Formation Mechanism and Variability of Tight Sandstone Reservoirs under Multi-Provenance Convergence: A Case Study of the Second Member of the Upper Triassic Xujiahe Formation in the Northeastern Sichuan Basin.

With the increasing exploitation of unconventional oil and gas resources in China, the tight sandstones of the Upper Triassic Xujiahe Formation in the northeastern (NE) Sichuan Basin have emerged as a critical exploration frontier. However, the complex controlling factors of reservoir development under multiprovenance convergence remain poorly understood. This study focuses on the second member of the Upper Triassic Xujiahe Formation (the T3x2) in the northeastern region of the Sichuan Basin as its research subject, through core observations, thin-section petrography, scanning electron microscopy (SEM), and light/heavy mineral analyses to clarify the differential controls exerted by various provenance systems. Results indicate that the T3x2 reservoir in the study area was jointly supplied by three distinct systems: the Longmen mountain, Micang mountain, and Daba mountain. Specifically, the Longmen mountain region is dominated by feldspathic lithic sandstones, featuring pore-type reservoir spaces with medium-to-fine pore throats. The Micang mountain region primarily comprises lithic sandstones and lithic quartz sandstones, characterized by fracture-pore type reservoirs with fine pore throats. The Daba mountain region develops feldspathic lithic sandstones dominated by micropore-throat systems. Porosity evolution analysis reveals that initial porosity was predetermined by provenance-related lithofacies. In the Longmen mountain and Daba mountain regions, porosity enhancement was driven by chlorite coatings and dissolution; however, porosity reduction in the Longmen mountain area was mainly caused by compaction, whereas the Daba mountain area was restricted by low-calcite cementation. Conversely, porosity in the Micang mountain region was preserved through dissolution and tectonic fracturing, with reduction primarily resulting from the combined effects of compaction and cementation. Furthermore, the timing and mechanisms of reservoir densification exhibit significant spatial heterogeneity. The Micang mountain reservoirs densified earliest during the Middle to Late Jurassic, primarily controlled by quartz frameworks and late-stage tectonic fracturing. The Longmen mountain and Daba mountain reservoirs reached the tight threshold later, from the latest Late Jurassic to the Early Cretaceous, with the former controlled by feldspar grains, chlorite coatings, and dissolution, and the latter governed by the synergy of chlorite coatings and dissolution. This study reveals the systematic control of provenance on lithology, pore-throat structure, evolutionary pathways, and densification history, providing critical geological insights for hydrocarbon exploration in multiprovenance convergent basins.