The upper portion of the São Sebastião Formation, in the Recôncavo Basin, Bahia, is a multi-layered artesian aquifer system composed of alternating sandstones and shales, with a thickness greater than 1.000 m, Electrical logs of 95 wells for both oil and water exploration in the region of the Camaçari Petrochemical pole have been studied with the aim of defining the structural configuration and the petrophysical characterization of the main sandstones bodies tapped by the wells. The geometry and internal structures of theses bodies, as clearly displayed in the SP and resistivity curves, suggest that they originated as point bar sands of a large fluvial environment. The water quality determined by chemical analysis and inferred from the logs is related to the general pattern of flow, the salt content being modified both by cation exhange within disseminated clays and by ionic filtering through confining shales. A thermal anomaly contour map constructed from borehole temperature data reveals a regional pattern of water transfer between sandstones by means of vertical leakages through the shale layers. Temperatures logs performed in nine water wells were used to estimate the rates of this leakage, which is in the order of 8.4 x 1O-7 to 1.2 x 10-6 em/s. Combined piezometric and pumping data of the Camacari well field attest clearly to the highly severe exploitation the aquifer is undergoing in this region. Considering the high level of surface water contamina tion by industrial and oil exploratory activities in the area, these data suggest an imminent risk of polution for this important groundwater reserve and calls for a rigorous control by government agencies.The upper portion of the São Sebastião Formation, in the Recôncavo Basin, Bahia, is a multi-layered artesian aquifer system composed of alternating sandstones and shales, with a thickness greater than 1.000 m, Electrical logs of 95 wells for both oil and water exploration in the region of the Camaçari Petrochemical pole have been studied with the aim of defining the structural configuration and the petrophysical characterization of the main sandstones bodies tapped by the wells. The geometry and internal structures of theses bodies, as clearly displayed in the SP and resistivity curves, suggest that they originated as point bar sands of a large fluvial environment. The water quality determined by chemical analysis and inferred from the logs is related to the general pattern of flow, the salt content being modified both by cation exhange within disseminated clays and by ionic filtering through confining shales. A thermal anomaly contour map constructed from borehole temperature data reveals a regional pattern of water transfer between sandstones by means of vertical leakages through the shale layers. Temperatures logs performed in nine water wells were used to estimate the rates of this leakage, which is in the order of 8.4 x 1O-7 to 1.2 x 10-6 cm/s. Combined piezometric and pumping data of the Camacari well field attest clearly to the highly severe exploitation the aquifer is undergoing in this region. Considering the high level of surface water contamination by industrial and oil exploratory activities in the area, these data suggest an imminent risk of polution for this important groundwater reserve and calls for a rigorous control by government agencies.