Organic and Inorganic Geochemical Heterogeneity and Cyclicity in Organic-Rich Carbonate Source Rocks

Abstract Organic-rich source rocks play a vital role in hydrocarbon exploration and serve as valuable records of past environmental conditions. This study investigates the Upper Cretaceous (Maastrichtian) Type II-S marine source rocks of Jordan (JSR) to examine organic and inorganic compositional variability corresponding to paleoenvironmental conditions during deposition. The identification of distinct geochemical cycles in these source rocks contributes to a deeper understanding of the depositional settings and paleoenvironmental dynamics influencing organic matter distribution. The study utilized a comprehensive analytical approach, including Spectral Gamma Ray (SGR), RockEval pyrolysis, XRD, XRF, ICP, SEM-EDX, and petrographic examination. These techniques were employed to evaluate the organic and inorganic geochemical composition and distribution of organic matter at both micro and nanoscale levels. A research well drilled in the Al-Lajjun Basin of central Jordan provided a pristine JSR core interval for the detailed geochemical analysis. The characterization of the samples was carried out to understand the lithologic and geochemical variation within the rock formation and correlate it with paleoenvironmental changes. The JSR is an organic-rich, type IIS, carbonate-dominated source rock interval, deposited during a significant nutrient upwelling event in the Cretaceous time period. It consists of three distinct 3rd-order geochemical cycles that represent geochemical variability driven by paleoclimatic changes during deposition. Cycle 1 features organic-rich mudstone facies with high total organic carbon (TOC), high carbonates, and low silicate mineral composition. The organic matter, found in fossil chambers and the matrix, suggests high productivity with periodic euxinic bottom water conditions and minimal detrital input, aiding in excellent preservation. Cycle 2 transitions to silica-rich mudstone to wackestone facies, marked by increased biogenic silica content and high TOC. Enhanced nutrient upwelling during this time led to higher organic matter productivity and euxinic bottom water conditions, leading to significant organic matter preservation. Cycle 3, comprising foraminifera-rich wackestone to packstone facies, shows a mix of detrital and biogenic input, with organic matter mainly in the matrix. Despite lower productivity and preservation than the earlier cycles, conditions rem ained favorable for organic matter accumulation. The intercalation of these cycles indicates the changing environmental and ocean current dynamics at the southern margin of the Tethys Ocean, resulting in mineralogical variations, bottom water redox conditions, and differential dilution, affecting the source rock quality and organic matter preservation in these cycles. The identified geochemical cycles emphasize the dynamic paleoenvironmental conditions influencing both organic and inorganic geochemical composition within the JSR. Understanding this geochemical heterogeneity is essential for comprehending the depositional architecture and variable paleoenvironmental conditions during source rock deposition. These findings provide a strong foundation for lateral correlation and the characterization of geochemical heterogeneity and cyclicity on a regional scale.