Sedimentary characteristics and implications for hydrocarbon exploration in a retrograding shallow-water delta: An example from the fourth member of the Cretaceous Quantou Formation in the Sanzhao depression, Songliao Basin, NE China

Based on the analysis of core, logging, and testing data, the fourth member of the Cretaceous Quantou Formation(Kq~4) in the Sanzhao depression, Songliao Basin, is investigated in order to understand the sedimentary characteristics and hydrocarbon exploration significance of a retrograding shallow-water delta. The results show that during the sedimentary period of Kq~4, the Sanzhao depression with a gentle basement experienced stable tectonic subsidence and suffered a long-term lake level rise caused by paleoclimate changes(from semiarid to semihumid), the Kq~4 in the study area were dominated by a fining-upward deltaic succession and had relatively stable thickness. From the bottom to the top, the color of mudstone gradually changes from purplish-red to gray and grayish-green, the contents of caliche nodules decrease gradually, while the presence of pyrite in sediments becomes frequent. Channel sandstones mainly composed of siltstone and fine sandstone with developed high-energy sedimentary structures constitute the main sand bodies of deltaic deposits, but the scale of channel sandstones decrease upward. Despite the long-term lake level rise and fining-upward sedimentary succession,purplish-red mudstone, caliche nodules and thin channel sandstones are still broadly distributed in the study area, and thin channel sandstones can be found at the top of Kq~4 covered by the black oil shale of Qingshankou Formation. These assertations suggest that the study area was dominated by retrograding shallow-water delta deposits during the sedimentary period of Kq~4. In comparison with modern Poyang Lake, we infer that during the sedimentary period of Kq~4, the study area experienced frequent lake level fluctuations triggered by paleoclimate changes despite the long-term lake level rise, and the lake level fluctuations control the deposition of retrograding shallow-water delta. In addition, most of the thin channel sandstones distributed at the top of Kq~4 and covered by black oil shale are generally immersed in oil, indicating that the thin channel sandstones formed at the top of a retrograding shallow-water delta sedimentary succession are favorable targets for lithological reservoir exploration.