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