Palynostratigraphy and paleoecology of a Lower Cretaceous (upper Barremian-lower Aptian) succession in the Shushan Basin, northwestern Egypt: inferences on intra- and inter-basinal correlations

摘要

Recent national and international demand for additional resources of the fossil fuel resulted in extensive hydrocarbon exploration activities in Egypt to further explore possible petroliferous rock units that were conventionally deemed as barren or having non-commercial reserves such as the Alam El Bueib Formation. This increased the stress to enrich the lithological correlation of this formation across the north Western Desert to better understand its regional lithological characteristics and thus its potential regional reservoir/source rock quality. Palynostratigraphic analysis of Lower Cretaceous successions in the Tut-1X and Amoun-1X wells in the Shushan Basin, north Western Desert enabled the recognition of the upper Alam El Bueib Formation in both wells, in contrast to a previous study, which suggested non-deposition of this formation in the Amoun-1X well. This highlights the important role of the palynology as a key stratigraphic tool, which helped effectively in recognising this problematic monotone elastic formation, where other marine microfossils are lacking. This also shows that recognition of this formation based on lithological criteria only was impractical and misleading. Marker terrestrial and marine palynomorph taxa recovered from the upper Alam El Bueib Formation enabled dating it as late Barremian-early Aptian and were used to identify two palynological biozones that can be traced out regionally across the Shushan, Matruh, and other nearby basins. These biozones are the early Aptian Hurospora florida-Afropollis operculatus-Pseudoceratium securigerum Assemblage Zone and the late Barremian Stellatopollis barghoornii-Pseudoceratium anaphrissum-Cyclonephelium vannophorum Assemblage Zone. An undifferentiated interval was recorded just below the upper Barremian strata in the Tut-1X well due to very low palynomorph recovery and absence of marker forms. Composition of terrestrial and marine palynomorph assemblages suggests the upper Alam El Bueib Formation was deposited in fluvio-deltaic to proximal inner shelf settings. The Alam El Bueib Formation shows notable regional variations in thickness and considerable lateral and vertical changes in lithology due to local tectonics and relative difference in its depositional settings in the Shushan and Matruh basins. Regionally, the Alam El Bueib Formation shows dual reservoir/source rock characteristics, where thick and poorly fossiliferous sandstone units act as reservoir rocks, while moderately fossiliferous finer elastics and carbonates already acted as intraformational source rock intervals in some parts of the Shushan and Matruh basins.