Abstract: The tuff reservoir in the lower part of the first member of Cretaceous system in A’nan sag has become one of the key fields for tight oil exploration in Erlian Basin in recent years. Based on core observation，thin section authentication，reservoir physical property analysis and X-ray diffraction analysis，using the experimental methods such as high pressure mercury injection，nitrogen adsorption，field emission scanning electron micros copy and CT scanning，the characteristics of lithology，physical property，reservoir space type and pore struc ture of tuff reservoir of Cretaceous system in A’nan sag in Erlin basin were analyzed. Favorable tight tuff reservoir zones were predicted according to favorable reservoir formation mechanism. The results show that：（1）The tuff tight reservoirs in the lower part of the first member of Cretaceous Tenggeer Formation of A’nan sag of Erlian Basi，are mainly composed of tuff，tuffite，tuffaceous mud and tuffaceous sandstone.（2）The range of physical properties of tuff reser-voirs in the study area is largely，with average porosity is 4.88%. Among them，the num ber of samples with ultra-low porosity and extra-low porosity accounted for 83.6%，with the average permeability is 0.067 mD. It is an ultra-low porosity and ultra-low permeability reservoir. The tuff reservoir space include dis solved pores and devitrified pores，and have stereoscopic network pore structures，the tuff is mainly composed of mesopores and macropores，the physical properties are the best. The main reservoir spaces of tuffaceous sand stone include dissolution pore and intergranular pores，have micro-thin pore-micro throat pore structures，and mainly develop mesopores，with good physical characteristics. Tuffite and tuffaceous mud mainly develop dis solved pores，intercrystal pores and microcracks，have isolated pore structures，the pore types are mainly meso pores，with poor physical properties.（3）Sedimentation，diagenesis and tectonism jointly control the formation of high-quality tuff reservoir in the study area，sedimentation provide the material basis for the formation of highquality reservoirs，which control reservoir formation and distribution，compaction and cementation reduces reser voir physical properties，dolo-mitization and devitrification improves reservoir performance，the dissolution is the key factor of effective reservoir forming，which has positive impact on improving the physical properties of reservoir.（4）The tuff reservoir can be classified into three types，class Ⅰ，class Ⅱ and class Ⅲ. the reservoir in class Ⅰ is mainly concentrated around trough area，the reservoir in class Ⅱ is distributed in the form of stripes along the western gentle slope zone，the distribution range of reservoir in class Ⅲ is the widest，the areas where develop the reservoir in class Ⅰsuch as the east blocks of A43 well，the north blocks of A27 well，the west blocks of H16 well and south west blocks of A11well are the next favorable targets for tuff tight oil exploration.

Key words: tuff, tight oil reservoir, ultra-low porosity and ultra-low permeability reservoir, dissolution, trough area, Tengger Formation, Cretaceous, A’nan sag, Erlian Basin