High-precision 40Ar/39Ar age of the gas emplacement into the Songliao Basin

The problem of determining an exact isotopic age of hydrocarbon emplacement is complex because minerals suitable for dating with common isotopic methods are often lacking in the sedimentary domain. However, the igneous quartz from the Cretaceous volcanic rocks that host the gas reservoir in the Songliao Basin (northeastern China), contains abundant secondary fluid inclusions with high concentrations of K and high partial pressures of methane trapped during gas emplacement. Quartz with abundant K-rich fluid inclusions provides an excellent closed system well suited for 40Ar/39Ar dating. Three igneous quartz samples were measured by stepwise crushing to release the inclusion-based argon gas. All three samples yielded well-defined isochrons with ages in close agreement, precisely constraining the gas emplacement at 42.4 +/- 0.5 Ma (2 sigma) below the Daqing oil field in the Songliao Basin, extending possible gas reservoirs from the upper Cretaceous to the middle Eocene.