THE IMPACT OF EARLY DIAGENESIS ON THE RESERVOIR QUALITY OF PRE-SALT (APTIAN) SANDSTONES IN THE ESPIRITO SANTO BASIN, EASTERN BRAZIL

Fluvial and estuarine sandstones of the Aptian Mucuri Member of the Mariricu Formation are the main reservoir rocks in the onshore portion of the Espirito Santo Basin, Brazilian eastern margin. These sandstones are contemporaneous with the major lacustrine pre-salt reservoir rocks which are currently the target of exploration offshore. The sandstones are texturally and compositionally immature, rich in feldspars, biotite, garnets and other heavy minerals, and are interpreted to have been derived from the erosion of igneous and medium-high grade metamorphic-plutonic terrains under dry climatic conditions. Early diagenetic processes have significantly impacted the reservoir qualities of the sandstones. Detrital feldspars were dissolved and replaced by kaolinite. Biotite was replaced and expanded by pyrite, calcite, kaolinite and/or smectite. The growth of authigenic rims and coatings of smectitic clays was the most important diagenetic process in most of the sandstones, except for those cemented by abundant poikilotopic calcite. Coarse-crystalline pyrite replaced primary constituents, particularly biotite and mudstone intraclasts, and all other diagenetic constituents. Mechanical compaction, although relatively limited, was the main process of porosity reduction, except in sandstones with abundant calcite cement. Primary intergranular porosity was commonly strongly reduced by calcite or smectite cementation and by compaction, but intragranular and mouldic secondary pores were generated by the dissolution of detrital feldspars and heavy minerals. Characterizing the diagenetic patterns of the Mucuri sandstones is important for increasing the recovery from producing oilfields and also for the development of analogous reservoir rocks elsewhere.