Sensitivity assessment of strontium isotope as indicator of polluted groundwater for hydraulic fracturing flowback fluids produced in the Dameigou Shale of Qaidam Basin

Hydrogeochemical processes that would occur in polluted groundwater and aquifer system,may reduce the sensitivity of Sr isotope being the indicator of hydraulic fracturing flowback fluids(HFFF)in groundwater.In this paper,the Dameigou shale gas field in the northern Qaidam Basin was taken as the study area,where the hydrogeochemical processes affecting Sr isotope was analysed.Then,the model for Sr isotope in HFFF-polluted groundwater was constructed to assess the sensitivity of Sr isotope as HFFF indicator.The results show that the dissolution can release little Sr to polluted groundwater and cannot affect the εSr(the deviation of the 87Sr/86Sr ratio)of polluted groundwater.In the meantime,cation exchange can considerably affect Sr composition in the polluted groundwater.The Sr with low εSr is constantly released to groundwater from the solid phase of aquifer media by cation exchange with pollution of Quaternary groundwater by the HFFF and it accounts for 4.6% and 11.0% of Sr in polluted groundwater when the HFFF flux reaches 10% and 30% of the polluted groundwater,respectively.However,the Sr from cation exchange has limited impact on Sr isotope in polluted groundwater.Addition of Sr from cation exchange would only cause a 0.2%and 1.2% decrease in εSr of the polluted groundwater when the HFFF flux reaches 10% and 30% of the polluted groundwater,respectively.These results demonstrate that hydrogeochemical processes have little effect on the sensitivity of Sr isotope being the HFFF indicator in groundwater of the study area.For the scenario of groundwater pollution by HFFF,when the HFFF accounts for 5%(in volume percentage)of the polluted groundwater,the HFFF can result in detectable shifts of εSr(Δ_(εSr)=0.86)in natural groundwater.Therefore,after consideration of hydrogeochemical processes occurred in aquifer with input of the HFFF,Sr isotope is still a sensitive indicator of the Quaternary groundwater pollution by the HFFF produced in the Dameigou shale of Qaidam Basin.

Investigation of Organic Matter Sources and Depositional Environment Changes for Terrestrial Shale Succession from the Yuka Depression: Implications from Organic Geochemistry and Petrological Analyses

Continental organic-rich shales are well developed in the Dameigou Formation within the Yuka Depression of the Qaidam Basin. Here, the Rock-Eval pyrolysis, biomarkers, organic petrology, and stable carbon isotope have been carried out on the Middle Jurassic Dameigou Formation source rocks from the northwest part of Yuka Depression, Qaidam Basin in order to study their thermal maturity, source of organic matter(OM), and palaeoenvironment changes. The Rock-Eval pyrolysis data(e.g., T_(max)), vitrinite reflectance, and biomarker-derived thermal maturity parameters(e.g., carbon preference index, Ts/(Ts+Tm), C_(29)Ts/(C_(29)Ts+C_(29)αβ hopane), C_(30) αβ/(αβ+βα) hopanes, C_(29)ααα 20S/(20S+20R) steranes, and C_(29)αββ/(αββ+ααα) steranes) suggest all studied samples stay between immature and low mature stage. The maceral compositions, stable carbon isotope compositions, n-alkane distributions, and biomarker-derived source parameters(e. g., C_(27)/C_(29)ααα 20R sterane, ternary diagram of C_(27)-C28-C_(29)steranes, C24 tetracyclic terpane) indicate both aquatic organisms and higher plants are the source of OM in the shales, but land plants are dominant. Generally low gammacerane concentration and environment-related parameters(e. g., cross-plots of C_(27)/C_(29)ααα 20R sterane vs. Pr/Ph) indicate these source rocks may be derived from lacustrine and fluvial-deltaic environments with fresh water, which is also supported by the variations of stable carbon isotopes from OM in the source rocks. However, the stable carbon isotope compositions of OM in the source rocks were influenced by multiple factors(e.g., source types and depositional environment) in the Yuka Depression. Slightly brackish condition is recorded in the upper part of the ZK6-1 well favor the formation of lacustrine algae, as confirmed by high contents of C_(27) steranes and short-chain n-alkanes. The variation of reducing to oxidizing condition of study area is possibly associated with the periodical flooded river-influenced aquatic condition during the deposition of the Middle Jurassic Dameigou Formation.