Sequence Stratigraphy and Sedimentology of the Bahariya Formation, Khalda Concession, Western Desert, Egypt

By

 Fred Wehr¹, John Youle¹, George Pemberton²

(1) Apache Egypt, Cairo, Egypt (2) University of Alberta, Edmonton

 The Bahariya Formation is a complex succession of mixed clastic-carbonate deposits of Cenomanian age that formed on a broad, tidally-influenced ramp on the southern margin of Tethys. In the Khalda area it records the punctuated transgression of the Tethys margin, overlain by shallow-marine carbonate sedimentation of the Abu Roash Formation. Detailed core studies combined with regional correlation indicate a complex interplay between relative sea-level change and sediment supply.

The Bahariya Formation is divided informally into two members. The Lower Bahariya formation (400-800 feet thick) is the principal reservoir unit, a relatively sand-rich succession of fine- to medium-grained sandstone and mudstone. Sedimentary structures, ichnofacies assemblages, and stratal geometries indicate the unit was deposited in an estuarine to shallow-marine environment with strong tidal influences. Reservoir-scale sequence stratigraphy of the Lower Bahariya is complex due to the presence of multiple erosion/onlap surfaces (marked by carbonate cementation and Glossifungites trace-fossil assemblages), abrupt lateral facies changes, and a complex diagenetic overprint.

The Upper Bahariya varies from about 130 to 300 ft thick and consists mostly of thin (2-6 m), sand-poor parasequences that can be correlated regionally. Parasequence boundaries are typically cemented, burrowed, and overlain with mudstones. Sedimentary structures, ichnofacies assemblages, and stratal geometries indicate deposition in mainly low-energy, restricted marine settings. Very-fine grained sandstones occur mainly in the upper 75 feet, mostly as thin bayhead distributary-mouth bar and shoreface deposits, but occasionally as isolated channel-fills. Upper Bahariya sandstones thicken into sand-rich deltaic deposits up to 150 feet thick in the southwestern Shushan Basin.