The average porosity and permeability of sandstone of Lower Shihezi Formation of Permian system in Shilijiahan area of Hangjinqi area are of 8.3 % and 0.89×10

-3

μm

2

respectively

causing the reservior to be a typical tight sandstone reservoir. Meanwhile

the microfractures play an important role in improving the physical properties of tight sandstone reservoir. Therefore

the researches of the reservoir in this area focus on the microfractures. By the observation of the microscopic thin section

and combi

ned with the mercury injection

core physical properties and structural characteristics

the types and genesis

formation stages

control factors and the influence of micro fractures on physical properties in this area have been studied. The results show that the tight sandstone of Lower Shihezi Formation in the study area can be divided into intragranular fracture

grain edge fracture and grain penetrating fracture according to the contact relationship of strata

and can be divided into unfilled fracture

incomplete filled fracture and complete filled fracture according to the filling degree. According to the genetic analysis

there are at least three stages of micro fractures in the study area. Their development degree are controlled by the combination of internal factors （sandstone particle size

rock debris composition

rock debris and interstitial content

contact relationship between particles

etc.） and external factors （tectonic stress）. The microfractures mainly affect the permeability of the reservoir. The multi-factor influence is a basic feature of micro fractured tight reservoir. The average throat radius plays a major role in controlling the permeability of the reservoir.