--> Hypogenic Karst Dissolution in Carbonate Rocks Has Implications for Karstified Carbonate Reservoirs

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Hypogenic Karst Dissolution in Carbonate Rocks Has Implications for Karstified Carbonate Reservoirs

Abstract

Dissolution along zones of preferential flow enhances anisotropy in geological media and increases its complexity. Many karst models tend to be descriptive and fail to predict localization and structure of karst systems. Conduit architecture is difficult to derive from borehole data, and it can be hardly mapped even with the advent of 3D seismic techniques. Therefore, analysis of the geometry of karst conduit systems remains elusive where direct access to karst conduits is not possible. The present study investigates the relationship between lithofacies, tectonics and karstification in the Neoproterozoic Salitre Formation, located in the central-eastern Brazil. This unit comprises several systems of caves including the Toca da Boa Vista and da Barriguda hypogenic caves, the largest in South America, and focus of this study. We focused on cave mapping and morphogenetic analysis, determination of petrophysical properties, thin-section description, micro-tomography, and isotopic analysis. The Salitre Fm, deposited in a epicontinental sea, comprises mud/wakestones, grainstones, microbial facies, and fine siliciclastic rocks. The caves comprise 3D mazes with a network/spongework pattern. Passages occur in several levels within ca. 60 m thick cave-forming section, limited at the top by lithofacies with low permeability and fractures. Cave development occurred in phreatic sluggish-flow environment with overall upwelling flow. It rose via cross-formational fractures and distributed laterally within the cave-forming section using geological heterogeneities to eventually discharge up through outlets breaching across the upper confining beds. Cave formations include an assemblage indicating a period of water table conditions. Sediment fill includes deposits of dolomitic sand originated from in situ weathering of cave walls and ceiling. Caves show no relationship with the surface. Our data indicate several events of porosity evolution, such as subaerial exposure, folds and fractures, hydrothermal events (exotic minerals assemblage), sulfuric acid dissolution, dissolution at the water table, condensation corrosion, and faults and fractures reactivation. The major enhancement of secondary porosity was due to hypogene speleogenesis. Hypogene karst process may have implications to carbonate reservoirs not only through porosity enhancement, but also through their influence on regional flow systems, cross-formational communication, and migration of hydrocarbons.