UNIVERSIDADE DE BRASÍLIA INSTITUTO DE GEOCIÊNCIAS PROGRAMA DE PÓS-GRADUAÇÃO EM GEOLOGIA

In this work, we show our new data, including facies analysis, petrography and isotope geochemistry obtained in Neoproterozoic carbonates of the Salitre Formation, in an outcrop located in the Achado village, 5 km east of the city of Irecê, Bahia. The main objective was to verify the relationship between lithofacies and fluid-rock migration using δC and δO carbonate data. Facies analysis was carried out in a section of continuous exposure, with wellpreserved facies layers, resulting in a stratigraphic column with 345m. The facies identified were grouped into three facies associations, distributed on a carbonate ramp system: (FA1) inner carbonate ramp dominated by tidal process and microbialites, (FA2) inner carbonate ramp dominated by waves and (FA3) mid carbonate ramp dominated by storm waves. Despite wellpreserved sedimentary structures, the entire sequence presents deformation features, such as veins, fractures and faults. Based on lithology, sedimentary features and isotope variations, the column was divided into three sections, namely: 1) lower; 2) middle; and 3) upper. The lower section is characterized by limestones, where δC ranges from -0.46‰ to +3.17, which reflects deposition when the basin was connected to the Neoproterozoic open ocean. The middle and upper sections are characterized by dolostones, and have δC values ranging from -3.41‰ to +13.69‰, representing deposition in a restricted basin. The diagenetic evolution of the succession is characterized by six distinct diagenetic stages: marine, authigenic, reflux, meteoric, burial and hydrothermal. The paleoenvironmental and diagenetic conditions were fundamental to establish the fluid flow in the studied carbonate succession. The dolostones of the middle and upper sections are characterized by frequent occurrences of veins, which suggests a lithological control of the distribution of fractures and faults. Such structures were the main conduits for fluid migration, which presented lateral spread due to facies control. In the dolostones, two groups of veins were identified which represent different sources of fluids. The first one is characterized by very negative δC (ranging from -8.99‰ to +0.61‰) and δO (ranging from 4.24 ‰ to -9.29‰) values, and radiogenic Sr/Sr values (ranging from 0.71056 to 0.73854), representing external hydrothermal fluids. The other group was formed in a closed system as a result of the fluid-rock interaction through a pressure-solution mechanism. Similar δC and δO values for veins and host rock support this interpretation. The multidisciplinary study involving facies analysis, diagenesis and isotope geochemistry allowed for the identification of the probable sources of fluids that percolated the basin, as well as of the isotopic changes in the host rock due to fluid-rock interaction. The identification of fluid flow zones in carbonate reservoirs is extremely important, since such fluids may cause changes in the quality of the reservoirs, especially in relation to porosity and permeability.