LATE ALBIAN – EARLY CENOMANIAN BASIN EVOLUTION USING HIGH RESOLUTION SEDIMENTARY FACIES PREDICTION OF ABU GHARADIG PERICRATONIC BASIN OF THE NORTH WESTERN DESERT, EGYPT AND ITS HYDROCARBON HABITAT

Abstract

The Upper Cretaceous sequences of the north Western Desert of Egypt form the main hydrocarbonbearing sequences of the mature petroleum system in the pericratonic Abu Gharadig basin. One of the main targets is the sandstone-dominated Bahariya Formation of Late Albian-Early Cenomanian age. It overlies unconformably the Middle to Lower Albian fluvial-dominated sandstones of the Kharita Formation. The boundary between these two rock units (of different facies types) is actually difficult to be detected, using the limited conventional wireline logs. Certainly, the application of the high-resolution analyses; palyno-biostratigraphic, formation micro-images lithofacies extraction and core data, in addition to the different conventional wireline logs, supported the ability to differentiate and predict the important and lithologically obscured boundary between the Bahariya and the Kharita formations. The Bahariya Formation is distinguished into five depositional sequences (1 to 5), from bottom to top, in the selected four wells. The depositional sequence boundaries were identified lithologically by a basal mudstone facies (sequences 1, 3, 4 & 5), with the exception of a pronounced lime-muddy facies at the base of sequence no. 2. The lowermost sequence represents mixed flat facies, followed upwardly by sandstone-dominated tidal channels, accompanying the global sea level rise, and ended with a shallow marine carbonate bed. The second sequence starts at the base with sand-dominated mixed flat deposits denoting the early lowstand systems tracts of the next sea level rise, followed by shelfal mudstone facies. A non-depositional gap that supported by the presence of a hard ground terminated this sequence. The third cycle starts with the flooding event represented by mudstone and shaly facies, followed upward by a continuous sea level rise building vertically stacked shoreface facies and forming an offshore sandbar. The fourth sequence reflects relatively a sea-level fall and composed of a mixed flat, that intersected by tidal channels and creeks, and topped by shallow subtidal sediments. The fifth sequence begins with a repeated sandy mixed flat that affected by storm events resulted in the deposition of tempestites and lower shoreface facies (trough and hummocky cross stratifications, in addition to glauconitic sandstone facies) and ended up with the shallow subtidal carbonate facies at the base of Abu Roash Formation (Upper Cenomanian). The development of shoreface sandy facies of the sequences three and five represents the maximum rate of sea level rise, developing the offshore sand bar system. These bars, due to sea level rise events, represent the high-quality sandstone reservoirs, in addition to the tidal channels, that may represent a moderate to high-quality reservoir.