In this paper we first indicate the sources of oils (condensates) in gas reservoirs containing oil or condensate in Sichuan basin according to the components of C4—C7 light hydrocarbons, and then combine them with the carbon-isotopic data of natural gases to determine their gas sources. This method is reasonable especially for judging the different sources in a gas. The oil-(condensate-) bearing gases in Sichuan basin were originally thought indigenous. From the identification of gas sources by this method,however, we have found out that sources--mixed gases are widespread.In
view of gas migration，the accumulation of commercial gases of the Middle
and Lower Triassic formation in Sichuan basin is frequently related to
faults