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Deposition, Diagenesis and Porosity Preservation of Campos Basin Bioclastic Lacustrine Rift Reservoirs

Abstract

The main reservoirs of the Lower Cretaceous rift section (Lagoa Feia Group) from the Campos Basin, eastern Brazil, correspond to bioclastic (bivalves and ostracodes) grainstones and rudstones (commonly known as “coquinas”). The integration of seismic, sedimentologic and petrographic evidence has shown that the carbonate deposits occur in diverse geographic and/or stratigraphic positions, both on basement highs and lows throughout the rift section. Carbonate bioclasts are usually mixed in various proportions to stevensite ooids and peloids, and to siliciclastic grains. This mixture was interpreted as a product of re-sedimentation through gravitational flows, triggered by tectonic events along the lake margins. The diagenetic evolution of the rudstones was strongly influenced by the early diagenetic transformation of the bivalve bioclasts. Their aragonitic original composition is indicated by widespread early neomorphism preceding cementation and compaction. While some bioclasts were partially dissolved, generating intraparticle pores that were early cemented by drusiform calcite, most were replaced by coarse calcite mosaic containing ghosts of the foliated structure of the original shells. In contrast, the calcitic ostracode bioclasts are largely non-recrystallized, retaining their original microcrystalline texture. The early stabilization of the bivalve bioclasts to coarsely-crystalline, low-Mg calcite, together with partial eodiagenetic cementation by blocky or drusiform mosaic calcite, inhibited compaction, favoring preservation of interparticle porosity. However, rudstones with significant amounts of stevensitic and/or siliciclastic grains were strongly cemented due to the enhanced replacement and nucleation on such particles. Bioclasts dolomitization and dolomite cementation are localized features, but silicification and cementation by chalcedony and drusiform quartz are relatively common. Other minor diagenetic phases include smectite rims, selective dissolution of stevensite ooids and peloids, authigenesis of kaolinite (dickite?) and barite. Understanding the diagenetic evolution of the bioclastic rudstones is essential to decrease the exploration risks of similar reservoirs in new areas of the Campos Basin and to increase recovery from producing oilfields.