Depositional and diagenetic factors in the generation of the Santiago arenite reservoirs (lower cretaceous): Araçás oil field, Recôncavo Basin, Brazil

Abstract

The Santiago Member of the Pojuca Formation in the Araçás oil field consists of deltaic sandstone bodies interbedded with shales which were deposited in an interdistributary bay area through breaching of channel levees. The best reservoirs occur in crevasse mouth bar deposits and consist of quartzose feldspathic arenites and feldspathic arenites. The diagenetic evolution of the reservoir arenites consists of: destruction of primary porosity by compaction and burial calcite cementation; development of 35% secondary porosity by leaching of calcite cement; decrease in secondary porosity to 20% by late cementation (kaolinite, chlorite, pyrite); and preservation of secondary porosity by oil migration which halted further diagenesis. Framework grains display distribution patterns controlled by features of the depositional environments. Diagenetic processes in pure arenites, controlled by a dome-like compaction structure, directed the dissolution of cement and the generation of secondary porosity. Both cement and secondary porosity are concentrated in the dome center and display closed area patterns of distribution in which values vary in opposition to each other. Intercalation of matrix-rich microfacies in any reservoirs inhibits dissolution. This leads to a reverse situation in which porosity still displays a closed areal pattern of distribution but concentrates in the dome periphery. In all reservoirs, permiability distribution follows neither the distribution of framework grains, nor of cement, nor of porosity, indicating that it results from a complex and as yet not understood interaction of factors.