Seismic sequence stratigraphy of the Permo- Triassic Karoo Group and its implication for hydrocarbon exploration, southwestern Ogaden Basin, Ethiopia

Abstract

The Ogaden Basin is the largest sedimentary basin in Ethiopia and one of the underexplored basins in eastern Africa. It has an area of approximately 350,000 sq. km and an estimated sediment thickness in excess of 7 km in the deepest part. The Karoo sequence which lies at the base of the sedimentary section is a Permian to Triassic age terrigenous clastics deposited in tensional to trans-tensional tectonic setting. The sequence has an estimated total thickness of 3000 m and contains proven gas reserve in excess of 6 TCF. Previous studies focused primarily on the sedimentology and lithostratigraphy of the section. This work provides the first integrated sequence stratigraphic analysis of the Karoo section based on seismic reflection and well data. In this study, the Karoo super-sequence is divided into four second-order sequences and at least four higher order sequences in the uppermost sequence. The transgressive systems tract and the overlying highstand systems tract separated by a maximum flooding surface constitute the bulk of these sequences. The dominant depositional systems in systems tracts are shallow to deep lacustrine, fluvio-deltaic and possibly sublacustrine fans and turbidites. Our study suggests that the major control in creation of accommodation space is related to wrench tectonics which gave rise to narrow pull apart sub-basins and their surrounding steep flanks. These structures are oriented NE-SW to E-W. Organic rich lacustrine shales commonly associated with transgressive systems tract and prodelta to lacustrine shales of highstand systems tract, charging deltaic and fluvial sand reservoirs of the highstand systems tract as well as potential targets such as turbidites, and sealed by transgressive surfaces make up potential hydrocarbon plays of the Karoo section. This study is one of the first detailed analysis of the Karoo sequence using sequence stratigraphic approach to predict an enhanced hydrocarbon potential in the Ogaden Basin.