Isotopic geochemical characteristics and identification indexes of shale gas hydraulic fracturing flowback/produced water

China is rich in shale gas resources, which are mainly distributed in densely populated southern marine carbonate areas. The geological and surface conditions are complex, the ecological environment is fragile, and water resources are scarce or unevenly distributed. Therefore, large-scale fracturing mining has high pressure of water resources utilization and serious risk of water environment pollution. In this paper, the geochemical characteristics of traditional and non-traditional stable isotopes such as hydrogen, oxygen, boron, lithium, strontium in shale gas hydraulic fracturing flowback/produced water (FP water) were comprehensively analyzed. The results show that, the hydrogen and oxygen isotopic composition of the FP water in Sichuan Basin, China has similar evolution trend with the produced water of conventional wells from Cambrian, Permian and Triassic Xujiahe, Jialingjiang and Leikoupo formations, but different from that of the Sinian conventional produced water. It indicates that the FP water in Sichuan Basin is a mixture of fracturing injected fluid and formation brine retained in Silurian shale. The saline end member is close to the formation water of Cambrian, but with higher δ¹¹B values. The FP water in Sichuan Basin has δ¹¹B values close to that of the Marcellus FP water, and both are derived from the evaporated seawater. The δ¹¹B values of FP water in Sichuan Basin have overlap with that of the conventional produced water from different strata, so it can not be precisely distinguished. However, the δ¹¹B and B/Cl values of the FP water in Sichuan Basin can be clearly distinguished from the river and the FP water from non-marine facies shale in Qaidam Basin. The FP water in Sichuan Basin has slightly higher δ⁷Li values than that of the Marcellus FP water, but has overlap with that of the Yangtze River. The average value of ⁸⁷Sr/⁸⁶Sr of FP water is 0.7197 in Weiyuan and 0.7193 in Changning, which is much higher than that of the produced water from conventional wells in different formations. This is because the Silurian shale is affected by terrestrial siliceous sediments and the underlying Sr-rich fluids. As a result, ⁸⁷Sr/⁸⁶Sr values measured in the Silurian strata in southern Sichuan Basin are high, which makes ⁸⁷Sr/⁸⁶Sr values become an effective index to distinguish shale gas fracturing FP water from conventional gas well produced water and shallow groundwater in Sichuan Basin.