ISOTOPE GEOCHEMISTRY OF NON-HYDROCARBONS IN NATURAL GASES FROM SANSHUI BASIN,GUANGDONG PROVINCE, CHINA
Du Jianguo Liu Wenhui National Key Laboratory of Gas-Geochemistry, Lanzhou Institute of Ceology, Academia Sinica, Lanzhou, 730000
Heat flow and the origin of non-hydrocarbons in natural gases are discussed in terms ofthe isotope geochemical characteristics of Ar, He, CO_2 and N_2 in natural gases from the San-shui Basin, Guangdong Province, China. ~3He/~4He ratios have a range of (1.60--6.39)×10~(-6),and ~(40)Ar/~(36)Ar ratios range from 450 to 841. The δ~(13)C(PDB) values of carbon dioxide rangefrom --20 to --2‰ δ~(15) N(air) ratior have a wider range of --57 to +95‰. The isotope geo-chemical characteristics of non-hydrocarbons ndicate that most of the He, Ar and N_2 in thegas reservoirs are enriched in non-hydrocarbons derived from the upper mantle, and carbondioxide was derived mainly from carbonate strata. Non-hydrocarbons in gaseous hydrocarbonreservoirs consist mainly of crustal radiogenic He and Ar and some amounts of mantle-derivedHe, Ar, and carbon dioxide depleted in ~(13)C and nitrogen generated from the decompositionof organic matter in the strata. Based on the relationship between heat flow (Q) and ~3He/~4He ratio in natural gases, va-lues of Q for the study area have been calculated. The calculated mean value of Q (78 m-Wm~(-2)) is coincident with that for the areas which have experienced upper mantle uplifting inHuabei and the Tanlu rift zone (77 and 88 mWm~(-2), respectively). More than 60 percent ofLeat flow in the Sanshui Basin was derived from the mantle.