The post-depositional evolution of Mesoproterozoic to early Neoproterozoic tight sandstone reservoirs, São Francisco basin, Brazil

Abstract

This study presents the first comprehensive assessment of unconventional gas reservoirs in the Proterozoic sequences of the intracratonic São Francisco basin, providing depositional and post-depositional characterization and insights into reservoir quality. Detailed stratigraphic, petrological, and seismic analyses, integrated with 2D seismic data, were performed to evaluate the influence of diagenesis and hydrothermal fluids. The drill core was drilled in the central sector of the basin and within the southern border of the Pirapora aulacogen, in a 303 m-thick interval of the Paranoá-Upper Espinhaço sequence. The petrological analysis shows arkoses and sublitharenites as well as conglomerates, siltstones and mudstones successions deposited in deltaic, fluvial braided and marine depositional systems. Six sedimentary environments were identified, from the base to the top: (i) deltaic plain, (ii) delta front, (iii) braided fluvial, (iv) delta front, (v) prodelta and (vi) basin plain. The integration of seismic data review three depositional domains, characterized by lower frequency arranged to transgressive-regressive cycles identified by gamma ray, resistivity and density logs. The studied strata review eo - and mesodiagenetic alterations, being also influenced by intrusion-related hydrothermal and contact metamorphism. The main diagenetic events resulted in cementation of calcite (0.84%), chlorite (1.23%) and silica (6.19%), which largely obliterated porosity at the shallow burial depths. The main secondary porosity is intergranular, with minor contributions to intragranular, fracture, moldic and vugular. Diagenetic processes as well as the influence of hydrothermal processes highlight the complexity of post-depositional processes in these units. The presence of natural helium and hydrogen within the basin also suggests that these reservoirs should be considered in future exploration campaigns targeting unconventional energy resources.