Abstract: The Eastern Sag of Liaohe Depression is one of the areas with the earliest biogas discoveries in China. The shallow biogas has not attracted much attention in the early exploration stage due to the good exploration results in the middle and deep formations, and there is a lack of understanding of source rocks and distribution law of shallow biogas. The geochemical characteristics of shallow gas (reservoir) in drilled wells are systematically analyzed, and a key index of carbon isotope is used to classify shallow gas (reservoir) in Eastern Sag into four types, including primary biogas, bio-transitional zone gas, transformed biogas and mixed type gas. Furthermore, the genesis of shallow gas is discussed, and it is indicated that the shallow biogas is dominated by coal measure genesis. The in-depth study supports to obtain geological understanding that the distribution of shallow biogas was distinctly controlled by coal measure strata, high-quality clastic rock reservoirs, two sets of regional cap rocks in the upper sub-member of the third member of Shahejie Formation (Es₃¹) and the first member of Shahejie Formation (Es₁), and the late formation of structures and late gas charging were conducive to the large-scale gas enrichment in shallow formations. Guided by the above understanding, high-yield gas wells and large-scale reserves have been discovered, opening up a new exploration field of coal measure genetic biogas in Liaohe Depression.