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

Sulaiman A. Abushagur

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.

Paleocene rocks crop out almost continuously along the southwestern and western margins of the basin. Abrupt changes in the lateral facies occurred between the platforms and the deeper parts of the basin. The carbonates, evaporites, and shales of the Paleocene series were deposited in an epicontinental sea that extended across the Sirte basin.

The Paleocene section of the Sirte basin was controlled by a segment of transgressions and regressions of an epeiric sea across a deep, persistent shale basin. Two depositional episodes are traceable and extend throughout much of the basin.

The first cycle is represented by deposition of the Hagfa transgressive shale in the western and central parts of the basin during Danian time. This sequence was followed by deposition of a shallowing-upward, regressive carbonate and evaporite sequence (Thalith, Farrud, and Mabruk Formations) during Montian time. Eastward, shelf carbonate deposition (Defa and lower Sabil carbonates) took place along the shelf during Danian and Montian times.

The second cycle is represented by a deeper depositional environment during the early Landenian. This is represented by a transgressive shale sequence deposited along the western (Dahra shale), central (Khalifa shale), and eastern (Shterat shale) parts of the basin. During the middle and upper Landenian, the sequence progressively shallowed. Limestones and dolomites (Zelten Formation) were formed within a restricted shelf in the western part, while basinal and near-basinal deposition dominated the eastern part of the basin. In the last stages of the Landenian, the general depositional environment became even more shallow than that represented by the underlying unit along the western part of the basin.

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.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.