Isotopic Geochemistry of Non-hydrocarbons in Natural Gases from the Sanshui Basin,Guangdong Province,China

This study is focused on geothermal heat flow and the origin of non-hydrocarbons in naturalgases in terms of the isotope geochemical characteristics of Ar,He,CO;and N;in natural gasesfrom the Sanshui Basin,Guangdong Province,China.;He/;He ratios are of（1.60-6.39）×10;,and;Ar/;Ar ratios of 450- 841.The carbon isotopic composition(δ;CPDB)of carbon dioxide ranges from-20‰ to-2‰.δ;N（air）ratios have a wider range of-57‰-+95‰.The isotope geochemical characteristics of non-hydrocarbons indicate that He,Arand N;in the gas reservoirs enriched in non-hydrocarbons were derived largely from the upper man-tle.Non-hydrocarbons in gaseous hydrocarbon reservoirs consist mainly of crustal radiogenic;Heand;Ar and some mantle-derived He and Ar,as well as of;C-depleted carbon dioxide and nitro-gen generated as a result of thermal decomposition of organic matter in strata.Carbon dioxideenriched in;C was derived largely from carbonate rocks and partially from the lower crust and up-per mantle.Based on the relationship between geothermal heat flow（Q）and;He/;He ratio in naturalgases,the Q values for the area studied have been calculated.Similar Q values are reported fromthe upper mantle uplift area(77 mWm;)in Huabei and the Tancheng-Lujiang Rift Zone(88 mWm;).More than 60 percent of geothermal heat flow in the Sanshui Basin may have beenderived from the upper mantle.