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With the increasing number of shale gas wells being hydraulically fractured with recycled flowback and produced water to reduce the consumption of fresh water, geochemical indicators of hydraulic fracturing flowback fluids (HFFFs) from such wells need to be found to evaluate the potential pollutants caused by shale gas development. To fill this knowledge gap, we analysed 64 HFFF samples from 3 Changning wells (fracked with recycled wastewater) to form a time series and compared them to HFFF samples from 3 Weiyuan wells (fracked with fresh water). We also made comparisons of HFFFs from two types of wells in the Appalachian Basin with reported data. The results indicated that regardless of the fracturing fluid being used, the water‒rock reaction caused by the hydraulic fracturing process resulted in the release of exchangeable phase elements, such as lithium, boron, with relatively depleted δ7Li and δ 11B, and strontium with relatively enriched 87Sr/86Sr values on the shale surface, into the HFFFs. The composition of the injected fracturing fluid affected the strength of the water‒rock reaction, but the effect is negligible, as the fracturing process can still form the unique Sr, Li, and B isotope ratios of the HFFF. Thus, the elemental signatures (B/Cl and Sr/Cl) and isotopic fingerprints (δ 11B, δ7Li, and 87Sr/86Sr) derived from HFFFs can be used to distinguish between HFFFs from wells hydraulically fractured by either fresh water or recycled wastewater and flowback water from conventional oil and gas wells. The results are important for the environmental evaluation of the large number of wells being fracked with recycled wastewater each year. [Display omitted] • δ 11B and 87Sr/86Sr can be used as indicator for HFFF in Changning field. • Hydraulic fracturing caused the release of minerals enriched in 10B, 6Li, and 87Sr. • Shale formation water composition determined the ratio between Na, Ca, Cl, and Br. • Fracturing fluid composition impact the major elements of HFFFs in the first dozens of days. [ABSTRACT FROM AUTHOR]

According to the compositional and isotopic analyses of He, CO 2 and CH 4 of 95 gas samples from the Ordos Basin and 76 gas samples from the Bohaibay Basin in China, this study carried out a detailed investigation on the geochemical characteristics of helium and CO 2 from the cratonic and rift basins, especially the cratonic Ordos Basin and rift Bohaibai Basin in China. Compared to the rift Bohaibay Basin, the cratonic Ordos Basin has lower level of helium and CO 2 , lower R/Ra ratio and lower δ 13 C CO2 maximum value. When the content of CO 2 is < 10%, both cratonic and rift basins have organic CO 2 associated with the hydrocarbon generation process and inorganic CO 2 from the thermal decomposition of carbonates or dissolution of organic acid. Rift basin can also have CO 2 > 20% and δ 13 C CO2 of − 6 ± 2‰ or − 4‰ –− 9‰, which is of volcanic-magmatic or mantle-derived origin, while cratonic basin has no such type of CO 2 . The cratonic basins (Ordos, Sichuan) have CO 2 < 5% and relatively smaller variation range of δ 13 C CO2 , i.e., 25.6‰ (− 0.8‰ –− 26.4‰). The rift basins have higher level of CO 2 , R/Ra ratios and δ 13 C CO2 values. The content of CO 2 can reach 100%, the R/Ra ratio can be up to 6.45 and the variation range of δ 13 C CO2 is 34‰ (7‰ –− 27‰). Bigger variation range of δ 13 C CO2 implies stronger tectonic activities, which is not favorable for the gas accumulation. While smaller variation range means more stable structure, which is favorable for the discovery of large gas fields. Sample in the cratonic basin in China generally has CH 4 / 3 He of 10 9 – 10 12 and R/Ra < 0.1 and in the rift basin it has CH 4 / 3 He of 10 5 – 10 13 and R/Ra of 0.1– 6.45. It is concluded that methane with CH 4 / 3 He ≤ 10 8 is of inorganic origin and methane with CH 4 / 3 He ≥ 10 11 is of organic origin. The regression equation for the CH 4 / 3 He-R/Ra in the cratonic basin is: y = 3E + 11e − 46.28x and it is: y = 4E + 10e − 1.791x in the rift basin. Both Ordos and Bohaibay basins found gas wells with He > 0.05%, which has significant commercial values and provides a clue for the exploration of helium-rich gas fields. Traps with R/Ra > 2 is a favorable clue for the discovery of CO 2 gas fields. Seven CO 2 gas fields have been found in the Bohaibay Basin, but no systematic development has been carried out and only some liquid CO 2 has been injected for tight oil from the Huagou CO 2 gas field. [ABSTRACT FROM AUTHOR]

The Zizhou gas field is a large, tight sandstone gas field in the Ordos Basin, China. Based on the analysis of the geological background, gas components, light hydrocarbon composition, carbon and hydrogen isotope values of alkane gases, and the geochemical correlation between gases of the Zizhou gas field and analogs from other fields in the basin, we conclude that : (a) most natural gases in the Zizhou gas field are dry with a small content of wet gas; (b) the gases have similar carbon isotopic and light hydrocarbon compositions to the analogs from other fields, as well as much more positive δ 13 C values and high contents of C 5–7 i -alkanes and methylcyclohexanes, indicating typical coal-derived gas; (c) carbon isotopic reversal in some gas samples is due to mixing of coal-derived gases of different maturities; (d) the gases are sourced from mature to high-mature Carboniferous–Permian coal-measures; and (e) the gas reservoirs represent continuous accumulations. [ABSTRACT FROM AUTHOR]