Isotopic Geochemical Traces of Gaseous Hydrocarbons for Natural Gas Migration

A lot of theoretical and practical studies have been done on applying the carbon and hydrogen isotopic compositions of gaseous hydrocarbons to indentify the type of natural gas and trace the type and maturation of potential gas source rocks. However, there are few studies on the application of the isotopic character of natural gas to gas migration. The main argument point on the above case is whether the isotopic fractionation occurs druing gas migration. In this paper, according to the characteristics of the carbon isotopic composition from acidlysis gas, canned gas and natural gas of Ordos and other oil - gas bearing basins, the changes of carbon isotope compositions of gaseous hydrocarbons during gas migration are discussed and the following conclusions are obtained.1.While natural gas migrates a long distance through micro fractures and the pore system in strata, a certain isotopic fractionation for methane occurred but little isotopic fractionation for heavier hydrocarbons.2. A relatively good concordant and comparative relation exsits between the carbon isotope compositions of heavier hydrocarbons from acidlysis gas of source rocks and those from natural gas orginated from the same source rocks. The correlation of direct gas and its source rocks was carried on from this coincidence.3. The carbon isotope ratios of methane from canned gas are equal to those of natural gas at gas production layers and show much heavier than the normal increase tendency with depth near gas production layers.The exsitence of gas layers may be indicated according to the vertical isotope characters of methane from canned gas.