Diagenesis and diagenetic evolution of Paleocene sandstone seservoirs in Lishui West Sag, East China Sea shelf basin

Many types of reservoirs are developed in Paleocene in Lishui West Sag, East China Sea shelf basin. Diagenesis, diagenetic evolution and pore evolution of Paleocene sandstone reservoirs in Lishui West Sag are studied by means of core slice identification, physical property testing, and carbon and oxygen isotope testing of four typical wells. The results show that: (1) the Paleocene sandstone reservoirs in Lishui West Sag are mainly composed of shallow sea delta facies medium and fine-grained lithic sandstone, with low compositional maturity and structural maturity. The mineral composition is mainly composed of lithic, followed by quartz and feldspar. (2) Sandstone experienced diagenesis such as mechanical compaction, cementation between carbonate rock and clay minerals, feldspar dissolution, etc. The carbonate cement of sandstone reservoir in the lower section of the Lingfeng Formation was formed in a freshwater diagenetic environment, while the carbonate cement of sandstone reservoir in the Mingyuefeng Formation and the upper section of the Lingfeng Formation was formed in a mixed water diagenetic environment, and the restored diagenetic temperature was between 76~141℃. (3) The sandstones in the lower member of Mingyuefeng Formation and the upper member of Lingfeng Formation of Paleocene are in early diagenetic stage B, and the sandstones in the lower member of Lingfeng Formation and Yueguifeng Formation are in middle diagenetic stage A. (4) Sandstone reservoirs have experienced pore evolution processes including compaction porosity reduction (−18.3%), early cementation porosity reduction (−3.9%), dissolution porosity increase (+4.8%) and late cementation porosity reduction (−6.7%). Each set of sandstone reservoirs has experienced differential diagenetic evolution processes, and diagenetic evolution types characterized by mechanical compaction, cementation between carbonate rock and clay minerals, and unstable clastic dissolution are developed respectively. Conventional high-quality reservoirs are developed in the lower member of Mingyuefeng Formation and the upper member of Lingfeng Formation. The sandstones of the lower member of Lingfeng Formation and Yueguifeng Formation are mainly exploring targets of unconventional, tight sandstone reservoirs, which are more difficult to explore. The research results are of great significance for in-depth understanding of Paleocene sandstone reservoirs in Lishui West Sag and guiding oil and gas exploration.