Due to a lack of suitable minerals, the gas/oil emplacement ages have never been accurately obtained before. CH4-CO2-saline-bearing secondary inclusions are found in quartz from the volcanic rocks of the Yingcheng Formation, the container rocks of the deep CO2 gas reservoir in the Songliao Basin. The inclusion fluid was trapped into microcracks in quartz during the gas emplacement and accumulation, providing an optimal target for the 40Ar-39Ar stepwise crushing technique to determine the CO2 gas emplacement age. 40Ar-39Ar dating results of a quartz sample by stepwise crushing yield a highly linear-regression isochron with an age of 78.4±1.3 Ma, indicating that the accumulation of the deep CO2 gas reservoir in the Songliao Basin occurred in the late Cretaceous. This is the first time to report an exact isotopic age for a CO2 gas reservoir, which indicates that the 40Ar-39Ar dating can serve as a new technique to date the oil/gas emplacement ages.