Reservoir heterogeneity and genetic mechanisms of the oligocene Huagang formation in southern Xihu Sag: implications for deep hydrocarbon exploration and development

This study aims to clarify the characteristics and genetic mechanisms of low-permeability to tight sandstone reservoirs. We carry out the research by integrating multiple data sources: core observation, cast thin sections, X-ray diffraction (XRD), scanning electron microscopy (SEM), high-pressure mercury injection (HPMI), and well logging data, focusing on the Paleogene Huagang Formation (E 3 h) of Structure Y, Xihu Sag. Results show that the E 3 h was deposited in a sandy braided river delta-lacustrine system, including subaqueous distributary channels, interdistributary bays, and shallow lake microfacies. Five major lithofacies are identified, with the massive medium sandstone lithofacies (MMLF) and massive fine sandstone lithofacies (MFLF) exhibiting the best physical properties (porosity 6%–15%, permeability 0.1–13.5 mD). The sandstones are mainly feldspathic lithic quartz sandstones, with interstitial materials dominated by clay minerals (avg. 4.34%) and matrix (avg. 3.68%), showing overall low porosity (5%–15%) and low permeability (avg. < 1.2 mD). Pore types are primarily residual intergranular and dissolution pores, with pore structure significantly controlled by lithofacies. Compaction is the main cause of porosity loss (avg. reduction rate 78%), while secondary pores formed by late organic acid dissolution (avg. dissolution pore surface porosity 1.6%) effectively improve reservoir quality, and abundant carbonate cementation (avg. 2.53%) leads to tightness. The reservoirs are in the late middle diagenetic stage A to B, having undergone an evolution from weak acidic leaching to acidic dissolution and finally to acid-alkaline transition cementation. Four diagenetic evolution types are classified, and reservoir quality is dually controlled by depositional environment (grain size, compositional maturity) and diagenesis (compaction, dissolution, cementation). The classification criteria, evolution mechanism, and proposed development adaptation scheme provide technical support for optimal well placement, reservoir reconstruction, and efficient exploitation of deep hydrocarbon resources in the Xihu Sag and similar offshore basins.