Origin of Nature Gas and Gas Filling Pattern in West Sag,Liaohe Depression

Natural gas origin is classified by chemical and isotopic compositions of natural gas in the west sag of Liaohe depression.We discuss the gas filling pattern by means of conditions of gas accumulation.The low-mature coal-type gas and oil-biodegraded gas are dominant in the west sag,such as Shuangtaizi area and Gaosheng as typical case.The primary gas pool of Shuangtaizi was formed since the Dongying Phase,and the coal type gas sourced from the middle-upper part mudstone in three member of Sha Formation was resolved from oil and accumulated in shallow strata series or migrated into gas trap along faults.The oil-biodegraded gas was formed before 24Ma and accumulated at top of heavy oil layers by short distance migration in the Gaosheng area.