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

摘要

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 Espirito Santo Basin, eastern Brazil. Kaolinization (δ~(18)O = +13.3‰ to +15.2‰; 5 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 δ~(18)O= -7.2‰; δ~(13)C = +9.3%J and pyrite. Progressive sediment burial (present depths = 1530-2027 m) resulted in the formation of poikilotopic calcite, ferroan dolomite-ankerite (average δ~(18)O= -7.9‰; δ~(13)C = +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 δ~(13)C with decreasing δ~(18)O and increasing temperature. From a composition closer to marine (≈0‰), one trend goes towards positive δ~(13)C 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 δ~(13)C 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.