Chemical composition and stable carbon isotopic studies were undertaken for 27 gas samples from deep strata of theXujiaweizi Depression in the Songliao Basin to investigate their origin. Gas molecular and carbon isotopic compositions showgreat variety. Methane is the main component for all studied samples and its content ranges from 57.4% to 98.2% with anaverage of 90.1%. Gas wetness ranges from 0.8% to 16.7% with an average of 2.7%. The main non-hydrocarbon gases arecarbon dioxide and nitrogen with an average of 4.0% and 3.2%, respectively. Carbon isotope data suggest that these deep stratagases are mainly coal-type gases mixed with minor amounts of associated (oil-type) gases. Coal-type gases are characterized byheavier carbon isotopic values and drier chemical compositions. These gases were generated from the Lower CretaceousShahezi Formation coals interbedded shales with type III kerogen during the postmature stage of hydrocarbon generation. Oiltypegases are characterized by lighter carbon isotope and higher wetness, which were generated from the Lower Cretaceousshales with type II kerogen in the shallow strata during the early mature stage of hydrocarbon generation. Mixing of twodifferent gases causes unusual carbon isotopic distribution patterns, with lighter isotopic values in higher numbered carbons inmost gases. The discovery of coal-type gases in the Songliao Basin provides new prospects for the exploration in this region.