Primary and diagenetic controls on the quality of Albian carbonate reservoirs of the Tubarão Martelo area, southern Campos Basin, Brazil

Abstract

Albian carbonate reservoirs have been an attractive target for oil exploration and production in the Campos Basin for several decades. However, the heterogeneous reservoir quality in the sourthern portion of the basin has represented a considerable challenge. The deposits in the Tubarão Martelo area are high energy oolitic calcarenites, and moderate energy oncolitic and peloidal calcarenites and calcirudites and hybrid arenites. Nine reservoir petrofacies were defined in the Quissamã Formation limestones. Their main diagenetic processes were eogenetic calcite cementation and replacement by blocky dolomite, mechanical and chemical compaction, eogenetic and mesogenetic dissolution. Primary composition and texture were less important than diagenesis in conditioning reservoir quality. Partial early cementation by calcite rims and sintaxial overgrowths limited the compaction of some oolitic and oncolitic calcarenites, preserving interparticle primary porosity. However, interparticle porosity was totally obliterated where cementation was abundant, as observed in the oncolitic calcirudites. Hybrid arenites also show very limited porosity, owing to strong compaction. The Búzios Member dolostones locally show good porosity, but their permeability is very limited, owing to dolomite cementation (overdolomitization), which severely reduced the intercrystalline porosity, isolating their moldic pores. The main phase of dissolution in the Quissamã limestones occurred during eogenetic infiltration of meteoric waters. A second dissolution phase took place during burial, by the percolation of evolved fluids through fractures and stylolites. Understanding the depositional and diagenetic factors that controlled the reservoir quality of Campos Basin Albian carbonates is paramount for the exploration of new accumulations, as well as for increasing oil recovery from the producing fields.