How Burrows Shape Carbonate Reservoir Quality: Selective Dolomitization in the Lower Marrat Formation

Summary This study investigates the role of bioturbation in controlling early dolomitization within the Lower Marrat Formation (Toarcian), central Saudi Arabia. Integrated ichnological, petrographic, mineralogical, and geochemical analyses of outcrop samples reveal that dolomitization is preferentially confined to burrow fills and associated halos, contrasting sharply with the predominantly calcitic host matrix. Elemental and isotopic data indicate that burrow networks acted as high-permeability conduits that focused Mg-rich, slightly evaporative, low-temperature fluids, driving early dolomite precipitation. Microbially mediated processes were present but played a secondary role. These results demonstrate how trace-fossil architecture strongly influences fluid flow, dolomite distribution, and reservoir heterogeneity in mud-dominated carbonate systems.