Isotope Characters of the Large to Middle Sized Gas Fields in West China and Prepared Gas Resource for "Transportation ofWestern Gas to East China" Progect

The increase of gas reserves is basically ensured by Large to middle sized gas fields. By end of 2000, reserves of 29×10 8 m 3 of natural gas has been probed in China. 75% of reserves is distributed in West and mainly in four large basins.Predominantly based on isotopic data, properties of typical large to middle sized gas fields in these basins are geochemically discussed. The coal type gases, esp., higher mature coal-type gases, in some pre continental subbasins in Trim basin are present day and potential gas resources. In Sichuan basin dominant gas resource is post mature thermal cracking gas. Two worldwide large gas fields have been probed in Ordos basin, namely, Changqing gas field and Suligemiao gas field, former is formed by mixing of coal type gas and oil-type gas, latter is coal type gas. The Sanhu large to middle sized gas fields in Qaidam basin is coal type biogenic gas. These large to middle sized gas fields presently guarantee gas resource for the transportation of west gas to East China project. The determination of natural gas origins of gas fields provide valuable scientific information for exploration of natural gas reserves. Key words:large to middle sized gas field;C isotope;geochemistry