Abstract: The lower member of the Pinghu Formation at the W-well Area of Xihu Sag is an important oil-bearing formation. It is significant to accurately understand the sedimentary evolution characteristics and reservoir distribution to guide the next-step exploration and development. In this paper, the sedimentary facies, evolution process and controlling factors of the lower member of the Pinghu Formation are studied by using core, logging, drilling and seismic data. The results show that the lower member of the Pinghu Formation (P12~P9 sand layer) can be divided into a third-order sequence, mainly developed by tidal delta and tidal flat deposits. The P12 sand layer, which corresponds to the lowstand systems tract, was mainly deposited by deltaic position while sediment supply was sufficient. The P11 and P10 sand layers, which correspond to the transgressive systems tract, were mainly deposited by tidal deposits . The P9 sand layer, which corresponds to the highstand systems tract, was mainly deposited by deltaic position again .Through the analysis of sedimentary evolution process, we found that sediment supply, sea level oscillations, and the paleogeomorphic characteristics controlled the evolution of sedimentary facies in well W area. Firstly, the ancient landforms directly control the depositional accommodation and determine the location of sedimentation. Secondly, abundant sediment supply and lower relative sea-level benefit delta development, by which distributary channel and mouth bar sand bodies develop. On the contrary, the reduction of sediment supply and rise of relative sea-level tend to restrict the development of delta while benefiting the enhancement of tidal power, by which tidal sand flat, tidal channel and tidal sand bar develop. In the study area, the relative dominance between delta and tide is controlled by the relative sea level and sediment supply: in the P12 and P9 sedimentary stages, the sufficient sediment supply and the relatively low sea level, delta dominated the study area. On the contrary, during the transgressive period corresponding to the P11-P10 stage, the sediment supply was weakened, and the relative sea level rose, tidal deposition dominated the study area while the development of deltas was easily limited. However, the interchange of delta and tidal deposition in response to the change of relative sea level and sediment supply is also controlled by paleogeomorphology, which can be reflected in the P12 sand layer. Due to the presence of palaeonasal uplift in the middle of the study area, the eastward progradation of the delta from the western was limited to a certain extent, resulting in differences in sedimentary facies types between the east and west sides of the palaeonasal uplift in the P12 sand layer: The western fault trough zone is dominated by delta deposits, while the eastern fault step zone is dominated by tidal deposits. During the P11-P9 deposition period, the influence of the central palaeonasal uplift was weakened, and the sedimentary facies types in the study area were relatively uniform (P11-P10 was mainly dominated by tidal deposition; P9 was mainly dominated by deltaic deposition).This study provides a reference for further understanding of the temporal and spatial distribution characteristics of favorable reservoirs in the study area and its peripheral zones. In the P12 sand layer, there are differences in sedimentary facies types along the eastern and western wings of the palaeonasal uplift. The western fault trough belt is dominated by delta deposits, and the main sand body types include distributary channels, mouth bars, sheet sand and related sand bodies, which are spatially distributed around delta lobes. The eastern fault-step area is dominated by tidal deposits, and the main sand body type is tidal sand bar which is oriented seaward. In P11-P10 and P9 sand layers, the genetic types of sand bodies in the study area are relatively simple, which are mainly tidal and deltaic sand bodies, respectively.

Key words: Xihu Sag, Development Well Area, River and tide joint control, Sedimentary facies, Main control factor