Diagenesisof Lacustrine Deltaic Sandstones and Its Impact on Reservoir Quality

Sedimentation during stage of Chang-8 deposition was controlled by a lacustrine delta system.The Chang-8 member is an important finger-bar reservoir-sandstone in Qingyang area. Sandstone reservoirs can be classified as compositionally immature feldspathic lithareniteand lithic arkose. The main diagenetic minerals of thesandstones include: authigenetic clayminerals, calcite cement, quartz overgrowth, albiteand later ferroan carbonate. Authigenic quartz overgrowths and micro-crystals contain a variety of organic fluid inclusions that indicate the petroleum filling history. Petroleum filling history showsthat the significant later filling process affectedreservoir quality.Porosity preservation caused by grain-coating , authigenic chlorite. Wide porosity variations in sandstones correlate with an abundance of grain-coating chlorite and consequent inhibition of quartz cementation. Secondary porosity has created mainly by feldspar, rock fragment and clay matrix dissolution. The dissolution mechanisim is closely related to organic acids and transformation of clayminerals in adjacentsource rocks. Based on diagenetic events related to temperature, reservoir diagenetic stages can be dividedinto A-substage of later diagenesis. Reservoir quality is controlled mostly by sedimentary microfacies and diagenesis. Subaquoeus distributary channeland mouth barsandbodies commonly have high porosity and permeability, whereas interdistributary and distal sandsheets may have low petrophysical properties.The study shows that the Chang-8 sandstone locally retained excellent porosities controlled by diagenetic facies despite deep burial and thermal conditions.