Test resultes of downhole hydrocarbon geo-chemical exploration from five wells in the south of Songliao Basin show that the identification ratio of light hydrocarbon of top air in gas wells and oil-gas indication wells is higher than that in dry wells. The role of acidolysisabsorption hydrocarbon of rock cutting is only auxiliary in hydrocarbon evaluation. The ratio of light hydrocarbon to alkane of the top air in abnormal well sections suggests that the gas wells of the study area possess the characteristics of dry gas