Sequence stratigraphy and sedimentary evolution in the Lower Cretaceous of the Eastern Depression in the North Yellow Sea Basin

The Lower Cretaceous of the Eastern Depression in the North Yellow Sea Basin is a favorable reservoir for hydrocarbon exploration. However, scarce studies on its fine stratigraphic delineation and sedimentary evolution have restricted the deep understanding of the prospect of hydrocarbon exploration in the eastern depression. Through the core observation, logging and recording analyses, and well-seismic calibration in the eastern depression, it is concluded that the Lower Cretaceous mainly developed 3 third-order sequences, K₁SQ₁, K₁SQ₂, and K₁SQ₃ in bottom-up order. Only 2 third-order sequences, K₁SQ₁ and K₁SQ₂, are visible in the drilling, and the descending half-circle of the K₁SQ₂ sequence is always incomplete, with only the ascending half-cycle of the sequence remaining. The K₁SQ₁ was in the period of faulting extension, and the vertical lithological sequence exhibited a complete depositional cycle with progradation-retrogradation. In the early stage, there were fluvial phases which developed at mainly the basin margins, the fan-delta phases were in the southern rift zone of the basin, and the braided river-delta phase was in the northern slow-slope zone. Due to the uprising of the lake level, mudstone-dominated lake deposits were developed in the middle and late stages. In the advanced stage, small-scale braided river delta deposits developed in the northern slow-slope zone. The K₁SQ₂ was in the period of stabilized rupture, which inherited the depositional characteristics of the K₁SQ₁ period and contracted slightly. The K₁SQ₃ was in the period of faulting abortion, which was characterized by the development of a large area of fluvial phase deposits.