Isotopic Geochemistry of Nonhydrocarbons in Natural Gas from Sanshui Basin, Guangdong
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摘要: 根据广东省三水盆地天然气中氦、氩、二氧化碳和氮等非烃组分的稳定同位素地球化学特征,探讨了该区天然气的来源以及大地热流。测得天然气中3He/4He（R）值为（1.60—6.36）×10-6,比大气的3Ne/4He（Ra）值(1.4×10-6)大:氩的稳定同位素组成(40Ar/36Ar=450—841)较大气氩富40Ar;二氧化碳的δ13C（PDB）值在-20—2‰的范围内,δ13N（Air）值在-57—+95‰之间:根据（?）He,4He值求得研究区的大地热流值（Q）为72—82mWm-2。大地热流和非烃组分同位素组成的高值以及研究区特别发育的火山岩等地质资料表明三水盆地有较强的地球深部流体（物质的和热的）向上溢出。一些油气藏中相当一部分氦、氩和氮来自地幔,各种天然气中均混有地壳来源的非烃气体。贫13C的二氧化碳气主要为地层中有机质分解的产物;富13C的二氧化碳则主要来自岩石化学反应的产物,并混有深部来源的二氧化碳。相当一部分大地热流源于上地幔。Abstract: Based on geochemical characteristics of stable helium, argon, carbon and nitrogen isotopes for natural gas, origins of nonhydrocarbons and fluids are discussed. 3He/4He ratios in natural gas are 1.14-4.54 times as much as that in the air. Stable isotope compositions of argon of natural gas are rich in 40Ar (40Ar/ 36Ar=450-841).Carbon isotope compositions (δ13CPDB) of carbon dioxide vary from-20‰ to -2‰.The δ15 Nvalues of natural gases range between -57.7‰ and 95.1‰.The values (Q) of heat fluid in the Sanshui Basin calculated depend on 2He 3He ratios, change from 72 mWm 2to 82 mWm-2.The high values of heat fluid and isotopic ratios of He. Ar. C and N have a relation to ancient volcanos. The geochemical qualities of stable isotopes of He, Ar, C and N in natural gases indicate that there are strong deep (matter and heat) fluids in the Sanshui basin.Part of He, Ar and N in some reservoirs came from the mantle. Nonhydrocarbons originated from the crust are mixed in various kinds of natural gases. Carbon dioxide in natural gases is mainly originated from the crust, CO2 depleted in 13C is mainly originated from degradation of organic matter in strata; but CO2 enriched in 13C are from reaction between carbonate rocks and under ground water and thermal fluids, and mixed in CO2 from the deep earth. An amount of heat fluid was derived from the upper mantle.
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