Salinity anomaly in the Araças area,Reconcavo basin

Abstract

The Araçás field is located in the central-eastern part of the Recôncavo Basin, state of Bahia, Brazil. lts main structural feature is the Araçás Fault, which divides deeper leveis into two blocks. A salinity-depth plot shows that the salinity of formation water increases to 250,000 mgl1 NaCl at 2,300 m; salinity decreases fram this depth, reaching 49,000 mgl1 of NaCI at 3,250 m, in the Baipeba MemberlAliança Formation. This behavior canstitutes an inversion of the normal salinity gradient and is prompted by the anomalously low salinity of formation water observed in the reservairs of the ltaparica, Sergi, and Aliança Farmations in the lower block of the Araçás Fault. An isasalinity map of reservai, waters in the Sergi Formation shows that salinity decreases from W-NW to E-SE, from 280,000 mgl1 ta 60,000 mgl1 of NaCl. This marked reduction may be caused by the release of water during shale diagenetic processes, such as the dehydration of clay minerais in reservoirs and a shale diapir in the eastern part of the Araçás field. Other processes have probably contributed to this anomaly. The ltanagra Fault in the southeastern part of the area, other older faults, faults associated to diapiric movement, and permeable zones in conglomerates of the Salvador Formation may ail serve as means for meteoric water to percolate down to deep reservoirs.