Abstract:

The lower member of the Pinghu Formation (hereinafter referred to as the lower Pinghu member) in the well block W of the Xihu Sag is an important oil and gas-bearing system. An accurate characterization of its sedimentary evolution patterns and reservoir distribution is critical for guiding future exploration and development. Based on core, drilling, and geophysical data, this study analyzed the sedimentary microfacies, evolution processes, and dominant controlling factors of the lower Pinghu member. The results showed that the lower Pinghu member (sand groups P12~P9) could be divided into third-order sequences, mainly comprising deltaic and tidal flat deposits influenced by tidal processes. The P12 sand group, deposited during a lowstand system tract with relatively low sea level, was primarily composed of deltaic deposits, issueed by tidal deposits. During deposition of the P11 and P10 sand groups in the transgressive system tract, sediment supply weakened and delta development was curtailed. Thus, tidal flat environments became dominant in the study area. The P9 sand group, deposited during the highstand system tract, experienced increased sediment supply, tidal flat deposition reduction, and delta progradation towards the basin. Analysis of the sedimentary evolution process clarified that sediment supply, sea level fluctuations, and the paleogeomorphology controlled the microfacies migration and evolution in the well block W. Firstly, paleogeomorphology directly controlled the depositional accommodation and determined the spatial distribution of sedimentation. Secondly, abundant sediment supply and relatively lower sea level promoted deltaic development, leading to the formation of distributary channel and mouth bar sand bodies. On the contrary, the reduction of sediment supply and rising relative sea level restricted deltaic propagation while enhancing tidal power, resulting in the development of tidal flats, tidal channels, and tidal sand bars. In the study area, the relative intensity of deltaic and tidal processes was controlled by changes in relative sea level and sediment supply. During deposition of the P12 and P9 sand groups, sufficient sediment supply and relatively low sea levels favored delta development. On the contrary, during marine transgression stage corresponding to the P11-P10 sand groups, the sediment supply weakened and the relative sea levels rose. Under such conditions, deltaic deposits were vulnerable to damage, which favored the development of tidal sediments. However, the development of deltaic and tidal flat deposits in response to changes in relative sea level and sediment supply was also controlled by paleogeomorphology. During deposition of the P12 sand group, the presence of a nose-shaped paleo-uplift in the central part of the study area limited eastward progradation of the western delta. This resulted in differences in sedimentary facies types between the east and west sides of the nose-shaped paleo-uplift during deposition of the P12 sand group. The western fault trough zone was dominated by deltaic deposits, while the eastern fault step zone was dominated by tidal deposits. During deposition of the P11-P9 sand groups, the influence of the nose-shaped paleo-uplift weakened, and the sedimentary facies types in the study area were relatively uniform (P11-P10 was mainly dominated by tidal flat deposits; P9 was mainly dominated by deltaic deposits). This study offers insights into the spatiotemporal distribution characteristics of favorable reservoirs in the study area and adjacent zones. In the western fault trough zone of the P12 sand group and in the P9 sand group, deltaic sand bodies such as channels, mouth bars, and sheet sands are the favorable sand body types, and their exploration and development should be primarily guided by the deltaic depositional model. In eastern fault step zone of the P12 sand group and in the P11-P10 sand groups, the dominant sand bodies are tidal sand bars or tidal channels extending seaward and parallel to the shoreline, and their exploration and development should follow the tidal depositional model.

Key words: Xihu Sag, development well block, river-tide joint control, sedimentary microfacies, main controlling factor