Abstract：The Guang'an gasfield is a large one newly discovered in the Sichuan basin in 2006 and has identified reserves of more than 500×108 m3. It is also the largest gasfield ever discovered in the Upper Triassic strata in the Sichuan basin up to now. Because the gas generation intensity of Upper Triassic hydrocarbon source rocks in the study area is less than 20×108 m3/km2, theoretically, no ideal conditions for the formation of a large or medium-sized gasfield are available in the area, and consequently the genetic type and source of natural gas in the gasfield are a problem that needs urgent solution. An intensive analysis of the components and carbon isotope data of the gas shows that the gas is coal gas. According to a correlation of gas sources, the possibility that the overlying and underlying source rocks provide gas is ruled out. In other words, the formation of the Guang'an gasfield is due to self-generation and self-accumulation of gas in the coal measures of the Upper Triassic Xujiahe Formation. Finally, an analysis of the gas migration pathway reveals that the proximal lateral migration is the main cause for the formation of the large Guang'an gasfield.