Mineralogical Constraints on Hydrocarbon Charging, Gas Alteration, and Accumulation Processes in the Pishan 1 Well, Southwestern Tarim Basin

The Middle Permian Qipan Formation in the Pishan 1 well, located in the Kekeya structural belt of the southwestern Tarim Basin, has recently delivered high-yield oil and gas production. Deep-buried hydrocarbon accumulations in this area have experienced complex multi-stage charging and post-entrapment fluid alteration, giving rise to pronounced hydrocarbon property contrasts among different sand groups within the same stratigraphic interval. This study systematically examines crude oils and natural gases from Sand Groups I, II, and IV of the middle Permian Qipan Formation in the Pishan 1 well, employing bulk oil physical property measurements, whole-oil gas chromatography, saturated and aromatic hydrocarbon geochemical parameters, natural gas compositional analyses, and fluid-inclusion petrography, together with previously acquired quantitative grain fluorescence data to unravel the genetic linkages and differentiation mechanisms among these sand groups. The results demonstrate that crude oils from the three sand groups share a unified source-rock affinity, all being derived from lacustrine source rocks of the Permian Pusige Formation. The n-alkane distribution patterns, carbon preference index (CPI) and odd–even predominance (OEP) values, and sterane assemblages consistently point to similar organic matter inputs and thermal maturation histories. Relative to Sand Groups I and IV, oils in Sand Group II display marked gas enrichment and compositional lightening, manifested by higher gas–oil ratios, lower densities, and reduced wax contents; the associated gases also exhibit elevated dryness coefficients and thermal maturity. Geochemical evidence indicates that these anomalous characteristics of Sand Group II are predominantly controlled by late-stage gas washing imparted by highly mature natural gas. By integrating fluid-inclusion petrography, solid-bitumen evidence, and hydrocarbon geochemistry, the hydrocarbon accumulation history of the Pishan 1 well is interpreted as a relative multi-stage sequence comprising an early oil-charging stage, a subsequent light-oil/condensate charging stage, and a later natural-gas overprint. Early oil charging was primarily governed by reservoir quality, whereas the intensity of late-stage gas invasion was jointly modulated by structural position, fracture intensity, and bitumen sealing. Sand Group II ultimately evolved into a condensate gas reservoir subjected to extensive gas washing. This study elucidates the mechanisms responsible for fluid property heterogeneity at the single-well scale within deep accumulations and underscores the critical influence of selective late-stage gas intrusion, conditioned by reservoir heterogeneity, on hydrocarbon accumulation and fluid differentiation. Quartz-hosted fluid inclusions and grain-surface fluorescence provide complementary mineral-scale records of hydrocarbon charging and subsequent fluid alteration within the sampled reservoir interval. The findings provide a well-scale perspective on fluid differentiation and multi-stage hydrocarbon accumulation in deep reservoirs of the southwestern Tarim piedmont belt.