Abstract: The Hesigewula Sag on the western margin of the Great Khingan is a newly-discovered sag where the hydrocarbon generation potential of its potential source rocks has not been systematically studied.Based on the analysis of organic petrology and organic geochemistry of 160 core samples from three parameter wells and some coalfield exploration wells in the Hesigewula Sag, and combined with tectonic and sedimentary characteristics, the hydrocarbon generation potential of Cretaceous mudstone in the Hesigewula Sag is systematically analyzed.The genetic types of natural gas in Well Hedi 1 are also discussed.The total organic carbon of Lower Cretaceous dark mudstone in Hesigewula Sag is high.The average TOC of K₁bt₁mudstone is 2.20% and the K₁bt₂ is 3.67%.The type of the organic matter is Ⅱ₁-Ⅱ₂.The degree of thermal evolution is relatively low, and it is at the stage of immature-low maturity.The carbon isotope of methane in Well Hedi 1 is relatively light, with contents ranging from -70.5‰ to -68.2‰.Combined with the hydrogen isotope characteristics of methane, it is considered that the natural gas in Well Hedi 1 is biogenic gas, which is produced by the mixture of carbon dioxide reduction and acetic acid fermentation.Because the core samples are generally collected less than 1000 meters at depth, it is presumed that the deep part of the depression(more than 2000 meters) has entered the peak period of oil generation, which has a certain exploration potential.