Seismic Stratigraphy Analysis and Hydrocarbon GenerationPotential of the Late-cretaceous–tertiary Formations From the Coriole Subbasin (Somalia Coastal Basin)

Abstract

The Coriole basin is the fifth largest sedimentary basin developed on the onshore of Somalia, between larger Mudugh basin in the north-east and Juba-Lamu basin, at the south-west. It also has a relatively thick infill (> 5 km) of sediments belonging to the Mesozoic and Cenozoic which might have created favourable conditions for the development of the Petroleum system elements (source, reservoir, seal, trap and migration). The major purpose of this research was to create a seismostratigraphic framework and determine the structural style of the Coriole basin that controls the hydrocarbon entrapment configuration within the thick Cretaceous to Recent sedimentary infill through the interpretation of 2D seismic data of a total length of 205 km. This kind of research has never been carried out before, and therefore, it has both academic and practical interest. To achieve these objectives, we integrated the interpretations from seven (7) 2D seismic profiles with well top formation data obtained from Afgoi-2 well. This enabled to map the extent of eleven (11) seismic horizons. In order to determine the maturity of the Upper Cretaceous rocks and Tertiary rocks and to evaluate the time of hydrocarbon expulsionmigration- accumulation, a series of Time Temperature Index (TTI) burial history curves were generated and analyzed. TTI results demonstrated that the Jesomma-Auradu Equivalent Formation began to expel some crude oil approximately 9.48 Ma (Tortonian stage Late Miocene), implying that these formations with average TOC (0.5 wt%) values may have possibly generated normal-light oil and wet gas condensate products, and are still in the early oil generating phase (Waples, 1985). The overlying stratigraphic layers are generally immature where the Obbia and Scebeli, and also Somal formations have TTI values ranging from 1.3003 to 3.089. TTI modeling was expanded on other neighboring well sections, including Merca-1, Coriole-1, Afgoi-1 and demonstrated common temporal source rock maturation trends and hydrocarbon expulsion dynamics. The TOC values from Afgoi-2 were used to identify three source rock intervals. These are: the Jesomma-lower Auradu formation interval (9190'-13710') with an average TOC value of 0.86 wt % and Seriole-Auradu transition – lower Somal Sand Member interval (4920'-7330') with an average weight TOC content of 0.90 wt % and Somal formation with an average TOC of 0.39 wt%. According to our TTI calculations, the first two intervals are mature and still capable of producing hydrocarbons (light oil products). Their thicknesses increase towards the Somali Coastal Basin's offshore area. The third source rock (Somal-Scusciuban-Somal Transition Formations) interval is mostly immature and has no HCproducing potential. This integrated study revealed that the Coriole section of the Somalia Coastal basin has three hypothetical (fictional) petroleum systems: a) Jesomma-Auradu Equivalent (Sagahley-Marai Ascia) Petroleum System; b) Scebeli/ Obbia-Auradu Petroleum Systems and c) Somal Petroleum Systems. Among them, the most promising in generating hydrocarbons is the first one, the Jesomma-Auradu-Jesomma Equivalent Petroleum System, consisting of mature source rock intervals and good quality reservoir. The Jesomma and Sagahley Marai Ascia formations offer two types of reservoirs: sandstone and carbonate. The generated isopach and isochore maps enabled us to map the deformations with potential traps and identify the possible primary and secondary hydrocarbon migration pathways. The results demonstrated that the area has been affected by wrench tectonism. This research offers an integrated approach that helps better understand the reasons and explain why the Afgoi-2 well failed to make any commercial hydrocarbon discovery.