The southern margin of the present Gabes-Sabratha basin roughly parallels the coastline of northwest Libya and southeast Tunisia and is contiguous to the northern margin of a Lower Cretaceous subaerial delta platform. The Lower Cretaceous Chicla and Cabao formations, exposed along the Jebel Nefusa escarpment and in the subsurface of the onshore Ghadames basin in Libya, represent a thin (0 to 200 m), laterally extensive, fluvio-deltaic sequence. A detailed study of 15 measured outcrop sections indicates each formation is a transgressive fluvio-deltaic package. A complete package can be subdivided into three major sedimentary facies from base to top: (1) braided fluvial delta plain (facies 1) or braided fan delta plain (facies 1a), (2) delta front-prodelta slope (facies 2), and (3) prodelta shelf (facies 3).

Figure

The Chicla and Cabao fluvio-deltaic packages were derived from two sediment source areas: (1) a large, undetermined area far to the south, and (2) a smaller, proximal area, 50 to 100 km inland of the Libyan coast.

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A system of large to small northwesterly flowing braided streams, fed from the quartz-rich southern source, deposited clean, laterally persistent, reservoir sands (braided fluvial and delta front facies) and impervious sealing shales (prodelta slope and prodelta shelf facies) across the slowly subsiding Saharan Platform. These deposits can be traced northward in the subsurface, across the entirety of western Libya, into the outcrop in the Jebel Nefusa, the northern limit of onshore control. Deposition ultimately extended northward into the more rapidly subsiding, organic rich, Gabes-Sabratha basin.

Simultaneously, the locally sourced reservoir facies were deposited, north-northwesterly, from the region of the Garian basement high. Coarse conglomerate quartz sands (braided fan delta plain facies) were transported a short distance to the Jebel Nefusa and northward, forming a wedge of clean, reservoir quality sands (braided fluvial and delta front facies) and sealing shales (prodelta slope and prodelta shelf facies) lying below well control in the Gabes-Sabratha basin area immediately adjacent to the Libyan coast due west of Tripoli.

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