CARBON ISOTOPES OF HYDROCARBONS AND CARBON-DIOXIDE IN NATURAL GASES IN CHINA

130 natural gases in the continent of China were determined for the ab undance and carbon isotopes of hydrocarbons and carbon dioxide. The de lta(13)C (PDB) values of methane range from -68 to -24.4%, and the lar gest isotopic variability for hydrocarbons is of delta(13)C(1) < delta (13)C(2) < delta(13)C(3) < delta(13)C(4) < delta(13)C(5). These charac teristics suggest that the gaseous hydrocarbons originated principally from thermal and/or microbial decomposition of organic matter. The wi de variations of delta(13)C(n) distribution patterns (n = 1, 2, 3, 4 a nd 5) can be explained by (1) extensive mixture between thermogenic an d microbial gases, or between thermogenic gases produced during differ ent thermal stage, and/or (2) preferential microbial oxidation. There is no direct evidence for the presence of abiogenic hydrocarbon in nat ural gases in China. One sample from Songliao basin shows a heavier de lta(13)C(1) (-24 parts per thousand) and reverse isotopic distribution pattern (delta(13)C(1) > delta(13)C(2) > delta(13)C(3)). Concerning t he occurrence of mantle-derived He, Ne, Ar and Xe in the south Songlia o basin, we propose that the methane mantle derived and heavy hydrocar bons (ethane, propane, butane, etc.) formed from polymerization of the mantle-derived methane. The delta(13)C values for carbon dioxide rang e from -13.9 to + 13.5 parts per thousand suggesting the multiple orig ins. Gases with CO2 concentrations greater than 10% have a narrow rang e of delta(13)C values, from -8 to -2 parts per thousand, and show hig her He-3/He-4 ratios (> 0.1 times the atmospheric value), suggesting t hat a mantle-derived component is diluted by CO2 derived from carbonat e and/or organic matter. Positive delta(13)C values (+4 - + 14 parts p er thousand) indicate CO2 reduction to CH2. CO2 with trace concentrati on in CH4-rich gases can be attributed to microbial decomposition of o rganic matter in sedimentary rocks. (C) 1997 Elsevier Science Ltd.