Santonian–Paleocene marls and fine grained carbonates have been sampled in Hasbayya locality, south Lebanon, in order to evaluate their organic matter (OM) content, petroleum source rock potential, and assess their depositional environment. Methods included total organic carbon (TOC), total inorganic carbon (TIC), total sulphur (TS), Rock-Eval pyrolysis, organic and inorganic petrography, X-ray fluorescence (XRF), gas chromatography–mass spectrometry (GC–MS) and Curie-point-pyrolysis-gas chromatography–mass spectrometry (CP-Py-GC–MS). TOC, Rock-Eval, and vitrinite reflectance (VRr) results reveal excellent immature petroleum source rocks within the Campanian–lower Maastrichtian interval with TOC up to 11.6 wt%, hydrogen index (HI) up to 872 mg/gTOC, Tmax up to 433 °C and VRr average of 0.36%. Biomarker ratios and maceral analysis suggest a marine depositional environment with a dominance of algal as well as submicroscopic OM. Original sediment composition and redox sensitive geochemical parameters suggest deposition of the OM rich intervals under an oxygen minimum zone (OMZ) that was emplaced and controlled by primary productivity and nutrient supply. Pyrolysate composition shows an important content of organic sulphur compounds (thiophenes) increasing with TOC and thus indicating the presence of Type II and Type IIS kerogen in the analysed sample set, which is consistent with the presence of immature solid bitumen in the Hasbayya region. The data produced in this study, coupled with regional correlations, allow us to construct a conceptual depositional model for the Upper Cretaceous OM rich rocks of the eastern Mediterranean suggesting deposition under a productivity belt localized along the inner and outer shelf leading to a decrease in source rock quality and a shift in kerogen type toward the deeper parts of the Levant Basin.