Diagenetic evolution of the Oligocene Huagang Formation in Xihu sag, the East China Sea Shelf Basin

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.