In Tabamiao area of northern Ordos Basin, natural gas is widespread in Ordovician.However, the commercial gas is mainly produced from the weathered crust lying at the top of Ordovician. In order to track the origin of gas in Ordovician,associated condensate and indigenous gas in Ordovician carbonate source rock in upper and lower Paleozoic were correlated from such geochemical characteristics as component, compounds and isotope.Detailed study showed that (a) the commercial gas in Ordovician weathered crust ...