Geochemical Constraints on Differential Organic Matter Enrichment in the Permian Fengcheng Formation, Western Depression of the Junggar Basin, NW China

ABSTRACT The Lower Permian Fengcheng Formation is a major source rock in the Western Depression of the Junggar Basin, but variable maturity and heterogeneous saline lacustrine deposition obscure original organic matter abundance and enrichment controls. Vitrinite reflectance, Rock‐Eval pyrolysis, biomarkers, and measured present‐day total organic carbon (TOC) constrained thermal maturity and organic matter type and supported reconstruction of original total organic carbon (TOC 0 ), which represents the organic carbon content prior to maturity‐related loss during hydrocarbon generation and expulsion. Elemental proxies and QEMSCAN mineralogy constrained paleoclimate, terrigenous input and weathering, salinity, redox conditions, and paleoproductivity. Within sag regressions and separate hierarchical cluster analyses (HCA) assessed sample level covariation and multivariate associations, whereas the comparison of the three sag datasets identified the first‐order spatial control. TOC 0 increases from Mahu (0.83 wt.%) through Penyijingxi (1.30 wt.%) to Shawan (1.50 wt.%). The sampled intervals record arid to semi‐arid conditions, generally weak source area weathering, oxygen‐deficient waters, and differences in terrigenous input, salinity, hydrological restriction, and paleoproductivity. No single proxy consistently explains sample level TOC 0 variation across all sags; separate HCA reveal different local associations. At the sag scale, only P/Al, P/Ti, and Cu/Ti reproduce the TOC 0 increase from Mahu through Penyijingxi to Shawan. Ti/Al shows the opposite trend, whereas paleoclimate, salinity, and redox proxies do not. Higher primary productivity therefore best explains the first‐order TOC 0 differences; depositional position, terrigenous input and associated dilution, salinity, and redox conditions modified preservation and local enrichment. Three modes are recognized: low paleoproductivity with strong terrigenous input and associated dilution in the Mahu Sag, mixed paleoproductivity and salinity‐related preservation in the Penyijingxi Sag, and high paleoproductivity with low terrigenous input under favorable preservation conditions in the Shawan Sag. This framework supports the evaluation of favorable source‐rock intervals in mature to overmature saline lacustrine systems.