Sedimentary characteristics of 1st member of Yaojia Formation in Zhaoyuan-Taipingchuan region of Songliao Basin

Through integral studies on cores, seismics, well-logs and laboratory sediment analysis a new sequence stratigraphic framework of the Late Cretaceous first member of the Yaojia Formation in the Songliao Basin, in China's Zhaoyuan-Taipingchuan (Zhy-Tpch) region, has been reconstructed. In this study, the first member of Yaojia Formation is divided into 3 three-order depositional sequences, each composed of three systems tracts, i.e. the lowstand systems tract (LST), the transgressive systems tract (TST) and the highstand systems tract (HST). Using a dense network of cored boreholes data and data acquired from a combination of spontaneous potential and electric resistivity logging methods, the paper studies the characteristics and distribution of sedimentary environments within the cyclic patterns (systems tracts) of the major depositional sequences, and ascertains that deposition of sediments in a predominantly shallow lake delta environment which prevailed in the region during the Late Cretaceous period was responsible for the creation of the first member of the Yaojia Formation. Results show that this formation is mainly composed of three sub-facies namely, the delta plain, delta front and prodelta (the shore shallow lake). Each sub-facies is further subdivided into eleven sedimentary environments including distributary channels, natural levees, crevasse splays, inter-distributary channels, flood plains, subaqueous distributary channels, inter-distributary bays, subaqueous crevasse splays, mouth bars, distal bars and sheet sands. The delta plains and delta fronts are widely developed. Deposited with frequent bifurcations, the distributary channels gradually moved towards the center of the lacustrine basin where prodelta sedimentation had hardly occurred. Meanwhile, subaqueous channels were frequently developed within the shallow water areas of the delta fronts. Sandstone facies in the distributary and subaqueous distributary channels mutually interacted and stacked up to form dendritic-like sands. Distributary channel sands formed the most favorable reservoirs available today and are considered to be the most important exploration targets for the oil industry. Overall, this study is crucial for improving the understanding of the geological and paleo-geographic record in the Zhy-Tpch region including the facies types, relationships and cyclicity in response to changes in depositional environments (i.e. sedimentation and base-level changes) that prevailed in the region during the Late Cretaceous period and hence important for the predictive aspects of the economic petroleum exploration and development in the Songliao Basin.