Abstract: Dolomite reservoirs in M5_5 of lower Ordovician in eastern area of the Sulige gas field is characterized by big thickness of single well,quick pinching out in horizontal direction,which is distinctively different from the characteristics of dolomites reservoir in M5_(1-4) with thinner individual layer and good horizontal continuance in the Jingbian gas field.The reservoir lithology of this area is mainly of fine medium dolomite and fine powder crystal dolomite,which is the result of the combination of evaporation pump dolomization,reflux permeation dolomitization and deep-burial dolomitization.The dolomite types are mainly of fine or medium dolomites and fine or silty crystal dolomite,the pore types are mainly intercrystal pore and intercrystal dissolved pore.Through the research of petrology,geochemistry and diagenesis,the genesis of dolomite reservoir in this area was studied.The grain bank deposition is an important condition for the formation of favorable reservoirs,the burial dolomitization is a precondition,and the burial phase dissolution is a key factor for reservoir formation.