Abstract: The S_Ⅰ oil formation in the B90 well block of Daqing Oilfield has large oil-gas reserves. However, the undetailed previous studies on its sedimentary microfacies restricted further exploration and development. To improve the potential exploitation efficiency, this study uses the closed skeleton profile control correlation method for fine stratigraphic division, constructs the closed skeleton profiles across the whole area with 5 transverse profiles and 5 longitudinal profiles, and establishes the high-frequency isochronous stratigraphic framework of S_Ⅰ oil formation. Besides, the sedimentary microfacies of 5 sedimentary time units are studied based on core and well logging data. The results show that narrow and isolated underwater distributary channels and large area of sheet sand deposits are developed in the area, and the sheet sand is mainly controlled by stream action and wave modification. The delta front subfacies are developed in the S_Ⅰ oil formation, with microfacies such as underwater distributary channel, sheet sand body, sheet sand, inner edge of sheet sand and outer edge of sheet sand. The sedimentary period of the S_Ⅰ oil formation is a small rapid evolutionary sequence from water regression to water transgression.