The Upper Cretaceous (Maastrichtian) Belqa Group in Jordan consists predominantly of organic-rich carbonate mudrocks that represent excellent source rocks but are still immature to just entering the oil window. Therefore, they offer a unique opportunity to study lithology, source rock composition, and geochemistry prior to the onset of maturation and expulsion of hydrocarbons. Our study examines the lithological and geochemical characteristics of these high-quality source rocks. To define the lithofacies and analyze the heterogeneities, 23 samples were investigated for their composition and texture using optical microscopy, SEM/EDS, XRD, and XRF). Geochemistry analyses were based on total organic carbon (TOC), Rock-Eval pyrolysis, and vitrinite reflectance. Four lithofacies were defined in this study: Strongly bioturbated calcareous mudstone (LF1), Slightly laminated calcareous mudstone (LF2), Parallel laminated calcareous mudstone to wackestone (LF3), Fine to coarse silicified wackestone to packstone (LF4), all containing significant organic content (TOC varies from 6 to 20 wt.%) and Sulphur contents (1.3-4.7 wt.%). Kerogen types are predominantly II-S with high HI (Hydrogen Index) values of >700 mg HC/g TOC. XRD results show that the rocks are mainly composed of calcite (75% on average) and quartz (10% on average). Minor amounts of clay (< 7%), pyrite (up to 6%), gypsum (up to 4%), and dolomite (up to 3%) also exist. Our results evidenced the high heterogeneity in the lithofacies of the MCM formation samples. Despite the traditional interpretation of low-energy suspension settling, we demonstrated the presence of bottom currents during the deposition of the Upper Cretaceous source rocks in Jordan (ripples, rip-up mud, reworking material, and fossil-rich lenses) and insight into the bottom water conditions (anoxic and oxic). The integration of the data obtained through the different techniques used in this study, together with the unique characteristics of these source rocks, allowed the characterization of the geological and geochemical properties of the defined lithofacies associations in these immature, Upper Cretaceous source rocks. This, in turn, helped to better evaluate the occurrence and distribution of the different litho- and organic facies and estimate hydrocarbon source potential.