Petroleum source rock potential and geochemical characteristics of the Upper Cretaceous Shiranish Formation in the Western part of the Zagros Fold Belt, Kurdistan Region of Iraq

Abstract This study provides the first comprehensive organic geochemical evaluation of the Upper Cretaceous Shiranish Formation as a moderate potential source rock in the WZFTB, northern Iraq. A total of 102 cutting rock samples from five wells were analyzed to assess hydrocarbon generation potential, organic matter type, thermal maturity, and depositional environment. Multi-proxy geochemical approach combining Rock-Eval pyrolysis, vitrinite reflectance, GC-MS biomarker analysis, and organic petrography were performed. Total organic carbon (TOC) values range from 0.34 to 2.08 wt%, indicating poor to good organic richness, with an average of 0.72 wt% falling within the low to moderate, based on genetic potential fair category. Hydrocarbon generation potential is considered low to moderate, based on genetic potential (GP) (0.51–11.09 mg HC/g rock) and extractable organic matter (EOM) (48.06 and 445.48 mg HC/g TOC). Kerogen is dominated by Type II/III, supported by HI values (85 to 625 mg HC/g TOC) and liptinite - rich macerals. T max (420–443 °C), and vitrinite reflectance (0.60–0.74% VRr) indicate early to peak thermal maturity. Reworked vitrinite with higher reflectance (0.80–1.10% VRr) suggest allochthonous organic matter input. This interpretation is reinforced by bimodal S2 pyrograms, which demonstrate heterogeneity in organic matter. Furthermore, biomarker maturity proxies (including CPI, Ts/(Ts + Tm), C₃₁ 22 S/(22 S + 22R), C₂₉ 20 S/(20 S + 20R), and ββ/(ββ + αα)) consistently confirm early to peak oil generation conditions. Framboidal pyrite, sulfur enrichment, and biomarker distributions, point to suboxic to anoxic marine carbonate deposition, with episodic terrestrial influence. Spatial variability is notable, with wells CH-3 and TT-22 showing higher source potential. Although the Shiranish Formation exhibits fair TOC content and low to moderate hydrocarbon generation potential compared to major regional source rocks such as the Sargelu and Chia Gara formations, it is classified herein as a secondary source rock. Notably, localized fracturing within its carbonate facies may enhance its capacity as a self-sourcing reservoir. These results advance understanding of petroleum systems in northern Iraq and highlight the value of integrated multi-proxy geochemistry for assessing complex carbonate source units.