Sedimentology and sequence stratigraphy of the second member of Shuangyang Formation, Y45 Block, Moliqing oilfield, Yitong Basin, China

Detailed stratigraphic analysis of the second member of Shuangyang Formation of Y45 Block of the rift Yitong Basin in northeast China has revealed that the study area, previously interpreted as fan delta systems, actually originated mainly in relatively deep lacustrine setting with nearby line source. Two depositional systems have been recognized: slope apron and slump fan. The regional correlation of the lithofacies within the different depositional systems has led to a high-resolution stratigraphic framework of 20 small layers. Sequence boundary SB1 marks the top of the target interval and is characterized by thick and continuous mudstone overlying sand-rich lacustrine sediments. Sequence boundary SB2 marks the bottom of the target interval and is characterized by high-channel-ratio lacustrine deposits overlying stably distributed shales. Maximum flooding surface (MFS) marks the end of a transgressive tendency and the onset of a regressive tendency and is characterized by surrounding shale-rich zone. Two system tracts constitute the target interval together which is classified by SB1, SB2, and MFS. The base-level fluctuation has a strong tectonic component in accordance with the regional tectonic settings that during active subsidence stage, the base level rises rapidly and during relatively stable stage, the base level rises slowly and is usually overtaken by sediment supply rate.