Fracture stratigraphy controls karst localization within a carbonate sequence with terrigenous input: Implications for conduit-seal layer development in a karst system

Abstract

The present study uses a multidisciplinary approach to investigate the impact of fractures on karst system localization that affects carbonate units containing terrigenous input. The study area is the Lapinha Cave external cliffs in the Neoproterozoic Salitre Formation, São Francisco Craton, Brazil. We performed structural and stratigraphic field investigations of the external cave portions with an unmanned aerial vehicle (UAV) and laser detection and ranging (LiDAR). We also collected uniaxial compressive strength (UCS) and gamma-ray (GR) data and performed petrographic and petrophysical analyses. The fracture stratigraphy characterization results indicate that sedimentary facies controlled the fracture distribution. The studied sequence consists of an impure carbonate sequence that comprises three main sedimentary facies: (i) mudstone (MD), (ii) packstone-grainstone containing mudstone and siliciclastic intercalations (PG), and (iii) mixed carbonate-siliciclastic unit composed of grainstone and sandstone (CS). The MD, PG and CS facies displayed the highest, medium, and lowest fracture intensities and densities, respectively. The MD and PG facies behaved as ideal reservoirs or geofluid conduits related to intense deformation. Two principal fracture sets along these MD and PG facies striking N–S and E‒W, respectively, were documented. In contrast, the heterogeneous and less deformed CS facies displayed a geofluid barrier behavior. Overall, the results produce a guideline for investigating carbonate sequences characterized by terrigenous input affected by different fracture concentrations and distributions.