Diagenetic controls on reservoir quality in deep ultra-thick sandstones: a case study from the Huagang Formation, Xihu Depression, East China Sea

Tight sandstone reservoirs represent a pivotal unconventional oil and gas production target. The upper section of the deeply buried Huagang Formation in the Xihu Depression is abundant in hydrocarbons and forms a tight reservoir. It exhibits substantial diagenetic variability and strong heterogeneity in sand bodies, which is reflected in variations in physical properties and grain size. Through integrated geological analysis, including rock thin sections, scanning electron microscopy, X-ray diffraction, well logging and production test data, we investigated the diagenesis, reservoir formation mechanisms, and controlling factors of high-quality reservoirs within the superposed sandstone bodies exceeding 100 m thick in the deeper Huagang Formation. We also studied the formation characteristics of these ultra-thick sandstones, clarifying that diagenesis and post-depositional modification are crucial for developing high-quality reservoirs in this formation. Our findings indicate that the sandstone underwent compaction, cementation (by chlorite, calcite and quartz), dissolution (of K-feldspar and carbonate cement), and authigenic clay mineral cementation (such as illite, chlorite, kaolinite). Multiple dissolution zones are present within the thick sandstone units. The distribution of these dissolution zones is mainly controlled by temperature and sandstone composition. With increasing temperature, acidic fluids derived from coal-bearing strata and early hydrocarbon source rocks promoted feldspar dissolution. The thick sandstone units in different intervals of varying depths are at various diagenetic stages. The petrophysical zoning of the reservoir is collectively controlled by diagenetic facies dominated by sedimentation, compaction, and dissolution processes. These findings provide valuable guidance and reference for oil and gas exploration and development in this area, particularly within the ultra-thick sandstone layers. • High quality reservoirs are still present at depths of 4000–5000 m. • Thick-bedded reservoirs exhibit superior petrophysical properties compared to thin-bedded ones. • Clay mineral content is a key controlling factor on the development of deep, high-quality reservoirs.