Chemical and isotope composition of the oilfield brines from Mishrif Formation (southern Iraq): Diagenesis and geothermometry

This paper focuses on the geochemical composition and isotope geochemistry of brines in the Cenomanian-Turonian carbonate Mishrif reservoir of southern Iraq. Main dissolved constituents, trace elements, delta H-2 and delta O-18, Sr-87/Sr-86, mineral saturation indices and thermodynamic calculations were investigated in formation waters from the Mishrif Formation to obtain a better understanding of brine evolution and diagenetic effects over geological time. Previous published delta B-11 data were also reinterpreted as a geothermometer tool. The results are compared with previous published data for local oilfields and coeval formations in the Arabian Gulf. The Mishrif brine has a marine origin and is diagenetically modified to Ca-excess and Na-deficit. Formation waters are quartz supersaturated and are in equilibrium with chalcedony and calcite-dolomite in the temperatures range of 50-75 degrees C, which is also confirmed by calcite-water oxygen isotope fractionation and delta B-11 geothermometer. The potential role of clays in conditioning brine chemistry during diagenetic processes was highlighted by activity diagrams; in particular, their adsorption/exchange effect on sodium could explain the lower temperature obtained by the Na/Li geothermometer (42 +/- 6 degrees C). The delta H-2 and delta O-18 values show that oxygen isotope composition of the brines was isotopically more affected by interaction with limestone during diagenesis than seawater evaporation. The main effect is an O-18-enrichment on the brine starting from the SMOW value. Locally, dilution by present-day meteoric water was also detected (Rumaila South), which is shifted towards the local meteoric water line. The strontium isotope ratios range from 0.707713 to 0.707749 and correspond to a marine strontium of late Cenomanian-early Maastrichtian age, except for the Majnoon sample, which shows a more radiogenic value (0.708043). Radiogenic strontium and gypsum and anhydride saturation indices of the Majnoon sample could indicate the contribution of calcium and sulphate from the strontium-rich sulphate minerals of the Cambrian salt domes occurring in the oilfields of southern Iraq. The higher manganese concentration (4 mg/l) and the slightly higher temperature inferred by geothermometers (up to 74 degrees C) in comparison with present-day could indicate that the Majnoon brine is a hot fluid, probably related to a deeper structure such as the Zagros Foredeep Fault.