GAS-PRONE SOURCE ROCKS FROM CRATOGENE KAROO BASINS IN TANZANIA

Source-rock quality and organic matter maturity of coals and sediments from several Tanzanian Karoo basins (Permian-Triassic) have been evaluated by microscopic, pyrolytic and geochemical methods. The intracratonic Ruhuhu Basin (SW Tanzania) differs from two peripheral cratogene basins (Mikumi and Rufiji) with respect to the composition of organic matter, thermal evolution and probable hydrocarbon generation in a tensional tectonic stress field of the East African craton at the Palaeozoic/Mesozoic boundary. The Lower Permian sediments and coals and the Upper Permian sediments of the Ruhuhu Basin exhibit moderate source-rock properties. They were not subjected to significant subsidence and are moderately mature source rocks. They possibly experienced temperatures in the range 60–110d? and vitrinite reflectance values of 0.5—0.8 were attained, thus placing them within the “oil window”. Kerogen Type III is typical for all the basins, and productivity indices indicate moderate to advanced stages of evolution. The coastal basins differ in most parameters. The sediments do not have source-rock properties. High vitrinite reflectance values (1.0–2.0%) imply a higher degree of thermal maturation, and geochemical results confirm that organic matter in these sediments is postmature, and that hydrocarbon generation, if it occurred at all, ceased some time ago. The assumption of a stable geothermal gradient from Late Palaeozoic to recent times, combined with field observations, leads to the conclusion that no significant overburden has covered the Ruhuhu Basin. A mean subsidence of 1, 5OO-3,000 m, depending on basin type and stratigraphic position, seems realistic, and this overburden is provided by the sediments of the “younger Karoo cycle” (Triassic). No post-Karoo sedimentary cover was deposited in the Ruhuhu Basin. Migration probably occurred in Jurassic times, shortly after the end of the Karoo sedimentary cycle, and it is presumed that no significant tectonic activity during the Cenozoic affected pre-existing accumulations. The chances that commercial volumes of hydrocarbons (mainly gas) await discovery are fair