Depositional and diagenetic impacts on the porosity of post-salt carbonate reservoirs of southern Campos Basin, southeastern Brazilian margin

Abstract

Shallow marine carbonates constitute some of the most productive oil reservoirs of the world. In Brazil, offshore oil production began from Albian, post-salt marine carbonate reservoirs of the Campos Basin, which lost their importance with the increasing discoveries and production from Upper Cretaceous and Paleogene clastic turbidites and, more recently, from Lower Cretaceous pre-salt lacustrine carbonates. Nevertheless, Albian carbonate prospects are still attractive, mostly in deep-water areas. This study aimed at identifying the main depositional and diagenetic impacts on reservoir quality of the Albian carbonates in southern Campos Basin, through the integration of systematic quantitative petrography, scanning electron microscopy and petrophysical log analysis with the structural, stratigraphic and sedimentological context from wells located outside discovered accumulations in the area. The studied limestones are moderate-to high-energy deposits, represented by oncolitic, peloidal and intraclastic calcarenites and calcirudites, and by oolitic/oncolitic calcarenites, deposited in shoals controlled by the topography of the underlying salt surface. Their main diagenetic processes and products are micritization, rim, mosaic and syntaxial calcite cementation, grain dissolution, interparticle and grain-replacive dolomite precipitation, mechanical compaction, halokinesis-related fracturing, chemical compaction and stylolitization, mesodiagenetic dissolution, and late calcite cementation. Reservoir quality was affected by both the depositional sorting and composition, and the intensity of diagenesis. Preserved interparticle porosity ranges widely in high to moderate energy facies, represented by well-sorted oolitic/oncolitic calcarenites, due to the intensity of cementation and compaction. Oncolitic/peloidal/intraclastic calcarenites and calcirudites deposited in moderate energy environments show negligible primary porosity. Secondary porosity was generated both by dissolution of ooids and oncoids during episodic meteoric influx, and during burial, by dissolution along fractures and stylolites. Intercalated hybrid arenites with abundant feldspar grains derived from uplifted plutonic terrains and marginal alluvial systems are tight, due to intense compaction and cementation, showing similar evolution, except for K-feldspar overgrowths, dissolution and kaolinization of feldspar grains. The basal dolostones were formed by seepage reflux of brines through lagoonal and barrier carbonates and hybrid sands, deposited on the irregular topography at the top of underlying evaporites, and show low porosity and permeability, due to the combination of decrease in intercrystalline porosity by overdolomitization, and the filling of moldic and vugular pores by calcite, anhydrite and halite. The role played by burial diagenetic processes and by the underlying evaporites was not effectively identified and explored in the literature previously published on the Campos Basin post-salt carbonates. Understanding the factors and processes that affected the porosity of the Albian carbonates shall contribute to the prediction of reservoir quality in exploration areas, as well as to the improvement of recovery efficiency in the producing fields.