Fluvial-Estuarine Source Rock Model for Exploration in Continental Rift Systems: Western Desert, Egypt

Bosworth, William; Thompson, Melissa; Drummond, Michael

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 (e.g., Katz, 1990; Schlische and Olsen, 1990). Models of this type are based on relationships observed in many rifts, including the Western Branch of the East African Rift System, the Newark Basin, the South Sudan Rifts, the Reconcavo Basin and Central Sumatra. A corollary of these models is that the lakes in which organic material can be produced and preserved form during the main phase of syn-rift extension and associated subsidence, i.e., during the middle of the rifting history (Lambiase, 1990). 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 syn-rift fill. In the subsurface these strata are referred to as the Middle Jurassic Khatatba Formation, and were deposited in predominantly fluvial (overbank marshes) to estuarine environments. TOC content generally varies from ~1-3% in the shale intervals. The crude oils derived from these are generally waxy and with a wide range of API gravities depending on several factors, chiefly maturity. Thin coal seams are also commonly present and contribute mostly gas. The deeper stratigraphic position of the Western Desert source rocks results in different exploration strategies than those applied to the lacustrine model. For example, the Jurassic source rocks are capable of sourcing oils to pre-rift reservoirs even along basin axes due to simple proximity of source and reservoir. Also, the Jurassic source rocks are mature in some areas where the Early Cretaceous main subsidence phase strata would not be. The fluvial-estuarine source rock model offers an additional exploration strategy in continental rift systems.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013