz, Lacustrine Systems Analysis
Lake Tanganyika is a complex series of half-grabens , the deepest parts of which lie predominantly in Congolese waters. Present day depths mirror sediment thicknesses and thus the location of the depocentres does not seem to have moved through time. Two distinct seismic sequences are seen on the Project Probe seismic, acquired in the 1980s, termed the Magara and Kigoma sequences. Extrapolation of current sedimentation rates suggests a Late Miocene initiation of the N-S trending northern part of the lake, though older dates are possible. In two onshore Burundi wells, Neogene sands overlie Basement. A different picture seems to be evident for the NW-SE trending southern segment of the basin, part of which is exposed onshore in Congo as non-marine Permo-Triassic sediments. This section of the lake therefore seems to be older in origin, with a thinner Neogene fill.
Source rocks are considered near certain in Lake Tanganyika, These are being deposited at present day in the anoxic zone below 150m with organic contents of the lake sediments of 3-8%. The present day topographic and climatic setting of the lake has likely remained similar throughout its Neogene history and thus thousands of metres of source rock could have been deposited. Presence of such source rocks is confirmed by spectacular oil slicks on the Congolese side of the lake, along a basin bounding fault near a depocentre of circa 2500m sediment thickness. In the absence of knowledge of the geothermal gradient, this calibration point of 2500m sediment thickness is used to define likely oil kitchens. Onshore seeps are reported in Burundi in colonial records as well as tar balls frequently washed up on beaches. It can be seen from the structure maps that a structural nose running through Block D forms a focus for migration from two large depocentres, one to the west, confirmed by the seeps, and a deeper basin to the south, within Tanzanian waters.
Study of the seismic stratigraphy of both Lake Tanganyika and Lake Malawi indicates that during arid periods, lake level probably lay up to 600m below current depths. Thus in the water depths such as exist in Block D, there were likely periods of shallow water deposition. Study of current depositional environments indicate many possible reservoirs during such lowstands, including beaches, bars, shelf sands and coquinas limestones (as in the Tupi field, Brazil), the latter most likely over the many fault blocks .The rapid changes in lake level, much amplified over any marine setting, likely led to deposition of hemipelagic shale seals during lake lowstands
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