Cyclic Transgressive and Regressive Sequences and Their Association with Hydrocarbons, Sirte Basin, Libya: ABSTRACT

Abstract

The Sirte basin was developed in north Africa between the Tethys Sea and the Saharan shield during Late Cretaceous time and was the site of mixed siliciclastic and carbonate deposition throughout the Tertiary. A series of bioclastic limestones and shales was deposited around the basin rim. Shales were confined to the low-energy zones in the basin, whereas carbonates were deposited on the shelf areas. The Farrud Formation (equivalent to the Beda Formation in the central part) is the main reservoir for oil found in the western portion of the basin. The faunal assemblages and lithologies recognized in this formation apparently reflect a very shallow marine depositional environment. Source rocks are developed in organic-rich, transgressive shales (Dahra and Hagfa). Moldic, fenestral, and intraparticle porosities are the most common types recognized in the carbonate reservoirs of the Farrud Formation. Permeability is developed in part by processes such as dolomitization, leaching, and fracturing in the two progradational, regressive carbonate cycles, resulting in the exceptional Ghani field reservoirs. Hydrocarbons were trapped in these reservoirs due to the presence of a supratidal anhydrite cap rock.