Structural interpretation of the Rukwa Rift, Tanzania

Abstract

The Rukwa Rift lies between Lakes Tanganyika and Malawi in the western limb of the East Africa rift system. Because little was known about the rift's structure or hydrocarbon potential, Petro‐Canada International Assistance Corporation completed a 2150 station gravity survey as part of an assistance program for the Tanzanian Petroleum Development Corporation. The survey covered an area 165 km × 375 km, which included the entire rift valley and lake plus regional control on either side. Outcrops of Carboniferous‐Triassic conglomerate, coal, and limestone, as well as Cretaceous sandstone, occur along the southwestern edge of the rift. The younger section is presumed to be dominated by alluvial material. In the absence of any density control, the gravity data were modeled using clastic sedimentary fill, which yields minimum depth estimates. Alternate models with more shale in the section have also been tried. A rift model with two shale pulses corresponding to interrift times yielded maximum depths of about 10 km. An all‐shale model failed to converge because of insufficient mass contrast. The final interpretation was based on the gravity models and aeromagnetic data acquired in an earlier survey. The Rukwa Rift is a half‐graben bounded to the northeast by a listric normal fault (strike 130 degrees) with 7 km of throw. A younger fault system forms the southwestern side of the valley and creates a major structure with 3 km of relief. The divergent strike of the younger faulting appears to be related in some way to right lateral shear in the Rukwa region. The Rukwa Rift has all the elements needed to be considered highly prospective for oil from a lacustrine source. There is strong evidence to suggest that the history of the Rukwa Rift is long and complex, providing ample opportunity for establishment of such an environment. The analogy of the Sudan rifts and the reports of oil seeps elsewhere in the western rift system support such a hypothesis. All the other elements of structure, reservoir, seal, maturation, and timing can be reasonably inferred from the available information. Of course, seismic and drilling are needed to provide firm stratigraphic control to confirm these inferences.