An integrated petrographical, petrophysical and organic geochemical characterization of the Lower Turonian Abu Roash-F carbonates, Abu Gharadig field, Egypt – Inferences on self-sourced unconventional reservoir potential

Abstract

Marine carbonate source rocks are ubiquitous worldwide and many of these can act as self-sourced unconventional reservoirs. This work focuses on the Lower Turonian Abu Roash-F Member from the prolific Abu Gharadig field, Western Desert, Egypt which consists of argillaceous and bioclastic limestones. Thin section petrographic analysis indicated the presence of various microfacies assemblages dominated by planktonic foraminifera in association with abundant calcispheres indicating an open-marine, deep-shelf depositional setting. The X-Ray Fluorescence data exhibits overall poor detrital influx throughout the AR-F while the Mo and V enrichment in the lower part represents Late Cenomanian/Early Turonian (C/T) oceanic anoxic event (OAE2) associated with the global sea-level rise. Geochemical analysis suggests that the studied AR-F has good to very good total organic carbon (TOC) content (up to 3.57 wt%) consists of Type-II kerogen (HI > 400 mgHC/gTOC), thermally mature (Tmax ∼ 438.5 °C) and presently within the oil generation window. The TOC-rich (>2.5 wt%), highly anoxic lower AR-F showed higher oil production probability which is inferred as a sweet spot zone. Petrophysically AR-F is very tight and impervious with 0.9–2.6% porosity and negligible permeability (<0.0033 mD), dominated by nano- and microporosities. Micritization is found to be the most dominant diagenetic process reducing the AR-F storage capacity, followed by calcite cementation filling the bioclast tests and fractures, chemical compaction (stylolites), and minor dolomitization. 2D CT scanning revealed some minor vuggy or fracture pores but those were isolated and lacks in connectivity, which therefore did not improve the flow capacity of AR-F. Therefore, stimulation is necessary to bring this tight carbonate reservoir into production. This study offers critical insights on the AR-F Member which is an excellent source rock and at the same time holds potential as a promising unconventional reservoir.