The researchers studied the sedimentary facies of the Chang 8 oil-bearing bed in Xifeng oilfield. They analyzed core and logging data to understand the types of sediment and structures in the reservoir. They found that a river-dominated delta system exists in Chang 8, with specific microfacies like distributary channels and mouth bars. These sedimentary facies control the physical and oil-bearing properties of the reservoir, with submerged distributary channels and mouth bars being the most favorable. This information helps predict where to find oil in the reservoir and guides future exploration efforts.