Diagenesis of the Mucuri member sandstones, lower cretaceous in the Espírito Santo and Mucuri basins

Abstract

The diagenetic evolution of the sandy conglomerate sediments belonging to the Mucuri Member was regionally defined in the basins of Espírito Santo and Mucuri from samples collected in 24 wells at depths varying from 500m to 3,000m. Nine hundred meters of core samples were described, and 600 thin sections were prepared to be studied at the petrographic microscope and later at the scanning electron microscope (SEM). Samples were also analyzed using X-ray diffraction in order to determine the clay mineraIs content as well as carbon and oxygen lsotopic determination of the carbonate cements. The Mucuri Member consists of Alagoas and pre-Alagoas age sediments from alluvial-fluvial-deltaic environments, of arid climate, deposited during the rift phase of the Espírito Santo and Mucuri basins. The Aptian portion (Alagoas age) presents intercalations of typical/y marine shales. Thus, the Alagoas age sediments present formation weter with a distinctive composition from the pre-Alagoas age sediments (continental) responsible for different diagenetic changes in both packages. Moreover, the pre-Alagoas sediments had a burial history characterized by a longer residence time at greater depths than the Alagoas package, and thereby being more susceptible to mesodiagenetic processes. Most reservoirs of the Mucuri Member are of the Alagoas age, and because of better sampling a more defailed descripfion is given. These reservoirs were also deeply affected by eodiagenesis, with particular emphasis in calcite precipitation in zones of sulfate reduction and methanegenesis and authigenesis of smectites, which is the main agent responsible for the large amounts of clay mineraIs present in these reservoirs. Several other diagenetic processes were identified in the Mucuri Member being however less efficient in reducing the pore space. The progressive iIIitization of the c/ay mineraIs, the albitization of the feldspar and the precipitation of quartz overgrowths were the mesodiagenetic features found in the deeper reservoirs.