Abundant natural gas resources have been discovered in the Lower Triassic Feixianguan Formation and Upper Permian Changxing Formation in northeast Sichuan Basin

revealing good potential for natural gas exploration.Based on an analysis of the characteristics of hydrocarbon source rocks in northeast Sichuan Basin

we identify the major hydrocarbon source rocks by the comparision of the natural gas composition and isotope features of the Changxing and Feixianguan formations with that of the Carboniferous reservoirs.The Changxing and Feixianguan formations were densely sampled to measure their organic carbon contents and to quantitatively calculate bitumen contents of the reservoirs.Meanwhile

the total gas volume from oil-cracking in Puguang ancient oil reservoirs were calculated based on the principle of conservation of matter.The study shows that the Longtan Formation source rocks are the major source rocks for Changxing and Feixianguan formation gas reservoirs.The kerogen-cracking contributed more gas than secondary cracking of crude oil

thus gas from kerogen-cracking is the primary gas source.The total gas generated from oil cracking is 361 BCM in the Puguang gas pool

which is not large enough for the forming of the huge Puguang gas pool.This further proves the conclusion that the major gas for the Changxing and Feixianguan formation gas reservoirs comes from kerogen-cracking gas.