No abstract

The East China Sea Shelf Basin was a back‐arc basin located at the active continental margin of the western Philippine Sea Plate. This study explores facies and architectural changes from tide‐influenced deltas to tide‐dominated estuaries in transgressive–regressive cycles, as well as their controlling factors. Cores, wireline well‐logs and seismic data allow the sedimentary architectures and models of the depositional systems to be reconstructed. In the Xihu Depression of the East China Sea Shelf Basin, the stratigraphic sequences of the Eocene Pinghu Formation are interpreted to be dominated by repeated phases of deltaic progradation, but with intervening transgressive phases only thinly developed as bioturbated, open‐marine shelf deposits. The sequences of the overlying Oligocene Huagang Formation, in contrast, are interpreted as stacked, tide‐dominated estuary units, alternating with only poorly preserved regressive half‐cycles because of repeated, strong estuary down‐cutting. The intervening unconformity in the succession corresponds to the Yuquan tectonic movements, which triggered a change from extensional to compressional settings in the Xihu Depression. In the Late Eocene, extension of the Xihu Depression led to moderately high rates of subsidence (163 m Ma−1), and short‐term sea‐level falls led to multiple phases of deltaic progradations. After the Yuquan Movement, Early Oligocene compression brought overall lower rates of subsidence (110 m Ma−1), as well as sea‐level rise and stacked estuary development with significant tidal influence in the infill. The interaction of tectonics, sea‐level change and sediment supply determined the nature of the depositional systems on the shelf during the entire period, whereas the sedimentary processes were key to reworking and shaping the facies distribution, geomorphology and architectures in the back‐arc basin. This research provides an insight into spatial and temporal characterization of deltaic and estuarine systems, contributing to a better understanding of the mechanisms controlling a change in dominant coastline type, despite continued strong tidal influence.

No abstract

The deep and ultra deep clastic reservoir is characterized by strong reservoir heterogeneity and complicated reservoir-forming characteristics for its high degree of diagenetic stage and the complexity of diagenesis. In order to better study the diagenetic evolution of deep and ultra deep reservoir in the burial process, a fine subsection scheme of 36 diagenetic micro-stage (DS) in diageneitic process was proposed based on paleotemperature (T), vitrinite reflectance (Ro%) and proportion of smectite in illite/smectite interstratified minerals (I/S-S%). Taking the Oligocene Huagang Formation in the Xihu sag of the East China Sea Basin as an example, the diagentic stage IIA1–IIA2–IIB was identified mainly by means of formation temperature data (T), homogenization temperature of fluid inclusion (Th), vitrinite analysis and clay mineral X-ray diffraction method. On this basis, diagenetic evolution using the fine subsection scheme in geological time were conducted. This fine division of diagenetic stage could bring accurate insight into porosity evolution history, hydrocarbon charging periods, diagenetic and reservoir-forming characteristics of low-permeability and tight sandstone.

No abstract