Influência de descontinuidades estruturais e sedimentares na porosidade cárstica da Formação Jandaíra, Bacia Potiguar

Abstract

Epigenic karst in carbonate rocks commonly results in progressive dissolution by the action of meteoric waters over time. The generation of secondary porosity and permeability allows for the dissolution of carbonates in geofluid reservoirs. These two factors can impact reservoir storage capacity and fluid flow. The present study investigates (1) the control of structural discontinuities (tectonic stylolites and fractures) and sedimentary discontinuities (stylolites generated by diagenesis, cross-bedding, and plane-parallel) in the generation of karst dissolution cavities, from microscale (102 - 1010µm) to mesoscale (0.1 - 5 m), and (2) the relationship of porosities formed by diagenetic processes and their impact on the generation of epigenic karst from the Jandaíra Formation, Potiguar Basin, Brazil. The study is based on micro to mesoscale analysis based on field analysis based on drone images, stratigraphic logs, structural and rock strength data collected by Schmidt's hammer. Laboratory studies focused on microtomographic images, petrography and petrophysics. . The results show that the stratabound and non-stratabound bed-perpendicular stylolites can be enlarged due to the percolation of meteoric waters until they merge and form a single channel system that cuts all the layers. The bed-parallel stylolites are at the limits of the layers and, therefore, can act as horizontal fluid conduits when well dissolved and forming channel porosity. Dissolution can occur in all diagenetic stages the carbonate rock matrix. Telodiagenesis, is the most important due to the most recent aerial exposure. The dissolution in telodiagenesis generated different types of porosity due to the dissolution of various diagenetic features previously formed, such as cementation, dolomitization, and compaction. Dissolution is also localized to preexisting discontinuities such as stylolites, fractures, and bedding surfaces. In epigenic karst systems, high percentages of microporosity are commonly due to mold, intragrain/intrafossil, and intercrystal porosities. Such porosities can be connected to microscale fractures, veins, and stylolites, further increasing the porosity and overall permeability of the rock.