Controls on hydrocarbon accumulation in the pre-rift paleozoic and late syn-rift cretaceous sandstones in PTAH oil field, north Western Desert, Egypt: Insights from seismic stratigraphy, petrophysical rock-typing and organic geochemistry

Abstract

North Western Desert (NWD) of Egypt is one of the most petroliferous provinces in North Africa. Mesozoic rift basins in the NWD host several active petroleum systems in their syn- and post-rift sedimentary successions. Also, numerous oil reserves were recently discovered in the pre-rift Paleozoic reservoirs. However, the genetic link between the pre-rift hydrocarbon plays and the syn- and post-rift accumulations is still ambiguous. Additionally, controls on distribution of the optimum rock types in the pre-rift reservoirs need further investigation. Therefore, this study integrates geochemical, seismic-stratigraphic, and petrophysical approaches to better understand these issues based on oil, seismic, and well data retrieved from the Paleozoic pre-rift “Shiffah” and the Cretaceous late syn-rift “Alam El Bueib (AEB)” sandstones in PTAH oil Field in the NWD.Oil retrieved from Shiffah and AEB pay zones displays a similar n-alkane composition dominated by cycloalkanes and nC20+ alkanes. The oil samples retain biomarker signatures of pre-Cretaceous terrestrial organofacies where oleanane is absent, isoprenoid ratio (prestane/phystane) is high (>5) and C29/C27 steranes is greater than 1.5. Shiffah and AEB oils have a common source rock and display compositional characteristics similar to NWD oils derived from the Middle Jurassic Khatatba Formation fluvio-deltaic organofacies. Deep-seated rift-faults cross-cutting the entire syn- and pre-rift successions provide the paths for the upward and downward hydrocarbon migration from Khatatba organofacies to AEB and Shiffah sandstones, respectively. Distribution of the reservoir rock types (RRTs) controlled the thickness of the hydrocarbon column. Reservoir quality decreases from RRTI to RRTIII. RRTI rocks occur in the Shiffah fluvial basin-fill facies, whereas the best RRTs in AEB are classified as RRTII and are exclusively associated with the prograding deltaic facies. Correlation between seismic facies and RRT allows the prediction of their lateral and vertical distribution. Our results highlight existence of excellent reservoir rock types in the pre-rift succession connected with the Jurassic kitchen which charged the entire syn- and post-rift reservoirs.