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.

Natural gas accumulation involves complex multi-source mixing and post-genetic alterations that can lead to carbon isotopic reversals. Here, we report for the first time the co-occurrence of bulk and propane position-specific (PS) isotope reversals in conventional overmature gas reservoirs. We propose an aromatization-stabilization mechanism to explain these anomalous isotopic patterns. The Longdong area of the Ordos Basin provides a unique natural laboratory due to its well-defined thermal maturity gradient, the presence of multiple source rocks, and the coexistence of gases derived from different kerogen types. Integrated geochemical analyses of Cambrian-Carboniferous gases from six wells, reveal two key findings. First, the natural gases are in the overmature stage and exhibit both bulk carbon isotope reversals (δ¹³C₁ > δ¹³C₂ > δ¹³C₃ or δ¹³C₁ > δ¹³C₂ < δ¹³C₃), and Position-specific carbon isotope of propane anomalies (Δ_C-T = δ¹³C_center−δ¹³C_terminal<0‰; δ¹³C_center: isotopic composition of the central carbon in propane, δ¹³C_terminal: isotopic composition of the terminal carbon in propane). The propane in natural gas exhibits 25%–50% contribution from the isopropyl pathway. This is a formation mechanism theoretically restricted to low-maturity systems. This indicates preservation of early-formed branched precursors through cyclization/aromatization processes. We establish an “early-aromatization, delayed-cracking” model. This work highlights position-specific isotope analysis as a powerful tool for identifying post-genetic modifications in deep conventional and unconventional reservoirs.

Hydrocarbons are one of the important fluids within the Earth's crust, and different biotic and abitoic processes can generate hydrocarbon during geological periods. Tracing the sources and sinks of hydrocarbons can help us better understand the carbon cycle of the earth. In this study, an improved approach of adsorbed hydrocarbons extraction from sediments was established. The improved thermal desorption approach, compound-specific isotope analysis and position-specific isotope analysis were integrated to investigate the molecular and intramolecular isotope fractionation between trace hydrocarbon gases within sediments and geological hydrocarbon deposits. The isotopic compositions of the terminal position carbon of propane (δ¹³C_terminal) serves as a correlation indicator between trace hydrocarbon gases within sediments and geological hydrocarbon deposits. The tight sandstone gas from the Turpan-Hami Basin is a first case study for the application of this novel method to trace hydrocarbon origins. The results showed that the hydrocarbons in the tight sandstone gases in the study area most likely originated from humic organic matter (type Ⅲ kerogen) at an early mature stage. δ¹³C_terminal values of the thermally desorbed propane gases from different source rocks were distinguishable and the values of the tight sandstone gases significantly overlap with those of the Lower Jurassic Sangonghe source rocks, suggesting their genetic relationship. Overall, the results provided novel position-specific carbon isotopic constraints on origins of hydrocarbons.