The mound-shoal complex is the most important sedimentary facies for natural gas in the Dengying Formation of the Sichuan Basin,and microbial mound is an important component of the mound-shoal complex.Based on a comprehensive analysis of the cores,thin sections,and logging of 11 wells in the fourth member of the Penglai gas field,three types of microbial mounds are classified according to spatial morphology,and the internal structure and evolution rule are clarified respectively.The rock types of microbial mounds are divided into microbial dolomite,residual granular dolomite,breccia dolomite and crystalline dolomite.The microfacies such as mound base,mound core,mound cover,and mound flank are identified.Three types of microbial mound developed as follows:Vertical accretion microbial mound was formed in an environment where the construction rate of microbial mounds was close to the rate of sea level rise,with a growth sequence of"mound base→mound core→mound cover".The cumulative thickness of this type developed at the platform margin is generally 64-93 m.Lateral migration of the microbial mound was formed in environments where the construction rate of microbial mounds was greater than the rate of sea level rise,with a growth sequences of"mound base→mound core → mound flank"and"mound core → mound flank".The cumulative thickness of this type developed at the platform margin is generally 25-49 m.Small isolated microbial mound was distributed between mounds,with a thickness of generally decimeters and a small scale.The microbial mounds of vertical accretion and lateral migration concentrated at the platform margin in the second to third sub-members of the fourth member,with large total thickness and continuous horizontal distribution,which are the most favorable facies for the Dengying Formation in the Penglai gas field.