Superimposed Mesozoic Deformations and Hydrocarbon Play Concept in the Northern Western Desert, Egypt -- Example from Umbarka Area

By

Khaled Saied¹, Ahmed El Shamy², Adel R. Moustafa³

(1) Khalda Petroleum Company, Cairo, Egypt (2) Repsol, N/A, (3) Ain Shams University, Cairo, Egypt

 A detailed study of 2D and 3D seismic and borehole data of the Umbarka area illustrates the effect of two superimposed Mesozoic deformations on hydrocarbon potential and entrapment in the Umbarka field. An early phase of Jurassic and Early Cretaceous rifting led to the deposition of thick source rocks in a northward tilted half graben (South Umbarka basin). Late Cretaceous right-lateral strike-slip movement on the basin bounding fault (Umbarka fault) formed second order wrench structures (folds and faults) above the depocenter of the early rift basin. A combination of hydrocarbon migration from mature source rocks in the South Umbarka basin and the presence of good reservoir rocks in these wrench folds, that provided optimum trapping conditions, established the Umbarka field. Similar conditions apply to other fields in the northern Western Desert.

The Umbarka fault is oriented E-W to ENE-WSW and separates the northward tilted South Umbarka basin from the Umbarka platform. The latter had gentle northward tilt where its updip (southern) part was above the Jurassic sea level leading to erosion of its Paleozoic cover. Tectonic subsidence of the South Umbarka basin probably continued at a slow rate during the Cenomanian affecting the net sand distribution in the Bahariya formation. Second order wrench structures formed by Late Cretaceous dextral movements along the Umbarka fault formed WNW-ESE oriented right-lateral strike-slip faults (Riedel / field shears). Dextral slip on these faults led to the development of NE-SW oriented folds in the syn-rift and post-rift sequences, which include the main reservoir rocks of the Umbarka field. The en echelon WNW-ESE oriented faults join at depth with the Umbarka fault and probably formed a migration pathway of hydrocarbons from the deep source rocks in the South Umbarka basin into the fold traps in the overlying reservoir rocks.