Karst dissolution along fracture corridors in an anticline hinge, Jandaíra Formation, Brazil: Implications for reservoir quality

Folded, fractured, and karstified structures have been recognized in several carbonate reservoirs. However, they are rarely fully identified from seismic or well data and have been poorly described. The present study investigates the geological and structural controls that regional anticlines may exert on solution-enlarged fracture corridors and epigenic karst concentrations in folded-fractured carbonate units. We performed regional mapping based on 106 2D seismic lines, 51 well logs, unmanned aerial vehicle (UAV) imagery of four major outcrop sites (I, II, III and IV) and field investigations to parameterize fracture and karstic features (density, spacing, length, and aperture). The study area is Potiguar Basin, Brazil, where we identified the gentle NE-trending Apodi fold, ~10 km wide and ~20 km long. The fold formed along an inverted extensional fault during the latest Miocene-Quaternary stress field, and the envelope of the meandering trend of the Apodi River coincides with the NE-trending fold axis. The fracture pattern in the fold hinge zone consists of two orthogonal, syn-folding fracture sets: a NE-SW-striking hinge-parallel set and an orthogonal NW-SE-striking set. These fractures and superposed karstification are mostly concentrated in the hinge zone along fracture corridors in an area 1.5 km wide and 20 km long. We conclude that fracture corridors in anticline hinges are likely sites to have high fracture densities and wide apertures. These corridors provide reservoir spaces that are favorable for the formation of karst cavities, with significant increases in permeability and porosity along the fracture sets. These features act as important fluid pathways and storage areas where matrix porosity is low. The findings can be used for first-order prediction in karstified carbonate reservoirs.