Multi-Proxy Reconstruction of Organic Matter Accumulation and Its Controls in Oligocene–Pliocene Lacustrine Shales, Western Qaidam Basin (China)

Low-TOC saline lacustrine shales are widespread in Cenozoic basins of northwestern China and Central Asia, yet their organic matter accumulation (OMA) mechanisms under arid, saline conditions remain poorly understood. We analyzed TOC, major and trace elements, and C–O isotopes in 64 core samples from eight wells in the western Qaidam Basin, and reconstructed terrigenous input, weathering intensity, paleoproductivity, redox conditions, and salinity. TOC ranges from 0.04% to 1.10% (mean 0.32%), with a statistically significant upward trend from the Shangganchaigou (mean 0.25%) to the Shangyoushashan Formation (mean 0.43%) (Kruskal–Wallis, p = 0.010). The climate was dry–cold with progressive aridification (increasing δ18O and Sr/Ba). The water body evolved from closed, strongly reducing, and unstable to more open, weakly reducing, and increasingly saline. We propose that limited nutrient supply set a low baseline for organic enrichment. Weakened terrigenous dilution and enhanced water-column stability then drove the gradual TOC increase. This reveals that OMA can occur without concurrent productivity rise, sustained solely by long-term preservation improvement, offering new constraints for source rock evaluation in arid lacustrine basins.