Isotopic geochemistry of non-hydrocarbons in natural gases from the Sanshui Basin, Guangdong Province, China
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This study is focused on geothermal heat flow and the origin of non-hydrocarbons in natural gases in terms of the isotope geochemical characteristics of Ar, He, CO2 and N2 in natural gases from the Sanshui Basin, Guangdong Province. China.3He/4He ratios are of (1.60-6.39) × 10-6, and40Ar/36Ar ratios of 450–841. The carbon isotopic composition (δl3C PDB) of carbon dioxide ranges from -20‰ to -2‰. δl5N(air) ratios have a wider range of-57 ‰- +95 ‰. The isotope geochemical characteristics of non-hydrocarbons indicate that He, Ar and N2 in the gas reservoirs enriched in non-hydrocarbons were derived largely from the upper mantle. Non-hydrocarbons in gaseous hydrocarbon reservoirs consist mainly of crustal radiogenic He and40Ar and some mantle-derived He and Ar, as well as of13C-depleted carbon dioxide and nitrogen generated as a result of thermal decomposition of organic matter in strata. Carbon dioxide enriched in13C was derived largely from carbonate rocks and partially from the lower crust and upper mantle.
Based on the relationship between geothermal heat flow (Q) and3He/4 He ratio in natural gases, the Q values for the area studied have been calculated. Similar Q values are reported from the upper mantle uplift area (77 mWm-2) in Huabei and the Tancheng-Lujiang Rift Zone (88 mWm-2). More than 60 percent of geothermal heat flow in the Sanshui Basin may have been derived from the upper mantle.
KeywordsCarbonate Rock Kerogen Carbon Isotopic Composition Radiogenic Heat Lanzhou Institute
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