Abstract: The oil measures I to IV in the first member of the Nantun Formation in the Sudert structural belt, Buir depression are made up of the volcaniclastic rock reservoirs including tuffaceous sandstone and conglomerate, tuffaceous sandstone, tuffaceous mudstone and sedimentary tuff. The contents and grain sizes of tuff have consequences for the porosity and permeability of the hydrocarbon reservoirs. The tuffaceous sandstone and conglomerate from the oil measures I have moderate to high porosity and low to extra-low permeability. The tuffaceous sandstones from the oil measures II and III have moderate to low porosity and extra-low to ultra-low permeability. The tuffaceous mudstones from the oil measures IV have low porosity and extra-low to ultra-low permeability. The diagenesis of the above-mentioned tuffaceous sandstone and conglomerate, tuffaceous sandstone and tuffaceous mudstone include compaction, cementation and recrystallizaion, of which the former two act as the key factors resulting in the production of poor porosity and permeability. The devitrification pores caused by the polymorphic phase transition of minerals may facilitate to the development of the micropore throats.