Burial stylolites enhance karst dissolution and control cave geometry in heterolithic carbonates, Salitre Formation, Brazil

Abstract

Understanding the karstification process in fractured carbonates is a key factor in developing and exploiting fractured and karstified reservoirs. This study documents the relationship among the 2D geometry, stratigraphy and petrophysical properties of a cave system developed in a Neoproterozoic mixed carbonate-siliciclastic sequence. We applied a multidisciplinary and multiscale approach that combined structural geological surveys, cave imaging by Laser Detecting and Ranging (LiDAR), linear scanlines, stratigraphic logs, uniaxial compressive strength (UCS), mercury-intrusion porosimetry, and 2D image analysis. We found that bedding-parallel stylolites in dolomitic limestone consist of mm-to cm-thick high-porosity zones (up to 20% measured via 2D image analysis). They acted as fluid conduits that enhanced bedding-parallel dissolution. Field and thin section analyses showed that beds with close spacing and thicker stylolites are more karstified and dissolved than beds showing greater spacing or completely devoid of stylolites, indicating that stylolitic zones act as flow pathways in low porosity-permeability carbonates. Primary porosity, pore size distribution, density and capillary pressure of host rocks have no direct relationship with karstification intensity. We conclude that clusters of burial stylolites may control the hypogenic cave geometry. These findings have direct implications for subsurface permeability prediction in karstified carbonate reservoirs.