Heterogeneities of the fluvial-eolian reservoirs of the Sergi Formation, Recôncavo Basin

Abstract

The Sergi Formation can be subdivided into three sequences, bounded by unconformities. The depositional and diagenetic heterogeneities that affect reservoir quality of sequences I and II were identified. The integration of sedimentologic, stratigraphic and petrologic data revealed significant differences in the depositional and diagenetic characteristics of these sequences, which have decisively influenced the definition of the reservoir heterogeneity patterns and porosity and permeability distribution. This integrated analysis supplied parameters to construct reservoir quality and heterogeneity models that can be applied to develop the fields producing from Sergi Formation, as well as predict the quality of Sergi reservoirs under exploration. Sequence I reservoirs are made up of fine to medium-grained sandstones, well to moderately-sorted, deposited by fluvial-eolian systems. Sequence II reservoirs, in their turn, are made up of fine to conglomeratic, predominantly coarse-grained sandstones, deposited by braided fluvial systems. In addition to the depositional differences, there are systematic differ-ences in the dominant diagenetic processes. Sequence I sandstones are characterized by a more intense carbonate cementation and larger chlorite abundance, while Sequence II sandstones show mechanical clay infiltration as the dominant diagenetic process, which has inhibited further diagenetic events. Intraclastic lags at the base of ephemeral channel cycles of Sequence I are commonly pervasively cemented by calcite, con-stituting local flow barriers. The concentrations of mechanically infiltrated clays constitute flow barriers and deflectors, being the most important controls on reservoirs quality and heterogeneity within Sequence II.