Abstract: Based on the observation of core samples,thin sections and well log data analysis method, through the petrography,pore types and reservoir physical properties of the chang-91 oil reservoir in the northwest area of Jiyuan oilfield,this article discussed reservoir characteristics,the main controlling factors and the evaluation of reservoir in the study area.The research demonstrates that the main types of the pores in the reservoir are residual primary intergranular pores,intergranular-intragranular dissolved pores and clay minerals micropores.The average porosity and permeability of the the chang-91 oil reservoir is about 9.95% and 1.05mD respectively,so it is a typical reservoir with low porosity and permeability. Development of the reservoirs is obviously controlled by sedimentary evironment and diagenesis. Distributary channel is the best reservoir facies,even the porosity of the reservoir was reduced by compaction and cementation,but the rim cementation of chlorite protected the primary intergranular pores availably.Moreover,the corrosion of feldspar and laumontite also improved the performance of the reservoir.Five types of the reservoir have been distinguished according to the research of its sedimentary characteristics,physical characteristics,reservoir space and so on.Class I and II which distribution alongthe center of the distributary channel are favorable areas,classⅢandⅣ which distribution along the edge of the distributary channel are secondary favorable areas.