Potential for deep reservoirs in Serraria Formation Sergipe-Alagoas basin northeastern Brazil

Abstract

The paleogeography, provenance, depositional environment and the diagenetic and burial histories of Serraria Formation (Neocomian of Sergipe-Alagoas Basin, Northeastern Brazil) have been analyzed to stablish the principles for prospecting for deep reservoirs. The unit was deposited by braided fluvial streams crossing a wide cratonic sag from the north-northwest before the rupture of Gondwana. Four diagenetic domains were defined, two of which have potential for the development of deep reservoirs. Domain 1, including the offshore Caioba oilfield and adjacent onshore areas, suffered Neocomian subsidence and burial diagenesis, followed by faulting, tilting and uplifting still by the end of Neocomian and beginning of Barremian. Meteoric waters infiltrated through pre-Aptian unconformity and major faults and dissolved most of the carbonate cement and feldspar grams. The abundant secondary porosity was only partially reduced during the subsequent burial. Domain 2, stablished from the study of the deep (4,200 m) sandstones in the offshore Robalo Field, shows porosity preservation by early hydrocarbon saturation related to a very rapid rift subsidence. Deep reservoirs of the Caioba Model may be discovered following a detailed analysis of the subcropping areas and major faults along the pre-Aptian unconformity, and the burial histories of the main structural compartments. Deep reservoir of the Robalo Model are predictable in the eastern, deeper blocks of the basin, which kept the rift-phase structures. The search for deep Serrana reservoirs is very important in the attempt to reverse the hydrocarbon production decrease in the basin.