The methane carbon isotope of natural gas, fliud inclusion homogeneous temperature and Kalium—Argon isotope of autogenetic illite of sandstone reservoir are studied.  The time and process of Jurassic gas pool formation of Yinan Well 2 trap are determined. It is put forward that the gas pool formation of Yinan Well 2 is a continuos process and underwent two main stages of the inclusion formation in earlier period and gas pool formation in late period. The natural gas began to charge from Jidike to Kangcun stage that was the main time of inclusion formation. The gas reservoir was mainly formed after the middle Kangcun period. The natural gas migrates from the source rock into the sandstone by direct expulsion. Research shows that the gas pool of Yinan Well 2 is formed due to many factors of short migrating distance, little dessipating, excellent caps and good preserving condition. In conclusion, the time and the process of pool formation in Yinan 2 trap are advantageous to the primary gas reservoir in Kuche Depression. The crucial factor of gas reservoir is the properties of reservoirs.