Source rock evaluation of the Upper Cretaceous black shales in the Abu Tartur Plateau: implications for untapped hydrocarbon resources in south Western Desert, Egypt

Upper Cretaceous black shales of Egypt’s southern Western Desert are widely recognized but remain poorly constrained as unconventional hydrocarbon targets. Here, we integrate Rock-Eval pyrolysis, TOC, total sulfur, and major- and trace-element geochemistry to assess the Campanian–Maastrichtian Duwi and Dakhla formations at the Abu Tartur Plateau using paired borehole and outcrop samples. The Duwi Formation shows a sharp preservation gradient: unweathered borehole intervals retain fair to good source-rock potential, with TOC of 0.57–2.07 wt% and S2 up to 4.52 mg HC/g rock, whereas outcrop equivalents are severely depleted by meteoric oxidation, with TOC mostly <0.5 wt% and S2 <0.2 mg HC/g rock. High chemical weathering indices and pyrolyzable carbon data indicate that the Type III–IV kerogen signature of the Duwi facies represents primary refractory terrestrial organic matter preserved under oxic conditions, coupled with severe oxidative degradation of the marine organic fractions. The Dakhla Shale is organically lean in the studied outcrops, with TOC <0.5 wt% and unreliable Tmax values caused by near-complete loss of labile organic matter and mineral-matrix effects. Paleoredox proxies, including V/Cr <2, V/(V + Cr) <0.6, and Ni/Co <5, indicate predominantly oxic to suboxic deposition rather than persistent anoxia. Elevated sulfur contents in the Duwi Formation are therefore attributed to localized early diagenetic sulfate reduction, while anomalous Cr enrichment mainly records detrital input from Cr-bearing clay-rich or ferromagnesian sources. These results demonstrate that post-depositional oxidation exerts a first-order control on the apparent organic quality, kerogen type, and maturity signatures of Abu Tartur black shales. Weathered outcrops substantially underestimate original source-rock potential; consequently, only minimally altered subsurface Duwi intervals currently represent credible targets for future unconventional resource assessment in the southern Western Desert.