Study on factors of spatial heterogeneity of Ordovician crude oils in the Lunnan Low Uplift

The Ordovician reservoirs in the Lunnan Low Uplift of the Tarim Basin display pronounced spatial heterogeneity in crude oil properties, yet the key factors governing this variation-particularly the roles of multi‑stage hydrocarbon charging, secondary alteration, and accumulation conditions-remain poorly understood. To address this, a systematic geochemical characterization of oil samples from the LG15, LN54, and LG39 blocks was conducted, integrating bulk physical properties, light hydrocarbon compositions, maturity parameters, Q values, and diamondoid concentrations. Combined with regional constraints on charging episodes, alteration histories, and tectonic evolution, the data reveal a clear west-to-east gradient: oil density and viscosity decrease progressively from LG15 to LG39, while GOR rises sharply from 50 to over 8500 m 3 /m 3 . The results demonstrate that this spatial heterogeneity is primarily controlled by the interplay between charging history and subsequent post‑accumulation modifications. In the western LG15 block, two phases of oil charge were overprinted by severe biodegradation and only minor gas invasion, culminating in low‑maturity heavy oil. The central LN54 block records three oil charging events, accompanied by mild biodegradation and localized gas washing, which together formed light oil and volatile oil reservoirs. In contrast, the eastern LG39 block experienced four episodes of hydrocarbon charge; intense Late Himalayan gas invasion caused extensive n ‑alkane depletion and significant diamondoid enrichment, ultimately transforming the early accumulated oils into high‑maturity condensate gas reservoirs. Overall, this study demonstrates that the spatial heterogeneity of crude oil physical properties across the Lunnan Low Uplift is collectively controlled by a combination of fundamental geological processes. Specifically, variable hydrocarbon charging events with distinct thermal maturity levels lay the material foundation for differential oil property distribution, while diverse secondary alterations with varying modification intensities further reshape crude oil features. Superimposed on these fluid evolution processes is the regional tectonic evolution, which dominates hydrocarbon migration pathways and determines the preservation efficiency of subsurface traps. A clear understanding of such integrated genetic coupling mechanisms provides a robust theoretical basis for predicting hydrocarbon phase characteristics in deep carbonate reservoirs.