Meteoric-water diagenesis in late Cretaceous canyon-fill turbidite reservoirs from the Espيrito Santo Basin, eastern Brazil

Parameters controlling the diagenetic evolution of passive margin, marine turbidites, which are important targets of hydrocarbon exploration, are poorly constrained in the literature. This study aims to unravel the conditions of diagenesis and its impact on the reservoir quality evolution from late Cretaceous canyon-filling turbiditic sandstones of the onshore portion of Espírito Santo Basin, eastern Brazil. Kaolinization (δ18O = +13.3‰ to +15.2‰; δ D = −96.6‰ to −79.6‰) and dissolution of framework silicate grains is attributed to meteoric water incursion during eodiagenesis in response to a considerable fall in relative sea-level. Eogenetic alterations also include cementation by siderite (average δ18O = −7.2‰; δ13C = +9.3‰) and pyrite. Progressive sediment burial (present depths = 1530–2027 m) resulted in the formation of poikilotopic calcite, ferroan dolomite-ankerite (average δ18O = −7.9‰; δ13C = +2.9‰), minor amounts of quartz overgrowths and in partial dickitization of kaolinite. Isotopic values of calcite and dolomite-ankerite follow two trends of co-variance of δ13C with decreasing δ18O and increasing temperature. From a composition closer to marine (≈0‰), one trend goes towards positive δ13C values (up to +22.4‰ for calcite; +18.6‰ for dolomite-ankerite), indicating increasing input of carbonate from methanogenic fermentation. The other trend develops towards negative δ13C values (down to −17.2‰ for calcite; 15‰ for dolomite–ankerite), suggesting increasing contribution from thermal decarboxylation with increasing temperature and depth. Despite the presence of various cement types, mechanical compaction was more important than cementation in reducing depositional porosity in the onshore Urucutuca sandstones.