TYPE AND EVOLUTION OF RIFT BASINS IN NORTHERN KENYA AND ITS IMPORTANCE TO SOURCE ROCKS

The Eastern Branch of the East African Rift System (EARS) contains a number of basins with large coverage, low exploration degree and complicated tectonic evolutionary history. This paper focuses on the South Lokichar Basin, Kerio Basin and Turkana Basin, Northern Kenya. Based on regional tectonics, and sedimentary and geochemical data, rift types, tectonic evolution- and depositional filling histories of the basins and their importance to controlling over source rocks are discussed in this paper in details. The study shows that the South Lokichar Basin is a passive rift, while the Kerio and Turkana Basins are active then. It has experienced a complete cycle of rift evolution, including the initial, rapid and late rifting stages, whereas the Kerio and Turkana Basins are mainly dominated by volcanic events and characterized by episodic tensions. Two main volcanic events are found in the region, corresponding to two sedimentary cycles. Rift Evolution controls the quality of source rocks. Medium-deep lake deposits prevailed in a large area in the South Lokichar Basin in the rapid rifting stage.Thick sandstone then deposited in the late rifting stage, which made the source rock coming into mature. Good matching of the stages is favorable for the formation of high quality source rocks. Two volcanic events did cause the formation of medium-deep lakes in theKerio and Turkana Basins, however, the lakes were too short and too small to be basins for hydrocarbon formation. When strong volcanic eruption brought thick tuff to the lake, the source rock is only 20~50m in thickness. It is the main reason for the Kerio and Turkana Basin having no oil and gas discovered so far.