Fluid mixing induced by hydrothermal activity in the ordovician carbonates in Tarim Basin, China L. JIANG 1,2 , W. PAN 3 , C. CAI 1 , L. JIA 1 , L. PAN 4 , T. WANG 1 , H. LI 1 , S. CHEN 1,5 AND Y. CHEN 1 1 Key Lab of Petroleum Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China; 2 Department of Geology and Geophysics, School of Environmental Sciences, University of Liverpool, Liverpool, UK; 3 Tarim Oilfield Company, PetroChina, Korla, Xinjiang, China; 4 PetroChina Hangzhou Research Institute of Geology, Hangzhou, China; 5 Energy Resource Department, China University of Geosciences, Beijing, China ABSTRACT Permian hydrothermal activity in the Tarim Basin may have been responsible for the invasion of hot brines into Ordovician carbonate reservoirs. Studies have been undertaken to explain the origin and geochemical characteris- tics of the diagenetic fluid present during this hydrothermal event although there is no consensus on it. We pres- ent a genetic model resulting from the study of d 13 C, d 18 O, d 34 S, and 87 Sr/ 86 Sr isotope values and fluid inclusions (FIs) from fracture- and vug-filling calcite, saddle dolomite, fluorite, barite, quartz, and anhydrite from Ordovician outcrops in northwest (NW) Tarim Basin and subsurface cores in Central Tarim Basin. The presence of hydrothermal fluid was confirmed by minerals with fluid inclusion homogenization temperatures being >10°C higher than the paleo-formation burial temperatures both in the NW Tarim and in the Central Tarim areas. The mixing of hot (>200°C), high-salinity (>24 wt% NaCl), 87 Sr-rich (up to 0.7104) hydrothermal fluid with cool (60– 100°C), low-salinity (0 to 3.5 wt% NaCl), also 87 Sr-rich (up to 0.7010) meteoric water in the Ordovician unit was supported by the salinity of fluid inclusions, and d 13 C, d 18 O, and 87 Sr/ 86 Sr isotopic values of the diagenetic minerals. Up-migrated hydrothermal fluids from the deeper Cambrian strata may have contributed to the hot brine with high sulfate concentrations which promoted thermochemical sulfate reduction (TSR) in the Ordovician, resulting in the formation of 12 C-rich (d 13 C as low as À13.8&) calcite and 34 S-rich (d 34 S values from 21.4& to 29.7&)H 2 S, pyrite, and elemental sulfur. Hydrothermal fluid mixing with fresh water in Ordovician strata in Ta- rim Basin was facilitated by deep-seated faults and up-reaching faults due to the pervasive Permian magmatic activity. Collectively, fluid mixing, hydrothermal dolomitization, TSR, and faulting may have locally dissolved the host carbonates and increased the reservoir porosity and permeability, which has significant implications for hydrocarbon exploration. Key words: carbonates, dolomitization, hydrothermal fluids, meteoric water, Tarim Basin, thermochemical sulfate reduction Received 30 July 2014; accepted 20 November 2014 Corresponding author: Prof Dr Chunfang Cai, Key Lab of Petroleum Resources, Institute of Geology and Geo- physics, Chinese Academy of Sciences, Beijing 100029, China. Email: cai_cf@mail.iggcas.ac.cn. Tel: +86 108 299 8127. Fax: +86 106 201 0846 Geofluids (2014) INTRODUCTION In the past few decades, the question of hydrothermal activity due to tectonic movements has gained increasing attention and interest by petroleum geologists in carbonate reservoir exploration (Qing & Mountjoye 1994; Lavoie et al. 2005; Davies & Smith 2006; Saller & Dickson 2011). This is because not only the hydrothermal activity itself can cause faults and fractures that improve the reservoir quality, but also the hydrothermal fluid mixing and hydrothermal dolomitization could either dissolve cal- cite or led to replacement of calcite by dolomite that both created secondary porosity (Corbella et al. 2004; Lavoie et al. 2005; Smith 2005; Davies & Smith 2006; Wendte 2006; Wendte et al. 2009; Lapponi et al. 2014). Lower Ordovician carbonates in the Tarim Basin represent an important hydrocarbon reservoirs associated with karstifica- tion (most commonly in the Yinshan Formation-O 1-2 y), © 2014 John Wiley & Sons Ltd Geofluids (2014) doi: 10.1111/gfl.12125