New insights on the upper Silurian Fegaguira Formation 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 MMboe, with around 45 MMboe 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 of up to 400° API, organic matter content (up to 17 wt% total organic carbon) and petroleum potential (up to 60 mgHC g −1 rock) with mature Type II marine kerogen. The first and the second units, which are dominantly organic-rich mudstones, can be compared to the Mississippian Barnett, Miocene Antelope and Cretaceous Tuscaloosa shales of the USA. Evaluation of the brittleness index shows that the HSII.1 and HSII.2 units are mostly ductile and comparable to tight oil and gas reservoirs, while the third HSII.3 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 Bbbl. 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 Formation source rock should be considered as an additional unconventional oil-shale target for Tunisia.