Depositional characteristics of lake-floor fan of cretaceous lower Yaojia Formation in western part of central depression region, Songliao basin

Based on the integrated subsurface data, including those of over 600 m drilled cores, more than 30 drilled wells and 600 km2 three-dimensional (3D) seismic-reflection data of the study area, the characteristics of the lake-floor fan of lower Yaojia (姚家) Formation have been clarified. An evident lacustrine slope break and a steep slope belt developed in the west of Songliao (松辽) basin during depositional period of Qingshankou (青山口)-Yaojia formations (K2). The slope gradient was about 15 m/km. During the depositional period of lower Yaojia Formation, the lake shrank and the shore line of the western Songliao basin shifted to the lacustrine slope-break. The wedge-shaped sediment body, which is interpreted as the lowstand system tract of SQy1 (LSTy1), developed in the area below the slope-break. The LSTy1 is pinched out in the west of the study area. As to the thickness of LSTy1, it is thicker in the east with 50 m in its thickness than in the west. The LSTy1, rich in sandstone, can be divided into lower part LSTy1L and upper part LSTy1u based on two onlap seismic reflection phases, and core and logging data clearly. The various sediments’ gravity flow deposits developed and the complex of lake-floor fan formed in the LSTy1 under the slope-break in the western part of the central depression region. The lake-floor fan consists of various sediments’ gravity flow deposits, including: (1) turbidity deposits with characteristics of Bouma sequences; (2) sand-bearing muddy debrite dominated by mud and mixed by sand; (3) mud-bearing sandy debrites characterized by dominated sand and mixed by mud; (4) sandy debris laminar flow deposits with massive or inclined bedding, and (5) sandy slump deposits developed as deformational sedimentary structure. During the lower lake-level period (LSTy1L), the western clinoform region was erosion or sediment pass-by area; the terrigenous clastic was directly transported to deep-water area, converted to channelized sandy debris flow, and combined with slump derived gravity flow deposited in the lower part of the slope and the deep depression region, which then formed lake-floor fans. During higher lake-level period (LSTy1u), terrigenous clastic deposited in the upper part of the slope, and formed deltaic depositional system; while in the lower part of the slope, the deep depression area was filled by lake-floor fans which were mainly slump deposits. The sandstone bodies of the lake-floor fan are the favorable targets of subtle oil-bearing traps in the western part of the central depression region.