Abstract 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, δ2H and δ18O, 87Sr/86Sr, 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 δ11B 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 °C, which is also confirmed by calcite-water oxygen isotope fractionation and δ11B 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 °C). The δ2H and δ18O 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 18O-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 °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.

中文翻译：

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Mishrif 组（伊拉克南部）油田盐水的化学和同位素组成：成岩作用和地温测量

摘要 本文重点研究伊拉克南部Cenomanian-Turonian碳酸盐岩Mishrif油藏卤水的地球化学组成和同位素地球化学。研究了 Mishrif 组地层水中的主要溶解成分、微量元素、δ2H 和 δ18O、87Sr/86Sr、矿物饱和指数和热力学计算，以更好地了解卤水演化和地质时间的成岩作用。先前公布的 δ11B 数据也被重新解释为地温计工具。结果与之前公布的阿拉伯湾当地油田和同期地层数据进行了比较。Mishrif 卤水来源于海洋，并经过成岩修饰，导致 Ca 过量和 Na 缺乏。地层水是石英过饱和的，在 50-75 °C 的温度范围内与玉髓和方解石-白云石处于平衡状态，这也得到了方解石-水氧同位素分馏和 δ11B 地温计的证实。活动图强调了粘土在成岩过程中调节盐水化学的潜在作用；特别是，它们对钠的吸附/交换作用可以解释钠/锂地温计获得的较低温度（42±6°C）。δ2H 和 δ18O 值表明，与海水蒸发相比，成岩过程中与石灰石相互作用对卤水的氧同位素组成的同位素影响更大。主要影响是从 SMOW 值开始对盐水进行 18O 富集。在当地，也检测到现代大气水的稀释（Rumaila South），移向当地的大气水线。锶同位素比率范围从 0.707713 到 0.707749，对应于晚森诺曼阶至早马斯特里赫特时代的海洋锶，但 Majnoon 样本除外，其显示出更多的放射成因值 (0.708043)。Majnoon 样品的放射性锶和石膏以及酸酐饱和度指数可以表明来自伊拉克南部油田寒武纪盐丘富含锶的硫酸盐矿物的钙和硫酸盐的贡献。与现在相比，更高的锰浓度 (4 mg/l) 和地温计推断的略高的温度（高达 74 °C）可能表明 Majnoon 盐水是一种热流体，可能与更深的结构有关，例如扎格罗斯前渊断层。