Pore network characteristics as a function of diagenesis: Implications for epigenic karstification in shallow-water carbonates

Abstract

Karst processes may be critical for developing secondary porosity and permeability within carbonate reservoirs and aquifers. Karstification can significantly influence reservoir storage capacity and subsurface fluid flow. This study investigates the interplay between fluid-flow pathways and diagenesis in an epigenic karst setting in Cretaceous shallow-water carbonate rocks, Potiguar Basin, Brazil. The results indicate that dissolution occurred at all diagenetic stages, but the last stage is the most important because it is related to the most recent subaerial exposure. Dissolution produced during the last karst stage occurred in the vadose and phreatic environments, leading to the precipitation of isopachous, blocky and syntaxial cements. Dissolution affected the original pore network within the matrix and localized on preexisting discontinuities such as stylolites, fractures, and bedding surfaces. The resulting karst cavities are classified as vugs, channels and caverns, varying from a few millimeters to hundreds of meters in size. In epigenic karst systems, high porosity percentages are commonly due to moldic, intragrain/intrafossil and intercrystal porosities. The results of our study show how such porosities can be connected to fractures, veins and stylolites at the microscale. Karst dissolution can further increase porosity and the overall rock permeability due to aragonite and calcite cement dissolution during eodiagenesis, microfractures and stylolites dissolution during mesodiagenesis and all cements, fractures and stylolites during telodiagenesis.