Abstract: Jurassic gas pool in Well Yingnan 2, Yingjisu seg of Tarim basin is characterized by deep-basinal gas pool. Source rocks with higher hydrocarbon-generating ratio are distibuted widely in Paleozoice stratum with regionally developed deep seg. Hydrocarbon-generating center locates in the deep seg with down-dip tight sandstone successively distributed on the better quality of sandstone reservoir. The closure mechanism of gas pool's cap rock is resulted from the process of capillary pressure and water-sealed event that could augment the capillary pressure,becoming seal of oil and gas, preventing from the vertical diffusing of them which allow gas accumulated in the better quality of sandstone reservoir and finally formned the oil-gas pool. During the long geological history, gas would partially diffuse to upper strata from the pool due to its higher diffusivity. At the same time,source rocks in the sag generating hydrocarbon, which supplemented the diffused gas, forming a gas pool with dynamic balance of diffusion and supplement.

Key words: Tarim basin, Yingjisu sag, gas pool, reservoir-forming model, tight sandstone, cap rock, deep-basinal gas reservoir