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2018 AAPG International Conference and Exhibition

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Integrated Source Rock Evaluation Offshore Somalia

Abstract

Offshore Somalia remains one of the last truly frontier passive margins – only two exploration wells have been drilled in the entire offshore area. Identifying potential source rocks in unexplored margins requires integrating multiple datasets to build a robust geological model that fits observations from seismic data. We use gravity and magnetic data to reconstruct tectonic plate positions, onshore well information and regional geological understanding. All of these complement our seismic analysis to build a framework for source rock characterization of potential source intervals offshore, along with sequence stratigraphy models and slicks seen on satellite imagery. Pre-rift sources exist in Karoo coals and lacustrine shales (e.g. Permian Bokh Shale sourced the Calub gas-condensate field in Ethiopia). Karoo lacustrine sources are well developed in Madagascan onshore oil fields. Syn-rift shallow marine shales were deposited during the Toarcian OAE. In the early post-rift, Upper Jurassic marine shales are a key source in the Ogaden Basin and northern Somalia. Similar age organic-rich shales exist in the Rovuma Basin and the Seychelles. An acoustically quiet homogeneous unit can be mapped at this level in Somalia, interpreted as a deep marine organic-rich marl. In the Cretaceous, OAEs occurred in the Aptian and Cenomanian-Turonian, and localised source rocks may be deposited in restricted marine environments created by transform faults from the Indian plate movement past Somalia. Cretaceous and Tertiary shale décollements linked to slope failure off southern Somalia may also have source potential. Thermal maturity models for four source intervals identified by seismic characterization were created. Peak maturation maps for sources at Karoo, syn-rift, Late Jurassic and Early Cretaceous levels indicate that post-rift Early Cretaceous and Late Jurassic sources lie in the present-day oil window in the south where sediments are thicker, whereas pre- and syn-rift sources are oil-mature in the north. Defining source rocks in untested frontier margins is challenging. By integrating multiple datasets and geological models we propose scenarios in which organic-rich sediments may be deposited and matured. This is a useful tool in de-risking frontier passive margins where limited data exists.