Reservoir rock typing and origin of oil in the Cretaceous post-rift mixed siliciclastic-carbonate of Abu Roash Formation, Badr El-Din 2 field, north Western Desert, Egypt

Abstract

Mixed siliciclastic-carbonate depositional sequences represent highly prospective targets for hydrocarbon exploration. These sequences commonly host reservoir facies characterized by both intergranular and secondary porosity, and they may be actively charged by intra-formational source rock organofacies. In the coastal rift basins of the Western Desert, the Upper Cretaceous post-rift succession of the Abu Roash Formation was deposited in environments ranging from shallow to deep marine settings. However, the existence of an intra-Abu Roash petroleum system has not been well-documented to date. This study integrates organic geochemical data with detailed petrophysical analysis to elucidate the origin of the hydrocarbons and to characterize the reservoir rock types (RRTs) within the mixed siliciclastic-carbonate facies of the Abu Roash Formation in wells from the BED-2 Field. Geochemical data reveal that the Abu Roash oil was sourced from a mixed Type II/III kerogen, with a greater contribution from marine organic matter. The oil exhibits a regular sterane distribution pattern of C₂₉ > C₂₈ > C₂₇, indicative of derivation from a mixed marine and terrestrial source rock. Moreover, the Abu Roash oil shows a clear negative correlation with Jurassic and Cretaceous oil families from adjacent basins but demonstrates a strong geochemical affinity with bitumen extracts from Abu Roash mudstones.

Depositional facies analysis indicates that the Abu Roash mixed siliciclastic-carbonate facies were deposited in a shallow marine setting, characterized by alternating tidal channel sandstones, lagoonal carbonates, upper shoreface, tidal inlet, and tidal flat facies, capped by lower shoreface to offshore mudstones. Petrophysical analysis identifies five distinct RRTs, with the highest reservoir quality (RRT1) associated with tidal channel sandstones. In contrast, the lagoonal carbonates exhibit highly heterogeneous reservoir properties, acting as micro-conduits and baffles to fluid flow. Notably, approximately 60 % of the reservoir's fluid flow capacity is attributed to the tidal channel facies of RRT1, while lower shoreface to offshore mudstones serve as effective sealing rocks. The findings of this study confirm the presence of an intra-Abu Roash petroleum system in the study area, thereby opening new opportunities for hydrocarbon exploration across the broader north Western Desert province. Moreover, the results of this study underscore the broader exploration significance of mixed carbonate–siliciclastic depositional systems globally hosting high-quality reservoir flow units, source rock organofacies, and effective seal rocks.