Pore and pore-throat size types and distributions of two low permeability sandstone reservoirs (E1f3 reservoir in the Shanian area of North Jiangsu basin and J2x reservoir in the Niujuanhu area of Santanghu basin) were analyzed to understand their origins using a series of methods such as grain size analysis, thin slices, pore images, SEM, mercury-injection data. The results show that pore type in the E1f3 reservoir is predominantly carbonate cement and feldspar dissolved pores and that in the J2x is mainly tuffaceous and feldspar dissolved pores, with the former containing more large dissolved pores and the latter containing more small intercrystal pores.