Rare gases geochemical characteristics and gas source correlation for Dabei gas field in Kuche depression, Tarim Basin

Abstract

The total components and isotopes of rare gases as well as hydrocarbon gases for Dabei gas field are analyzed firstly, then their geochemical characteristics and genesis are discussed respectively, finally gas source correlation and their contributions are evaluated. The research results show that: (1) In Dabei gas field, methane is the main component, accompanying with a few N2 and CO2, as well as extremely low rare gases. In total component contents of rare gases, the He content is from 56.0 to 60.4 ppm with an average of 57.8 ppm and generally 1 order of magnitude higher than the air content value, while the Ne, Ar, Kr, and Xe contents are generally 1–3 orders of magnitude lower than the corresponding air content values. (2) The δ13C1 value in Dabei gas field usually ranges from −31.9‰ to −29.4‰. The δ13C2 value generally ranges from −24.2‰ to −19.4‰, which is obviously heavier than −28.5‰, The natural gases in Dabei gas field can be identified as typical coal-formed gas. (3) The 3He/4He value in natural gases is generally from (6.31–11.24) × 10−8 with an average value of 8.42 × 10−8. The 20Ne/22Ne value is from 9.563 to 9.734, and the 21Ne/22Ne value is from 0.0298 to 0.0307. The 40Ar/36Ar value is usually from 390 to 858 with an average value of 552, and the 38Ar/36Ar value is from 0.1955 to 0.2035. The 129Xe is relative loss, while the 132Xe is relative surplus. The comprehensive genesis identifications of He, Ne, Ar, and Xe indicate that rare gases are crustal derived genesis, mainly originating from the decay of related radioactive elements in crust. (4) The gas source correlation indicates that the natural gases in Dabei gas field are originated from humic coal series source rocks of Triassic-Jurassic, mainly contributed by Jurassic in vertical strata and coal rocks in type, coal rocks accounting for 63% and mudstones accounting for 37%.