Abstract
Light hydrocarbons (LHs) serve as a valuable analytical tool for elucidating natural gas genesis. However, the evolution of LHs and the applicability of related diagnostic indices under thermal cracking conditions remain insufficiently explored. In this study, a series of closed-system pyrolysis experiments were conducted on marine oil and reservoir limestone at high temperatures (350–500 °C) and high pressure (60 MPa) to simulate thermal cracking occurring in the subsurface. The results demonstrate that LHs content increases during the early cracking stage through the cracking of heavy components, then decreases rapidly in the later cracking stage as a result of its own thermal cracking. The thermal stability of individual components of LHs is controlled by carbon number and molecular structure, with high-carbon-number systematically degrading into lower-carbon-number species; n-alkanes dominate in the early cracking stage due to high yield, while cycloalkanes and aromatics become enriched later due to greater thermal resistance. It is found that the genetic correlation index K1 and the maturity indicator 2,4/2,3-dimethylpentane (DMP) maintain diagnostic value throughout all cracking stages. Most other parameters related to organic matter types, genetic correlations, and thermal maturity are only reliable in early cracking stage and become invalid at later stage. Thermal cracking complicates the interpretation of biodegradation and evaporative fractionation indicators. Three possible mechanisms, cyclization, aromatization, and demethylation, govern the LHs evolution, and the ratios of cycloalkanes to chain alkanes, aromatics to aliphatic components, and high to low-methyl-substituted cycloalkanes hold promise as indices of thermal maturity and cracking degree. The 2,2-, 2,4-, and 3,3-DMP isomers of C7 LHs exhibit exceptional stability, with ratios of their contents to n-heptane, 2,3-DMP, and 1,1-dimethylcycloalkane are potential indicators of thermal maturity and cracking level. The findings can be successfully applied to deep marine conventional gas in the Tarim and Sichuan Basins and shale gas in the Sichuan Basin, thus offering practical geochemical tools for exploration in these two strategic hydrocarbon frontiers.
Paper Information：
Chai, Z., Chen, Z.H, Luemba, M., Chen, Y., 2026. Evolution and diagnostic potential of light hydrocarbons in thermally cracked gas: Insights from crude oil pyrolysis experiments. Fuel, 406, 136967. https://doi.org/10.1016/j.fuel.2025.136967
