Abstract: Preliminary analysis on gas accumulation and reservoir forming period in Songnan gas field has been conducted using fluid inclusion technique. The study results indicate that there exists four phases of hydrocarbon gas filling, including depositional period of Quantou Formation, final depositional period of Qingshankou Formation, final depositional period of Yaojia Formation and the final depositional period of Nenjiang Formation. This study has revealed that the Songnan gas field is characterized by multi-periods of gas filling and reservoir forming. Due to the effect of the Neogene deep fault, the gas field is also filled by mantle-derived C0₂, becoming a mixed type of gas reservoir.