Chemical fractionation during the phase separation of hydrocarbons and its applications in tracing petroleum origins

As one of the important geological fluids within the Earth's crust, the generation, evolution, and migration of hydrocarbons are related to the formation of large-scale petroleum reservoirs. The cracking of kerogen to generate hydrocarbons is the primary pathway for petroleum formation. After generation, hydrocarbon with different carbon numbers have differential migration pathways, entering different phase states, thereby forming petroleum reservoirs. However, no effective approach can characterize the chemical fractionation of hydrocarbons from their formation to their entry into the gas-liquid phases, which impedes the in-depth understanding of phase separation and gas-liquid phase equilibrium processes during the initial migration after kerogen cracking. In this study, based on an established online thermal desorption hydrocarbon extraction method for mudstone, we investigate the chemical fractionation of hydrocarbons between the gas and liquid phases in a short-distance migration within a tight oil and gas system. The results show that hydrocarbons of the same carbon number also exhibit chemical fractionation between different phases. Compared with isomeric alkanes, normal alkanes tend to enrich in the liquid phase (oil and formation water), while aromatic hydrocarbons are more inclined to concentrate in formation water. For source rocks, the parameters of thermally desorbed hydrocarbons lie between the enriched and depleted phase states. A mathematical model based on the law of mass conservation further confirms that, in relatively confined petroleum systems, there should be a linear relationship between the normal/isomeric and aromatic/alkane ratios in the original hydrocarbons, oil, natural gas, and formation water. This result, along with corresponding experimental methods, provides a theoretical foundation for the study of phase separation and gas-liquid phase equilibrium mechanisms in the initial migration of hydrocarbons, and offers new approach for direct source rock identification.