Carbonate reservoir quality and permoporosity obliteration due to silicification processes in the Barra Velha Formation, Santos Basin, Southeastern Brazil

Abstract

Silicification is a diagenetic process commonly observed in sedimentary sections of the Aptian carbonate deposits of the Santos Basin that involves the replacement of carbonate minerals with silica. It is mostly associated with hydrothermal fluids percolating through faults and fractures. CO2 degassing favors an increase in acidity. Consequently, the fluids became subsaturated with calcite and supersaturated with silica. Given the increase in silica content, two distinct behaviors were identified: reduction and enhancement of permoporous properties. The aim of this work is to identify permoporous alteration due to silicification in the Barra Velha Formation. To accomplish this, a qualitative analysis was conducted correlating acoustic borehole images, nuclear magnetic resonance porosity and permeability, and elemental capture spectroscopy logs were performed, followed by a quantitative analysis based on crossplot evaluation. The Barra Velha Formation was divided into three intervals with respect to silica content. The common behavior observed is permoporosity reduction associated with silica increase, but locally, permoporosity enhancement associated with silica increase was also identified. Finally, we established a direct relationship between calcite dissolution and silica precipitation, a well-defined trend characterized by sonic log (DT) reduction and density increase, and an inverse relationship between acoustic impedance and permoporous properties. Finally, the quantitative analysis also favored improving the lack of reservoir qualities of the intrusive igneous interval.