Based on the mercury penetration analysis of more than 300 samples and analysis result s of more than 260 casts and more than 100 image pores , and referred to the classification criterion about clastic reservoir , especially low permeability reservoir , the low permeability sandstone reservoirs in Ordos Basin were divided into four types and four sub-types. The four types include low permeability reservoir ( I) , extra-low permeability reservoir ( II) , super-low permeability reservoir ( III) and tight reservoir ( IV) . Among them extra-low permeability reservoir and super-low permeability reservoir are the main reservoir types of Triassic Yanchang Formation in Ordos Basin. According to t he characteristics of t he pore structure and petrophysics and the classification criterion of the reservoir thickness , they are divided into two
sub-types respectively.