Microscopic characteristics and genetic mechanisms of the Jurassic Lianggaoshan Formation sandstone reservoirs, central Sichuan Basin

The study systematically characterized reservoir characteristics and highlighted primary factors controlling reservoir tightening based on cast thin-section petrography and petrophysical analysis of core samples from five wells, including well G119. The research results show that the Lianggaoshan Formation sandstones in the central Sichuan Basin exhibit fine-grained, well-sorted texture; however, lithofacies vary significantly across different systems tracts. The transgressive systems tract (TST) is characterized by quartz sandstones and lithic quartz sandstones featuring high quartz and low feldspar contents alongside unstable sedimentary lithic fragments, whereas the highstand systems tract (HST) predominantly develops lithic sandstones and feldspathic lithic sandstones. The quartz-rich sandstones of the TST is collectively controlled by the forebulge gentle slope setting, distal provenance, intense hydrodynamic winnowing, and "relay quartz supply" sourcing from the weathered horizons of the pre-existing alluvial plain. The sandstones exhibit an average porosity of 2.56% and an average permeability of 0.11 × 10^-3 μm². Intense compaction(accounting for an average loss of 83.5% of primary porosity) and early-stage pervasive calcite cementation(observed in 25% of samples) are the critical drivers of reservoir densification. Dissolution is poorly developed with a thin section porosity less than 0.5%), and this is primarily limited by the scarcity of soluble feldspar, rapid compaction rates, and insufficient organic acid supply from adjacent source rocks. By contrast, reservoirs relatively high in quality with an average porosity reaching 7.56%, are developed within distributary channel sandstones with low matrix content at the base of the TST. Excessively high quartz content increases the risks of wear to drill bit and hydraulic fracturing operations. HST sandstones, which possess higher feldspar and lithic contents than TST sandstones, offer better drillability and stimulation potential, serving as promising succession intervals for future exploration.