Other African Rifts
Published:December 01, 2015
Sedimentologic and hydrocarbon systems modeling of continental rift systems often incorporate deposition of organic-rich source rocks in deep, long-lived lacustrine settings as a central premise. A corollary of these models is that the lakes in which organic material can be produced and preserved form during the main phase of synrift extension and associated subsidence; i.e., during the middle of the rifting history. The deep-lake model has been very successful as an exploration tool but does not describe the relationships observed in all hydrocarbon producing continental rifts. In the Mesozoic basins of the Western Desert of Egypt, the most important and regionally extensive source rock occurs at the base of the synrift fill. These Middle Jurassic Khatatba Formation strata were deposited in broad fluvial flood plains with overbank swamps and small lakes transitional to estuarine or lagoonal environments. Total organic carbon content generally varies from ~1–3% in the shale intervals. The crude oils derived from these shales have variable wax content and a wide range of API gravities. Thin to locally thick coal seams are also commonly present and contribute mostly gas. Production tests of over 8000 barrels per day have been recorded from reservoirs sourced by these Khatatba oils. The deeper stratigraphic position and different lithologic facies of the Western Desert source rocks results in different exploration strategies than those applied to the lacustrine model. The Jurassic source rocks: (1) were deposited over very broad areas and not just along main basin depocenters; (2) were capable of sourcing oils to high-quality prerift Paleozoic reservoirs due to simple proximity of source and reservoir; and (3) are thermally mature in some areas where the Early Cretaceous main subsidence phase strata are not. The fluvial-estuarine source rock model offers an additional exploration strategy in continental rift systems.