Petrophysical and petrographic data correlation and the discrimination of magmatic environments and processes in Santos Basin, offshore SE Brazil

Abstract

The igneous rocks in the context of oil & gas exploratory sedimentary basins have been a paradigm and challenge for industry and academia due to the difficulty of reconnaissance and interpretation using indirect methods, e.g. geophysical data. We presented the combination of the petrographic and well log data of distinct magmatic sections from wells of Santos Basin, SE Brazil. Effusive and explosive felsic rocks, mafic volcanic subaerial and subaqueous and plutonic mafic rocks were recognized by petrography and petrophysics proprieties data, as radioactive (GR), electric resistivity (R), acoustic property (Δt), porosity (Φ) and density (ρ), showed different patterns along sections. This enables the division on magmatic subsections and allowed the recognition of rock types and igneous setting. The compositional influences are clearly distinguished by GR, as felsic and mafic rocks, presence of amygdales in mafic effusive rocks and inter-pillow intervals in subaqueous environments, can interfere with GR patterns. In plutonic environment, GR can show evidence of magmatic differentiation. Δt and Φ are directly proportional with similar pattern and both are inversely proportional to R and ρ. Felsic rocks, effusive or explosive, and inter-pillow intervals makes Φ and ρ curves closer. Less dense and more porous rocks, as explosive, tend to imprint the Φ and ρ pattern to the left of the log. Welding processes are distinguished by gradual decrease of Φ and increase of ρ. Otherwise, denser mafic rocks, volcanic or plutonic, tend to show separate Φ and ρ curves on the right of the log, while amygdaloidal portions are lighter and more porous than the massive one, showing the separate pattern on the left. The subaqueous mafic volcanic rocks imprint GR, ρ and Φ intermediate patterns between mafic massive and amygdaloidal. The recognition of the type of magmatic rock, the environment and the physical properties can aid to improve not only the decisions of exploration and production but also the elaboration of a geodynamic context for offshore sedimentary basins.