The Yushulin Oilfield in the Songliao Basin had large shallow-water deltas during the Lower Cretaceous period. By studying well cores and loggings, researchers identified different subfacies within the deltas, like delta plain and internal front. They found that the deltas initially formed from a northern source, then shifted southward. Eventually, both northern and southern sources contributed to the delta sediments. The deltas retrograded when the lake level rose, leading to the development of internal front, external front, and predelta in the area.