--> Exploring in the Kerio Valley Basin, Southern Kenyan Rift: The Lakes, Lavas, and Prospectivity in a Magmatic Rift Basin

2018 AAPG International Conference and Exhibition

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Exploring in the Kerio Valley Basin, Southern Kenyan Rift: The Lakes, Lavas, and Prospectivity in a Magmatic Rift Basin

Abstract

Tullow Oil has embarked upon a multi-basin assessment in Kenya, so far resulting in major discoveries in the South Lokichar Basin (SLB) and the confirmation of an active petroleum system in the emerging Kerio Valley Basin (KVB). The KVB fill is a complex succession of Middle Miocene to Recent sediments and volcanics that has attracted interest due to the source rock outcrops on its flexural margin (Tugen Hills). This paper presents the findings of an integrated geological and geophysical investigation where fieldwork in the excellent exposures of the exhumed basin periphery are incorporated with the well, seismic, and FTG data to help form our understanding of the tectonic, magmatic, sedimentary evolution of the basin and its implications for prospectivity. The KVB is bounded by the N- S trending Elgeyo fault, located at the transition between the Tanzanian Craton and the Mozambique Mobile Belt. The presence of NW-SE pre-cursive fabrics may influence the segmental development of the fault and the resultant morphology. Varying fault segment subsidence rates led to depocentre shifts during extension (~16-7 Ma). Contemporaneously, multiple eruptions of low viscosity phonolitic lavas occurred across the region interacting with the prevailing sedimentary systems. Post 7 Ma the extensional axis shifts eastward from the KVB leading to the formation of the Baringo Basin and footwall uplift of the Tugen Hills. Finally, a late stage local uplift occurs in the north of the basin resulting in a similar structural framework to the SLB, with potential traps located along the basin bounding fault, the flexural margin and in the uplifted north. The description of sedimentary logs and analysis of samples from the exposed Elgeyo footwall, the Tugen Hills and the Cheptuket Well allowed us to reconstruct the depositional environments of the basin. Ar-Ar age dates of the interbedded volcanic units provide a robust chronostratigraphic framework. Predominantly fluvial pre-rift depositional environments (~18-20 Ma) develop into a fluvio-lacustrine setting as the Elgeyo Fault begins to subside during a warm wet climatic period (~16-13 Ma). Highly variable lakes housed a plethora of depositional environments with evidence of alluvial fans, fluvial sandstones, sandy shorefaces, oolitic sands and organic rich, fossil laden muds. The KVB houses source, seal and reservoir along with multiple structures, the challenge remains to find the sweet spot.