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 quantity and quality of organic matter, depositional environment and thermal maturity. Several methods were applied, encompassing visual maceral examination and multi-geochemical analyses like TOC, Rock-Eval pyrolysis, pyrolysis-gas chromatography (PY-GC), liquid and gas chromatography (GC), and 1D basin modeling. Results demonstrated that the Cenomanian Naur-Shueib strata are non-source rock, while the Turonian-Maastrichtian strata (Wadi Essir to Ghareb fms) are mainly good-excellent source rocks. The hydrogen index results suggest the Cenomanian strata as having major contribution from mixed kerogen type II-III and minor contributions from kerogen type III, whereas the Turonian-Maastrichtian source rocks are dominantly contributed by the kerogen types II and I. These findings match the maceral examinations, showing dominance of vitrinite macerals in the Cenomanian beds, and amorphous organic matter (AOM) in the Turonian to Maastrichtian strata. The PY-GC and gross composition supposed that the Upper Cretaceous strata may have produced paraffinic oil with a high content of wax, 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 -C 17 , and Ph/ n -C 18 ratios alongside the low GORs suggest the Upper Cretaceous formations as 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 modeling reveals that the Cenomanian Fm is immature to marginally mature, whereas the Turonian-Maastrichtian source rocks are completely immature. This study and compiled published data expressed the Turonian-Maastrichtian strata as immature organic-rich source rocks, possibly prospective as unconventional hydrocarbon resources in northern Jordan, particularly the Maastrichtian Fm.