Geochemical and Petrographic Characteristics of Lithofacies from Upper Cretaceous Organic-Rich Source Rocks, Jordan

Abstract

Abstract The Upper Cretaceous source rocks of Jordan are organic-rich carbonate mudrocks that represent excellent source rocks but are essentially 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 aims to examine the lithological and geochemical characteristics of these high-quality source rocks. The study utilizes an integrated petrographic and geochemical approach to define the different microfacies in the studied source rock sequence and understand their intrinsic variability. Representative subsurface samples were collected from 13 vertical cores drilled in the Upper Cretaceous interval across Jordan for bulk elemental geochemistry and microfacies analysis. To define the different microfacies and examine the mineralogical and geochemical changes, the samples were investigated for their composition, texture and lithofacies (using both optical microscopy and scanning electron microscopy (SEM/EDS)), mineralogy (XRD/XRF), and geochemistry (total organic carbon, (TOC) and total Sulphur content using a LECO elemental analyzer, and maturity using Rock-Eval). Five laminated and non-laminated carbonate and siliceous lithofacies associations were identified, all containing significant organic content (TOC varies from 7 to 29 wt.%) and Sulphur contents (2-7.5 wt.%). The rocks are of type II-S kerogen with high HI (Hydrogen Index) values of >600 mg HC/g TOC. XRD results show that the rocks are mainly composed of calcite (15-94%, 75% on average) and quartz (0-53%, 10.47% on average). Minor amounts of apatite (average 5.9%, range 0 to 27%), clay (< 7%), pyrite (up to 6%), gypsum (up to 4%) and dolomite (up to 2.7%) are also exist. Our results indicate a predominance of the organic-rich calcareous microfacies association, evidenced by the abundant foraminifera grains and the high preservation of organic matter as disseminated throughout the matrix and/or filling interparticle micropores and microfractures. This reflects the deposition in a restricted basin under anoxic conditions. 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, organic-rich Upper Cretaceous source rocks. This, in turn, helped to achieve a better evaluation of the occurrence and distribution of the different litho- and organic facies and estimation of hydrocarbon source potential.