Abstract: Microscopy and scanning electron microscopy identifications of cast thin sections of sandstones from the Yanchang Formation of the Ordos Basin have been made in view of the characteristics of low and ultra-low porosity and permeability.Diagenesis effects such as compaction,cementation,dissolution and hydrocarbon emplacement have been proved in the sandstone reservoir of the Yanchang Formation and the diagenesis sequence has been made clear.The difference of diagenesis sequence between delta sandstone and gravity flow sandstone has been studied.Chlorite membrane is well-developed in delta sandstone and the compaction effect is weak.In gravity flow sandstone,chlorite membrane does not exist and the compaction effect is severe.Through the quantitative calculation of diagenesis,the evolution history of porosity has been restored.Early compaction and late cementation lead the decrease of porosity.After the early compaction,reservoir is not tight.And after the late cementation,porosity has decreased to below 12% while permeability is about 1×10^-3μm²,both referring to extra-low and ultra-low permeability reservoir.These perceptions have an important guiding significance in predicting relatively high permeability reservoirs and discovering oil fields.