New insights on the upper Silurian Fegaguira shale oil play in the Chotts Basin (Southern Tunisia)

Abstract

In Tunisia Chotts Basin, the upper Silurian (Ludfordian) Fegaguira Formation comprises, organic-rich black mudstones deposited during a major anoxic event. It is a prolific source rock having yielded a large volume of oil and gas from conventional reservoirs that reached about 69 MMB OE with around 45MM BOE as recoverable reserves still to be produced. Based on various investigations, the stratigraphy of the Fegaguira Formation is updated and its unconventional play potential is assessed. It is divided, in the present work, into three units (HSII.1, HSII.2, and HSII.3) characterized by gamma-ray values up to 400 API, organic matter content (up to 17wt. % TOC), and petroleum potential (up to 60mg HC/g rock) with mature Type II marine kerogen. The first and the second units which are dominantly organic-rich mudstones can be compared to the Barnett, Antelope, and Tuscaloosa. Evaluation of the brittleness index shows that the HSII.1 and 2 units are mostly ductile and comparable to tight oil and gas reservoirs, while the HSII.3 third unit, where organic-rich facies are juxtaposed to organic-lean limestone beds with natural fractures (porosity between 3 and 7% ), may be compared to the Niobrara B formation. Within the shale-oil fairway of the Chotts basin, the estimated recoverable oil is around 1.3 billion bbl. It is comparable to the recoverable oil estimated volume for the Middle Member of the Bakken in the USA. This study demonstrates that the Fegaguira source rock should be considered as an additional unconventional oil-shale target for Tunisia.