Geochemical Characteristics of Gas and Flowback Water in Lake Facies Shale: A Case Study From the Junggar Basin, China

The chemical and isotopic compositions of the natural gas and the co-produced flowback water from the XJC 1 well in Junggar Basin, China, were analyzed to determine the origin of gases in the Permian Lucaogou Formation (P(2)l) and the Triassic Karamay Formation (T(2)k) in the Bogda Mountain periphery area of the Southern Junggar Basin. The value of carbon isotope composition of the P(2)l lacustrine shale gas in the Junggar Basin was between the shale gas in Chang 7 Formation of Triassic (T(1)y(7)) in the Ordos Basin and that in the Xu 5 Formation of Triassic (T(3)x(5)) in the Sichuan Basin. The difference in gas carbon isotope is primarily because the parent materials were different. A comparison between compositions in the flowback water reveals that the P(2)l water is of NaHCO3 type while the T(2)k water is of NaCl type, and the salinity of the latter is higher than the former, indicating a connection between P(2)l source rock and the T(2)k reservoir. In combination with the structural setting in the study area, the gas filling mode was proposed as follows: the gas generated from the lacustrine source rocks of the Permian Lucaogou Formation is stored in nearby lithological reservoirs from the Permian. Petroleum was also transported along the faults to the shallow layer of the Karamay Formation over long distances before it entered the Triassic reservoir.

Automated summary

The study analyzed the chemical and isotopic compositions of natural gas and co-produced flowback water from the XJC 1 well in Junggar Basin to determine the origin of gases in the Permian Lucaogou Formation and the Triassic Karamay Formation. The carbon isotope composition of P(2)l lacustrine shale gas in Junggar Basin suggests a connection between the source rock and the reservoir. The gas filling mode proposed involves gas generated from lacustrine source rocks in the Permian Lucaogou Formation being stored in nearby lithological reservoirs before being transported to the Triassic reservoir.