Depositional model for the Early Cretaceous estuarine, fluvio-deltaic, and braided channel deposits of the Faregh Sandstone Member, eastern Sirte Basin, Libya: Reservoir quality evaluation

Abstract

The Early Cretaceous Faregh Sandstone Member constitutes major hydrocarbon reservoirs in the eastern Sirte region, Libya. Despite the member's considerable economic importance, high-resolution sedimentological and petrological studies are still limited, especially concerning their influence on reservoir formation and quality. Therefore, this study integrates core, petrographic and diagenetic analyses to examine the sedimentological features of the Faregh Sandstone Member of the Early Cretaceous Sarir Formation, with the objective of determining controls on reservoir evolution. Sedimentologic examination shows that the Faregh sandstones consist of six distinct petrofacies: quartz arenites, sublithic arenites, lithic arenites, lithic wackes, quartz wacke sandstones, and silty claystones. Core samples taken from various wells in the Faregh sandstones reveal a range of sedimentary structures. These include current ripples, both high- and low-angle tabular and trough cross-beddings, parallel laminations, and reactivation surfaces. Cores also reveal the presence of clay clasts and pebbles. Based on the petrofacies types and sedimentary structures, the various lithofacies of the Faregh sandstones are interpreted to constitute three depositional associations which represent estuarine, fluvio-deltaic, and braided river depositional settings. The estuarine facies association includes, estuarine channel and bar, bay, and tidal accumulations. The fluvio-deltaic facies association features crevasse splay, distributary channel, and mudflat to bay facies, while the braided fluvial facies association comprises braided bars, channels, and zones of channel abandonment. Diagenesis in the Faregh Member is featured by extensive quartz overgrowth cements, especially in the quartz and sublithic arenites, which often occlude intergranular pores and form interlocking textures. Moreover, kaolinite occurs in various forms, such as pore-filling, grain-replacive, and pore-lining—along with other minor clays, pyrite, hematite, partial dissolution, and rare fracture porosity. In finer-grained lithologies, such as wackes and silty claystones, diagenetic features include replacive pyrite and hematite, and moderate amounts of indeterminate clays. The reservoir quality is primarily influenced by sedimentation, with the estuarine and distributary channel and bar deposits identified as the optimal facies for the development of high-quality reservoirs. This study presents a conceptual model that supports enhanced exploration and development of the Faregh sandstones, a significant reservoir in the eastern Sirte Basin, while also offering guidance to reduce uncertainties in similar hydrocarbon systems.