Diagenetic impact on unconventional tight carbonate reservoir of AR-F Member from the Abu Gharadig Field, north Western Desert, Egypt: insights from integrated combined sedimentological, petrographic, and petrophysical dataset

Abstract

Upper Cretaceous organic-rich carbonate rocks of the Abu Roash F (AR-F) Member represent both source rock and reservoir units and are considered as an unconventional reservoir rock at the Abu Gharadig Field (AGF), Western Desert, Egypt. This study aims to investigate the diagenetic history and its impact on the porosity and permeability of the AR-F unconventional reservoir rock using the sedimentological, petrographic, and petrophysical results for 24 core samples of the AR-F carbonate Member from well AG-121X, located in AGF, Western Desert, Egypt. The petrographic results show that the AR-F Member is composed mainly of mudstone, wackestone/packstone, and packstone microfacies. These microfacies and the richness of such marine assemblages, including planktic forams, calcispheres, pelagic bivalves, echinoderms, with rare pelecypods and ostracods suggest that the AR-F carbonate facies were deposited in mainly deep sea to deep shelf and slope marine settings. The studied AR-F carbonate facies have relatively low total porosity in the range of 0.95–2.33%, which includes mainly grain-based (intergranular and intragranular) and non-grain-based (fenestral, fracture, and dissolution) pores. The core laboratory measurements also show that the AR-F carbonate facies is impermeable rock, with values of up to 0.0039 mD. Therefore, the existence of different types of porosities and permeabilities pointed out that the reservoir quality of the AR-F carbonate facies at the AGF ranges from poor to very poor quality. This could be attributed to different stages of diagenesis, including eogenesis and mesogenesis. These deformation stages, on one hand, contributed to reducing and destroying the reservoir porosity and permeability by a series of processes like micritization, cementation, mechanical and chemical compaction, and less pronouncing pyritization and kaolinitic precipitation. On the other hand, dissolution and fracturing enhanced, in a limited way, the porosity. According to the findings of this study, the AR-F carbonate succession is suitable for further exploitation as an unconventional tight resource of oil reservoir potential.