Geochemical characteristics of the organic-rich Upper Cretaceous carbonates in northern Jordan: implications for source rock evaluation and depositional settings

The Upper Cretaceous strata of Jordan possessed several organic-rich intervals. This study investigated the geochemical characteristics of these Upper Cretaceous source rocks in northern Jordan, including the quantity and quality of organic matter, depositional environment and thermal maturity. Several methods were applied, encompassing visual maceral examination and multi-geochemical analyses such as TOC, Rock-Eval pyrolysis, pyrolysis-gas chromatography (PY-GC), liquid and gas chromatography and 1D basin modelling. The results demonstrated that the Cenomanian Naur-Shueib strata are non-source rocks, while the Turonian–Maastrichtian strata (Wadi Essir to Ghareb formations) are mainly good–excellent source rocks. The hydrogen index results suggest that the Cenomanian strata have major contributions from mixed Type II and Type III kerogen and minor contributions from Type III kerogen, whereas the Turonian–Maastrichtian source rocks dominantly contain Type II and Type I kerogen. These findings match the maceral examinations, showing the dominance of vitrinite macerals in the Cenomanian beds, and amorphous organic matter in the Turonian–Maastrichtian strata. The PY-GC and gross composition analyses indicate that the Upper Cretaceous strata may have produced paraffinic oil with a high wax content, and displayed a predominance of naphthenic oils, with a less significant presence of aromatic naphthenic oils and aromatic asphaltic oils. The Pr/Ph, Pr/n-C17 and Ph/n-C18 ratios, alongside the low GORs, suggest that the Upper Cretaceous formations were mainly contributed by marine organic matter, indicating oil-prone kerogen deposited under strongly reducing conditions. Integrating VRo%, Tmax, PI, the gross chemical composition and 1D basin modelling reveals that the Cenomanian formation is immature to marginally mature, whereas the Turonian–Maastrichtian source rocks are completely immature. This study and compiled published data determine the Turonian–Maastrichtian strata to be immature organic-rich source rocks that may be prospective as unconventional hydrocarbon resources in northern Jordan, particularly the Maastrichtian formation.