Facies analysis, depositional architecture, and sequence stratigraphy of the upper Abu Roash “G” Member (Late Cenomanian), Sitra Field, Western Desert, Egypt

Abstract

The Abu Roash “G” Member constitutes the most important hydrocarbon reservoir–producing interval in the Sitra Field, Western Desert, Egypt. A multidisciplinary approach was applied to reconstruct a depositional model and stratigraphic architecture of the upper unit of the Abu Roash “G” Member from the Sitra field integrating core analysis and petrography. Petrographically, the studied sandstones are classified as quartz arenite and subarkose types. Sedimentological and ichnological analysis indicates that deposition took place in a tide-dominated embayment oriented northwards to the sea. Within this depositional model, ten facies and five facies associations are distinguished. The channelized cross-bedded sandstone facies (FA1a) reflects a tide-dominated channel environment. The bioturbated marine sandstone, with shell debris (FA2a) and bioturbated, massive to low-angle planar laminated sandstone facies (FA2b), characterizes the tidal-bar environment. The bioturbated sandstone facies (FA3a) and carbonaceous shale and laminated mudstone facies (FA3b) suggest accumulation in a lagoon/bay-fill environment. The wave and current-rippled sandstone facies (FA4a), flaser-bedded heterolithics (FA4b), and bioturbated muddy heterolithics (FA4c) indicate deposition in a tidal-flat environment. The sandstone with plant debris facies (FA5a) and carbonaceous mudstone and coal seams (FA5b) indicate a coastal plain/swamp environment. The upper Abu Roash “G” Member forms an overall transgressive succession of stacked transgressive-regressive cycles bounded by regional erosional surfaces, mostly produced by tidal ravinement. It consists of four high-frequency (fourth-order) depositional sequences bounded by regional erosion surfaces. These sequences are dominated by transgressive (TST) and highstand normal regressive (HST) systems tracts stacked into an overall retrogradational stacking pattern. This stacking pattern is interpreted as being the result of complex interaction between passive thermal subsidence and relative sea-level changes.