Abstract: For a reservoir at the medium-later development stage,a fine study on microfacies is crucial for establishing a proper reservoir model,ascertaining the distribution of re- maining oil and improving oil recovery rate.Through core observation,lithofacies analysis and log facies analysis,the sedimentary microfacies ofⅡoil-bearing strata,the 4th member of Aershan Formation,Ba19 block,Baolige oilfield, were studied systematically.Results suggests that the 4th member of Aershan Formation at the interest area is com- posed of fan delta deposit resulting from the braided river flowed directly into the lake,and it is further divided into 6 microfacies as subaqueous distributary channel,mouth bar, distal bar,front sheet sand,subaqueous distributary channel bay and front fan delta mud.The subaqueous distributary channel deposit and mouth bar deposit are the main reser- voirs.The evolution and the distribution of microfacies in main oil-bearing horizons have been studied.The investiga- tion on relationship between microfacies and oil-bearing in- dicates that the oil and gas accumulation is subject to micro- facies.As of the storage and preservation ability,the suba- queous distributary channel is the best,the mouth bar sec- ond,and the distal bar and front sheet sand body the worst. The results provide a geological foundation for the adjust- ment of reservoir development scheme and the dynamic anal- ysis.