Explorando os controles estruturais e estratigráficos no Sistema Cárstico Furna Feia

Abstract

The Furna Feia cave is the second largest cave in the state of Rio Grande do Norte and was developed along the Cretaceous carbonates of the Jandaíra Formation. The aim of this study was to analyze the geological indicators that affected the development of this karst system, combining surface and subsurface geological and geophysical data. Thus, a multidisciplinary approach was adopted, utilizing field visits, GPR, drones, LiDAR, gamma rays, porosity, permeability, uniaxial compressive strength, and petroglyph analysis. The cave is shallow, with a 766m linear development, meandering linear geometry, and its walls and ceiling exhibit domes and pockets. The identified facies associations were tidal bars at both the base and top of the cave, interspersed with lagoon facies association. Within this lagoonal interval, the rocks are intensely dolomitized. The base tidal bars indicate a paleocurrent towards the northwest, while the top tidal bars indicate a northeast direction. LiDAR data showed that the dolomitized interval is also the most dissolved, where the cave exhibits the greatest widening. This interval also showed the highest amount of fracturing in GPR data and field analyses. Contrarily, this interval presents the lowest porosity and permeability values in its matrix. Additionally, structural analyses also showed a tectonic influence on the development of this karst system. The Poço Verde Fault, trending northwest, affected the orientation of both the galleries and the pockets and domes found in the cave. Therefore, we conclude that dolomitization exerted the greatest control over the formation of the cave, as it allowed for the highest concentrations of fracturing and consequent horizontal fluid percolation for subsequent dissolution of the carbonates. Tectonics, on the other hand, guided this dissolution in preferred directions (northwest and northeast). These findings can be extrapolated and used as guidelines for carbonate reservoirs affected by the same processes and indicate a dual porosity behavior for these rocks.