Abstract: The sedimentary system of A reservoir in the Liaohe Western Depression was analyzed to clear up the uncertainty about the formation mechanism of the sand body in the study area. The characteristics of sedimentary facies, plane distribution, and the space-time evolution process are described in detail based on abundant data on the rock core logging and granularity. The results indicate that the fan-delta front subfacies reservoir developed in the study area can be divided into five microfacies, namely, the underwater distributary channel, distributary mouth bar, underwater distributary inter-channel sand, underwater distributary inter-channel mud, and frontal sheet sand. The underwater distributary channel is the main sedimentary microfacies and has a banded distribution in a northwest-southeast direction. This microfacies is affected by the water withdrawal cycle, with the base level descending generally. From bottom to top, the y₂₄^I sublayer shows a process of base level rise and fall, the y₂₃^I layer shows a process of base level rise, and the y₁₂^I layer shows a process of base level fall. The heterogeneity of the reservoir is controlled by the distribution of sediment, as well as evolution. Detailed research results of the sedimentary system provide a solid geological foundation for facies-controlled modeling and further study on the distribution of the remaining oil deposit. This information provides a critical basis for the adjustment of the development method.

Key words: Liaohe western depression, Yulou oil layer, fan-delta front, base-level cycle, reservoir heterogeneity