Oil and gas often contains a certain amount of rare gas, which is often used to distinguish the source or type of oil and gas, especially in the identification of inorganic genetic gas at present, rare gas has unique advantages. Xu Yongchang et al. (1996) made extensive research on rare gases in natural gas in China. In the analysis of several basins in eastern China, they found that the 3He/4He values of these rare gas fields, which have reached industrial level, are mostly several Ra (atmospheric 3He/4He ratio); 40Ar/36Ar has mantle characteristics (40Ar/36Ar = 450 ~ 3069), indicating that mantle-derived components are involved. Combined with the specific tectonic environment in these areas, they think that these mantle-derived rare gases are closely related to the fault system, and use the age accumulation effect of he and Ar to establish the he and Ar isotopic dating formula of natural gas. Rare gases are also used to determine the contribution of different sources in natural gas. The study of Ballentine( 1996) shows that the oil and gas in the North Sea are doped with mantle-derived he and ne. He calculated that 2.3% ~ 4.5% of 4He and 4.3% ~ 6.29% of 2 1Ne were mantle-derived. Sherwood et al. (1994) found that 39.8% of He and 58% of 2 1Ne were mantle sources in the study of natural gas in Pennonian and Vienna Basin in Hungary and Austria. Rare gases are also used to explore the location of the mantle exhaust center. Weinlich et al. (1999) found that the gas composition of the four exhaust centers changed regularly from the center to the edge, that is, the center was dominated by CO2 and changed from N2 to the edge; The central mantle source accounts for 60%, and the 3He/4He ratio is 5Ra. Edge 3He/4He is less than 2.0. Zhu Ming et al. (1997) and Liu Qiang et al. (2000) used Ar isotopes to identify the direction of oil and gas migration, and thought that the direction where the 40Ar/36Ar value became smaller might be the direction of oil and gas migration.
Isotopes of 1. argon
When studying the age aggregation effect of argon source rocks, the source rocks and oil-gas traps are regarded as a closed system, and the 40Ar formed by 40K decay in the source rocks migrates into the oil-gas traps together with the oil and gas formed by the source rocks. At this time, 40Ar/36Ar is a function of the age and K content of the source rocks. Further ignoring the influence of the change of K content, the K-Ar age of source rocks can be roughly estimated by using the 40Ar/36Ar ratio of natural gas.
40ar/36ar > of the crust; 300, and it becomes larger with the aging of rocks, that is, it has an age accumulation effect. The maximum value of 40Ar/36Ar in the upper mantle or MORB source region is 64000, mainly distributed between 1000 ~ 12000, and the low value of 40Ar/36Ar is generally explained as the result of atmospheric Ar mixing. The 40Ar/36Ar ratio of OIB (ocean island basalt) source region is between atmospheric value (295.5) and 8000, but generally lower than that of upper mantle or MORB (mid-ocean ridge basalt).
2. Isotopes of helium
Helium is an important natural gas resource, mainly from natural gas reservoirs. Helium mainly comes from α decay of U and Th. Because 3He has remained basically unchanged in geological history, the helium isotope in helium reservoir has a very small 3He/4He value. For example, the helium isotope value of Weiyuan gas reservoir, the only large helium reservoir in China, is about 2× l0-8. In the study of natural gas geochemistry, helium is a research method on the one hand-it can provide the historical track of geology and geochemistry of the research object, especially the crust and mantle characteristics of the object; On the other hand, helium is an important natural gas resource. Therefore, the geochemistry of helium plays a special role in the study of natural gas.
Among rare gases, he and Ar isotopes are the most studied and applied, and He isotopes are the most studied. Both the degassing of the upper mantle and the new deep mantle contain primitive he and solar Ne, which are not affected by the secondary atmosphere or crust, while the isotopic ratios of various rare gases in the atmosphere are quite stable, such as 3He/4He = 1.4× 10-6, 40Ar/36Ar=295.5, 20Ne/22Ne=9.80. The 3He/4He ratio of the earth has changed from 10-5 of mantle rocks and mantle fluids to 10-8 of continental crust source areas, which has changed by several orders of magnitude. The typical 3He/4He value of MORB is (8 1) Ra (Ra is the atmospheric 3He/4He value, 1.4× 10-6), with a narrow range of variation, while the 3He/4He ratio of OIB to mantle plume is greater than 8 ~ 8.5 Ra, that is, higher than MORB value. Many studies on He isotopes believe that high He isotope ratio is one of the main characteristics of mantle plume or large igneous province. The 3He/4He ratio of the earth's crust is generally in the order of 10-8, and its end member value is generally recognized as 2× 10-8. In particular, the 3he/4He end-member difference between crust and mantle is as high as three orders of magnitude, and a small amount of mantle-derived He is added to crust and rock, so it is easy to identify with the 3He/4He ratio, which also has obvious identification effect on the interaction between mantle plume and lithosphere.
3. Isotopes of neon
For a long time, because the problem of separating helium and neon by cryogenic separator has not been solved in experimental technology, accurate neon isotope values have not been measured in our own laboratory in the study of rare gases.
There are systematic differences in neon isotopic compositions between MORB and OIB. The MORB's 20Ne/22Ne≥ 12.5, 2 1Ne/22Ne≥0.07 are obviously higher than the atmospheric ratio (20Ne/22Ne=9.8, 2 1Ne/22Ne=0.029). The 2 1Ne/22Ne value of OIB is lower than that of MORB source region.
4. Xenon isotope
The research of Xe isotope started late and accumulated relatively little data, but it also has important tracing significance and research value. The crustal value of 129Xe/ 130Xe is about 6.5 (close to the atmospheric value). Staudacher et al. found that 129Xe in mid-ocean ridge basalt is "surplus", with an abnormal value of 129Xe/ 130Xe, which is 7% higher than the atmospheric value (6.48). This anomaly exists in all MORB samples in major ocean basins. The maximum values of MORB, 129Xe/ 130Xe and 136Xe/ 130Xe, are also considered to be representative, which were measured from the broken rocks of MAR, and were 7.73 and 2.57 respectively, which was further confirmed in the later MORB research. The existing research data show that the values of 129Xe/ 130Xe and 136Xe/ 130Xe in OIB source region or mantle plume are generally higher than those in MORB source region by more than 10%.
As early as 1970s and 1980s, excess xenon isotope 129Xe was found in gas wells in the United States and Australia relative to the continental environment. Excess 129Xe represents the decay products of extinct radionuclides 129I formed in the early crust. If there is an excess of 129Xe in natural gas, it means that the original composition of the earth has been added. Xu Sheng (1997) studied the xenon isotopes of 30 gas samples in the east. The results show that the values of main samples are124xe/126xe/130xe, 128Xe/ 130Xe. The maximum over-standard amount of samples in Wanjinta area of Songliao Basin is 4%, which is not very large compared with the over-standard amount of 129Xe l5% of MORB (mid-ocean ridge basalt). But it still clearly reflects the increase of mantle material in the deep part of the earth. Using these data, Xu Sheng further discussed some problems of material exchange between crust and mantle and earth evolution. Sun Mingliang (200 1) developed the measurement technology of heavy and rare gases krypton and xenon in natural gas and rare gases in rock inclusions, and carried out corresponding research work on this basis. Liu studied the isotopic composition characteristics of rare gases krypton and xenon in Ordos basin, and tried to use them for gas source correlation, and achieved positive results.