Structural Inversion, Reactivation and Extensional Detachment and Their Influence on the Formation and Preservation of Hydrocarbon Accumulations in Northern Western Desert of Egypt

Abstract

ABSTRACT The northern Western Desert Basins of Egypt experienced a complex evolutionary history and several transformation of tectonic stress field properties. An integrated analysis of geological and geophysical data reveals that structural inversion, reactivation and extensional detachment develop in the area, and have a significant effect on formation and preservation of hydrocarbon accumulations. Such an analysis is paramount for prospect evaluation, risk mitigation, and therefore improving the exploration success rate. The rifting in Jurassic and early Cretaceous formed several faulted-depression basins with boundary normal faults in the area. A few boundary normal faults were reactivated in late Cretaceous to Eocene period as reverse faults with dextral compressive features, giving birth to a series of inverted anticlines over them. Compressive wrenching movement on the boundary faults greatly weakened their lateral sealing capacity and accordingly enhanced the vertical conduit capacity of hydrocarbon migration from the Jurassic source kitchen to Cretaceous inverted anticlines along the boundary faults. This is why Cretaceous inverted anticlines show a high concentration of hydrocarbon accumulations whereas there are few oil discoveries in the lower Alam El Bueib (AEB) formation and Jurassic along the boundary faults. Reactivation of basin boundary normal faults in late Tertiary to present abounds in the area. Most of them are surface penetrating, which are vital to the existing hydrocarbon accumulations because the reactivation could not only make poor the preservation of the existing hydrocarbon accumulations and cause the redistribution of hydrocarbons, but also it would destroy the existing hydrocarbon accumulations. Some unsuccessful wells can be attributed to the reactivation of basin boundary normal faults in late Tertiary to present. Some prospects associated with the reactivation of basin boundary normal faults have the same or similar hydrocarbon preservation risks as the unsuccessful wells. By integrating seismic interpretation and lithologic assemblage and thickness variation of the AEB formation, an extensional detachment fault was recognized in Alamein Basin. The detachment, located in the mudstone-dominated AEB 5-6 intervals, makes hydrocarbon migration difficult from Jurassic source kitchen to Cretaceous traps because the vertical migration pathways have been cut off by it, and are unfavorable for the formation of hydrocarbon accumulation above it.