Abstract
Liquid hydrocarbons (LHCs) derived from humic organic matter (HOM) exhibit high potential for late-stage gas generation through recombination reactions. The biomarker composition and isotopic geochemical signatures of these LHCs can provide insights into the origin, maturity, and depositional environment of organic matter. Semi-open system thermal simulation experiments were performed on typical low-maturity humic source rocks from the Ordos Basin. The resulting LHCs were quantitatively characterized, followed by separation into compound-grouped fractions and subsequent analysis by GC–MS and stable carbon and hydrogen isotopic measurements. Based on the yield characteristics of LHCs, the applicability of biomarker parameters in HOM is systematically assessed, and the carbon and hydrogen isotopic fractionation mechanisms of alkanes are explored. The results demonstrate that: (1) LHCs are generated throughout the entire pyrolysis process of HOM, with the C15+ resins and asphaltenes fraction accounting for a relatively high proportion. The n-alkanes in HOM are dominated by short- to medium-chain homologues. Within the maturity range covered by pyrolysis experiments, the δ13C values of compound-grouped fractions are heavier than −27‰; those of n-alkanes are heavier than −29‰, and the corresponding δD values are lighter than −140‰, (2) the distribution of n-alkanes, relative content composition of steranes, and carbon isotopic compositions of polar fractions collectively indicate that the organic matter was predominantly derived from terrestrial C3 plants. Isoprenoid alkane ratios and the gammacerane index suggest that the source rock was deposited in a dysoxic freshwater environment, and terpane parameters exhibit a favorable correlation with thermal maturity across the oil-generation window, and (3) the kinetic isotope effect (KIE), along with cracking of polar compounds and C15+ aromatic hydrocarbons, causes the average δ13C values of n-alkanes to initially increase and subsequently decrease. Throughout thermal maturation, aromatic moieties and unsaturated cross-linked structures in HOM undergo hydrogen isotopic exchange with D-depleted formation water. Cracking via active free-radical reactions further amplifies this isotopic exchange, yielding n-alkanes with lighter average δD values. The distinct linear distribution patterns between δ13C and δD values of n-alkanes can be utilized to discriminate mixed-source oils and provide preliminary maturity assessment. This study enhances the understanding of the geochemical characteristics of LHCs derived from HOM and provides a scientific basis for the evaluation of hydrocarbon generation potential, gas-source correlation, and maturity in humic source rocks.
Paper Informaiton：
Haohao Wang, Ziqi Feng*, Zhen Qiu, Qin Zhang, Chenzhao Quan, Mingxiang Yang, Huayao Zou, Chaojin Lu, 2026. Geochemical characteristics of liquid hydrocarbons derived from humic organic matter: Insights from semi-open system thermal simulation experiments. International Journal of Coal Geology 324, 105068. https://doi.org/10.1016/j.coal.2026.105068.
