Coupling Control of Depositional Compositions and Diagenesis on Reservoir Quality of Tight Sandstones in Fuyu Oil Layer, Gulong Sag, Songliao Basin

Recent exploration highlights the Gulong Sag as a promising target for tight oil in the Fuyu oil layer. However, the distribution of high-quality reservoirs remains poorly understood due to complex depositional and diagenetic controls, posing significant exploration risks. To address this, this study integrates petrological, mineralogical, and pore structure analyses to elucidate the genetic mechanisms of reservoir quality and identify favorable exploration targets. The sandstones are predominantly fine-grained lithic arkoses and feldspathic litharenites, characterized by abundant volcanic lithic clasts and muddy matrix. Four diagenetic facies were identified based on diagenetic assemblages and pore evolution. The chlorite pore-lining facies and moderate compaction–dissolution facies exhibit the highest reservoir quality (avg. porosity of 10.42% and 9.79%, respectively), benefiting from chlorite coatings that inhibited quartz overgrowth and intense dissolution that created secondary porosity. In contrast, the tightly compacted facies and carbonate-cemented facies represent unfavorable reservoirs (avg. porosity of 7.02% and 6.17%, respectively) due to the occlusion of pore throats by muddy matrix and early carbonate cements, respectively. The distribution of these facies is governed by the coupling of sedimentary environment and diagenetic alteration. Benefiting from superior sedimentary conditions and proximity to source rocks, the first member of the Fuyu oil layer developed extensive favorable facies (chlorite pore-lining facies and moderate compaction–dissolution facies), making it the primary exploration target. This study demonstrates how depositional compositions and diagenetic processes jointly control reservoir quality in tight sandstones, providing important insights for the exploration of analogous tight oil reservoirs.