Abstract

Quantitative petrography and stable-isotope and fluid-inclusion analysis revealed that the diagenetic evolution of two Tertiary turbidite sandstone reservoirs from the offshore part of the Espírito Santo Basin, eastern Brazil, was influenced by the flow of meteoric water and salt-dome-related fluids, which had different impacts on their quality. Influx of meteoric water during early burial, in response to relative sea-level fall, resulted in extensive kaolinization and dissolution of framework silicate grains. Mesogenetic compactional fluids were progressively modified by the proximity of salt domes, which led to ubiquitous feldspar albitization and localized quartz, calcite, and saddle-dolomite precipitation. Fluid inclusions in quartz overgrowths indicate that the precipitating fluids had salinities predominantly in the range 8–13 wt% NaCl equivalent and temperatures largely in the 100–155°C range (higher than present-day formation temperatures—105° up to 115°C). The distribution of feldspar albitization suggests that the fracture systems along the margins of the salt domes acted as preferential pathways for such hot brines. However, the influence of the salt domes on the diagenetic processes of the reservoirs was relatively mild, despite their proximity, because pore-filling neoformed illite is absent, and occurrence of quartz cement is limited in the sandstones, which may be related to the late burial of the reservoirs and the intermittent behavior of the salt-dome-associated fracture systems as effective conduits for reactive fluids. We expect that this paper may contribute to the understanding and prediction of diagenesis and reservoir properties of turbidite sandstones influenced by meteoric and salt-dome-related fluids in offshore Espírito Santo Basin and in other similar areas.

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