Applications of contrasting hydrogen and carbon isotope fractionation of light hydrocarbons during evaporation: insights into the formation of light oils in the Tarim Basin, NW China

Evaporative fractionation plays a pivotal role in governing the geochemical composition and accumulation of deep and ultra-deep light oils and condensates (LOCs), but its identification using conventional light hydrocarbon parameters is often obscured by thermal maturity and source heterogeneity. Evaporation leads to a progressive depletion of deuterium (D) in residual oils, yet this inverse isotope fractionation has rarely been applied to geological reservoirs. To validate whether contrasting carbon and hydrogen isotope fractionation can reliably constrain LOC genesis, this study investigated carbon and hydrogen isotope ( δ 13 C and δ D) data of light hydrocarbons in LOCs derived from the Tarim Basin. The humic LOCs from the Dina oilfield have high and widely varying toluene/ n -heptane (Tol/ n C 7 ) ratios, consistent with intense evaporative fractionation. This interpretation was corroborated by a distinct negative δ 13 C– δ D correlation in methylcyclohexane (MCH), together with molecular–isotopic trends characterized by increasing Tol/ n C 7 ratios and concurrent 13 C enrichment and D depletion. In contrast, sapropelic LOCs from the Tazhong and Shunbei oilfields displayed low and narrow Tol/ n C 7 ranges that overlap with thermal maturity trends. LOCs from the Tazhong oilfield display a negative δ 13 C MCH – δ D MCH correlation and exhibited molecular–isotopic trends of decreasing n C 7 /MCH accompanied by 13 C enrichment and D depletion, diagnostic of evaporative fractionation. By contrast, LOCs from the Shunbei oilfield display a positive δ 13 C MCH – δ D MCH trend, with no coherent molecular–isotopic covariation, indicating overprinting by multiple alteration processes. Overall, the contrasting carbon and hydrogen isotope fractionation behaviors effectively distinguish evaporative fractionation from other secondary processes, providing a robust framework for tracing complex petroleum genesis in deep superimposed basins.