The Mesozoic rifting of Alamein Basin (Western Desert, Egypt): 3D evidence of transtension in the Southern Tethyan margin.

Abstract

The Alamein Basin is part of the prolific oil Western Desert region (Egypt), originated during the Mesozoic rifting of Northern Africa. Its complex evolution was linked to Gondwana break-up and comprises several rifting stages (Jurassic to Cretaceous) followed by Neotethyan passive margin evolution and inversion during the Syrian Arc deformation (Cenozoic).

The complexity of the fault system, showing different families, episodic reactivations and the presence of flower structures, has a big impact in the hydrocarbon trapping forming mechanisms.

This study combines onshore 3D seismic and well data to evaluate the structural Mesozoic evolution of the Alamein Basin. The 3D seismic interpretation and mapping of key seismic horizons (Masajid, Alam El Bueib, Alamein, Kharita-Bahariya, Abu Roash and Khoman Fms.) has allowed generate thickness maps, subsidence curves and make a detailed fault analysis.

In agreement with the main literature results revealed two initial rifting stages commonly identified in the Alamein basin (Middle Jurassic to Lower Cretaceous). Each stage is characterized by a syn-rift phase (Bahrein and Alam El Bueib Fms.) represented by a thickness increase towards the north and a post-rift phase represented by dominantly isopach formations (Masajid and Alamein Fms.), with only subtle changes of thickness.

A third additional rifting stage has been identified in the transition between Lower Cretaceous and Upper Cretaceous. This last stage is represented by a thin syn-rift phase (Kharita-Bahariya Fms.) showing thickness increase towards the north and a thick post-rift phase (Abu Roash-Khoman Fms.) developed as a result of thermal subsidence enhanced by transtensive faulting. The 3D horizon tracking clearly displays transtension to have occurred, leading to the development of a widespread wrench zone affecting the post-rift phase formations.

Fault directions obtained from four horizons plus a timeslice show a progressive clockwise variation of fault strikes orientations going from 130 to 155°. More than 1000 fault strikes were measured. On top of Abu Roash Fm. (post-rift phase of third rifting stage) the number of faults decreases and the observed faults are related to a different tectonic regime affecting Tertiary formations. Long lasting normal growth faults together with transtensive faulting could have enhanced shale formation in faults, thus hampering oil charge.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015