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

This study investigated the geochemical characteristics of the Upper Cretaceous source rocks in northern Jordan, including the quantity and quality of organic matter, the depositional environment and the thermal maturity. Several methods were applied, encompassing maceral examination and multi-geochemical analyses such as total organic carbon (TOC), Rock-Eval pyrolysis, pyrolysis–gas chromatography (PY-GC), liquid and gas chromatography (GC) and 1D basin modelling. Results demonstrated that the Cenomanian Naur-Shueib strata are non-source rocks, while the Turonian–Maastrichtian strata (Wadi Essir–Ghareb formations) are mainly good to excellent source rocks. The hydrogen index (HI) results suggest that the Cenomanian strata have mainly mixed kerogen Type II-III, and minor kerogen Type III, whereas the Turonian–Maastrichtian source rocks are dominantely contributed by kerogen Types II and I. These findings match the maceral examinations, showing a dominance of vitrinite macerals in the Cenomanian beds, and amorphous organic matter (AOM) in the Turonian–Maastrichtian strata. The PY-GC and gross composition suggest 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, along with the low gas/oil ratios (GORs), suggest that the Upper Cretaceous formations predominantly comprise oil-prone kerogen derived from marine organic matter (algae/bacteria) and deposited under strongly reducing conditions. Comprehensive 1D basin modelling, combined with thermal maturity indicators (vitrinite reflectance ( R o ), T max , the production index (PI) and the gross chemical composition), indicate that the Cenomanian formation is immature to marginally mature, whereas the Turonian–Maastrichtian source rocks are completely immature. This study and compiled published data demonstrated that the Turonian–Maastrichtian strata are immature organic-rich source rocks, possibly prospective as unconventional hydrocarbon resources in northern Jordan, particularly the Maastrichtian formation.