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 N 2 and CO 2 , 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 d 13 C 1 value in Dabei gas field usually ranges from À31.9% to À29.4%. The d 13 C 2 value generally ranges from À24.2% to À19.4%, which is obviously heavier than À28.5%, The natural gases in Dabei gas field can be identified as typical coal-formed gas. (3) Triassic-Jurassic, mainly contributed by Jurassic in vertical strata and coal rocks in type, coal rocks accounting for 63% and mudstones accounting for 37%.

Comparison on Rare Gas Geochemical Characteristics and Gas Originations of Kuche and Southwestern Depressions in Tarim Basin, China

“…(6) Finally, the separated rare gases were sent into rare gas isotope mass spectrometer to determine their isotopic compositions by method of peak height ratio on ion current signal intensity. The accuracies of molecular composition abundances of rare gases helium, neon, argon, krypton, and xenon were estimated to be ±3.36%, ±3.66%, ±1.32%, ±2.99%, and ±6.96% [23], and the accuracies of isotopic compositions of rare gases 3 He/ 4 He, 20 Ne/ 22 Ne, 40 Ar/ 36 Ar, 38 Ar/ 36 Ar, 129 Xe/ 130 Xe, and 132 Xe/ 130 Xe were estimated to be ±4.50%, ±1.32%, ±1.27%, ±1.39%, ±1.63%, ±1.84%, and ±2.13%, respectively.…”

“…Molecular composition abundances and isotopic compositions of rare gases were measured in the Key Laboratory of Gas Reservoir Formation and Development of CNPC. In order to reduce impacts of rare gases in the air on sampling and experimentation and obtain correct rare gas analyzed results, some special gas sample collecting methods were adopted [23]. The detailed analytic processes were listed as follows [15,23].…”

“…In order to reduce impacts of rare gases in the air on sampling and experimentation and obtain correct rare gas analyzed results, some special gas sample collecting methods were adopted [23]. The detailed analytic processes were listed as follows [15,23]. (1) A natural gas cylinder was connected to the injection port of the device through a pressure relief valve, and a mechanical pump, a molecular pump, and an ion pump were utilized to obtain low, high, and ultrahigh vacuum for the stainless steel pipeline separately.…”

“…The 132 Xe/ 130 Xe value of natural gases ranges around 6.628~6.809 with an average of 6.705 in the former and 6.754~6.828 with an average of 6.791 in the latter ( Table 2). The relatively depleted 129 Xe and the excessive 132 Xe in gas fields from the Kuche Depression as well as the Akemomu gas field from the Southwestern Depression indicate that xenon has chiefly crustal-derived genesis (Figure 9), while the Changshen and Xushen gas fields in the Songliao Basin have a relatively excessive 129 Xe rather than 132 Xe, showing that xenon is obviously mixed with mantle-derived genesis [23].…”

“…The molecular and isotopic compositions of rare gases in natural gases can provide a lot of useful geochemical and geological information. They have extensive prospects on studies of gas genesis, migration, origination, deep mantled fluid action, volcanic moment, and earthquake activities ( [1]; Xu et al, 1994;[5]; Liu and Xu, 1987; [18,19]; Xu, 1996; [15][16][17][20][21][22][23][24]). Because Rn is a radioactive element, the rare gas studies on oil and gas geology usually include helium, neon, argon, krypton, and xenon.…”