The Xujiahe sandstones in the Yuanba area are characterized by ultralow porosity and ultralow permeability and should belong to ultracompacted sandstone. Based on thin sections, cathodeluminescence (CL), scanning electron microscope (SEM) and reservoir property data, densification causes and exploration potential are unraveled through comparison of detrital components, interstitial materials and reservoir properties between the Xujiahe sandstones and other tight sands in the world. The results show that the Xujiahe sandstones contain high contents of ductile rock fragments, clay matrix and carbonate fragments. The interstitial materials are dominantly clay matrix and calcite cements, with strong siliceous cementation occurring in quartz sandstone. Secondary pores are dominated by clay matrix intercrystalline micropores, with pore throats commonly present as narrow, poor sorting and sheetlike slots. There are two reasons for the highly densification of the Xujiahe sandstones. First, the anomalously poor quality of sandstones is attributed to the high compaction and calcite cementation during the early diagenetic stage, which are functions of the primary texture and composition of sandstones. Second, the Xujiahe sandstones were buried deeply and experienced further compaction and calcitic/siliceous cementation to forming highly tight sandstone in mesogenetic stage. Comparison with typical tight sandstone gas abroad indicates that ultratight sandstone contain less effective porosity, which has limited accumulation capacity to nature gas. Therefore, only when the porosity of the Xujiahe in the Yuanba area reaches to 8% (e.g. feldspathic litharenite) can sandstone be favorable reservoir for highlytight sandstone exploration.