Siliciclastic content controls the sealing behavior of layers and karst development in a carbonate-siliciclastic sequence, Salitre Formation, Brazil

Abstract

Although caves formed in carbonate units play a crucial role in fluid flow, most subsurface voids are below seismic resolution. This study investigates how sedimentary facies control the vertical distribution and the development of karst features from small vugs to caves in a carbonate succession with terrigenous input. We analyzed the sedimentary succession and the potential of layers to behave as sealing and conduit units in the karst system. The study area is located in the Neoproterozoic Salitre Formation in the Una-Utinga Basin, São Francisco Craton, Brazil. We selected two caves and their hosting units as analogs of carbonate reservoirs affected by karstification. Our findings indicate that the caves have similar sedimentary and petrographic patterns, composed of three sedimentary facies at a 20–30 m vertical section from the bottom to the top. At the bottom, the lime mudstone facies contains 10% siliciclastic content, which is the pure conduit facies that host most cave passages. A heterogeneous calcarenite containing lime mudstones and siliciclastic intercalation facies occurs in the middle portion, where the siliciclastic content varies between 10-30% and 30–50%. This aforementioned facies exhibits conduit and sealing layers. At the top, a mixed limestone siliciclastic facies has siliciclastic content above 50% and a high degree of compaction. It behaves as a sealing facies. The conduit-sealing system has the same pattern at various scales: macro (observed in drone imagery), meso (Light Detection and Ranging imagery - LiDAR), and microscale (thin section). We conclude that the siliciclastic content of layers controls the degree of karstification at various scales in a carbonate-siliciclastic sequence.