PETROLEUM GEOLOGY OF THE NOGAL BASIN AND SURROUNDING AREA, NORTHERN SOMALIA: PART 1, STRATIGRAPHY AND TECTONIC EVOLUTION

In this study, 92 closely-spaced reflection seismic profiles (approximate to 4000 line-km) were tied to biostratigraphic and lithological data from six deep exploration wells in the poorly-known Nogal rift basin, northern Somalia, and were integrated with outcrop and aeromagnetic data to investigate the basin stratigraphy and tectonic evolution. Aeromagnetic data show NW-SE trending magnetic anomalies which are interpreted as plutonic bodies intruded during the Early Cretaceous, probably contemporaneously with a pre-Cenomanian uplift phase. The aeromagnetic data also suggest a change of basement type from Inda Ad Series metasediments in the SE of the study area to igneous and high-grade metamorphic basement in the NW. Biostratigraphic data and seismic reflection profiles define the Nogal Basin as a WNW-ESE striking half-graben, approximately 250 km long and 40 km wide, which formed as a result of mainly Cenomanian-Maastrichtian and Oligocene-Miocene intracontinental rifting. The depocentre contains at least 7000 m of Mesozoic and Cenozoic sediments and is located in the centre of the basin (east of well Nogal-1), to the south of the Shileh Madu Range. To the north, the basin is bounded by a major border fault along which significant variations in the thickness of sedimentary units are observed, suggesting that the fault controlled basin architecture and patterns of sedimentation. Oligocene-Miocene normal faults which resulted in north-tilted fault blocks are widespread within the main basin; smaller-scale sub-basins oriented NW-SE to WNW-ESE are observed to the NW of the basin and probably developed contemporaneously. The Late Jurassic rift phase which has been documented elsewhere in northern Somalia is either missing in the Nogal Basin or is preserved only in localised grabens in the western and central parts of the basin. This is probably due to the pre-Cenomanian uplift and erosion which removed almost the entire Jurassic and Lower Cretaceous successions over a wide area referred to as the Nogal-Erigavo Arch. A more pronounced rifting episode followed this erosional event in the Cenomanian-Maastrichtian and resulted in the deposition of well-sorted fluvio-deltaic sandstones (Gumburo and Jesomma Formations), more than 2000 m thick. In wells in the Nogal Basin, these formations are between two and three times thicker than in wells drilled in footwall locations, and include excellent reservoir rocks sealed by transgressive mudstones and carbonates. A final rifting event in the Oligocene-Miocene was related to the opening of the Gulf of Aden. A rift sag phase which accommodated the Early Oligocene continental sediments of the Nogal Group initially developed at the centre of the basin. This was followed by a period of strong rotational faulting and tilting, which reactivated the Cenomanian-Maastrichtian structures.