Diagenetic history of the Lower Jurassic Middle Member of the Marrat Formation in the Magwa Field, part of Kuwait's Greater Burgan Field

This study evaluates the diagenetic characteristics of the Middle Marrat reservoir, a partially dolomitized carbonate with thin anhydrite layers and clay (Magwa Field, Kuwait). Six cored wells were described and facies identified. By integrating the new petrographical data with geochemical analyses (stable and clumped isotopes), X-ray diffraction analysis and 1D basin modelling, a diagenetic model was constructed showing the phases and timing of diagenesis and processes affecting reservoir quality. Our analyses suggest that it underwent early marine, meteoric diagenesis and dissolution, developing secondary porosity. Burial compaction was accompanied by calcite cementation, dolomitization and multiple phases of fracturing. Later dolomitizations included fracture-related Fe-rich dolomite precipitation. The late-stage Fe-rich calcite and dolomites precipitated from deeper-sourced hydrothermal fluids. Carbonate clumped isotope (CI) temperatures from matrix samples and calcite (vein) cements range from 45 to 130°C, which we interpret to have precipitated during three diagenetic phases. The first phase represents diagenesis during depositional and shallow burial ( c . 40–60°C), which is mostly seen in the micritic matrix samples. The second phase is related to compaction and moderate burial (80–100°C), mostly recorded by mesogenetic dolomites and calcite-filling cements. The third phase, seen in calcite vein cements, is associated with deeper burial and upward migration of hydrothermal fluids along deep faults ( c . 100–130°C). Integrating CI data with basin modelling suggests that the main diagenetic phases occurred during the Lower Jurassic–mid-Upper Jurassic (phase 1), Lower Cretaceous (phase 2) and post-Cretaceous (phase 3).