Methane transportation in tight sedimentary rocks results in significant and complex isotope fractionation. Existing models and mechanistic studies of isotope fractionation cannot fully explain the actual observations in gas transport processes that occur in natural reservoirs and in the laboratory. Thus, geological applications based on transport-related isotope fractionation have been lacking in substantive progress. Here, we established a multi-scale model in which seepage in fractures, diffusion and adsorption/desorption in matrix pores, and concentration diffusion in kerogen structural pores are coupled. The results show that both diffusion and adsorption/desorption lead to significant isotope fractionation, while the contribution of pressure-driven seepage, is limited. The diffusion of kerogen-dissolved gas is of considerable significance to isotope fractionation in the later stage of transport, despite its weak contribution to gas production. The sequence of controlling factors of isotope fractionation during pure diffusion is as follows: diffusion coefficient ratio (D^⁎/D) > initial pressure (P₀) > others, where the value of D^⁎/D mainly depends on the average pore size of rocks. The isotope fractionation caused by adsorption/desorption is closely related to the Langmuir parameters of sedimentary rock. It is essentially a dynamic non-equilibrium fractionation during adsorbed gas transport, rather than thermodynamic equilibrium fractionation between adsorbed gas and free gas. The model developed herein determines the contribution of each single effect to the apparent isotope fractionation and provides a novel method for obtaining the Langmuir parameters of rocks under in-situ geological conditions. Under variable boundary conditions, this model can be used to evaluate the gas resource potential and recoverable reserves in unconventional reservoirs and demonstrates the great potential for monitoring the production status of shale gas wells.

致密沉积岩中的甲烷运输会导致明显而复杂的同位素分馏。现有的同位素分馏模型和机理研究不能完全解释天然油藏和实验室中发生的天然气运移过程中的实际观测结果。因此，基于运输相关同位素分级分离的地质应用一直缺乏实质性进展。在这里，我们建立了一个多尺度模型，其中裂缝的渗透，基质孔隙的扩散和吸附/解吸以及干酪根结构孔隙的浓度扩散是耦合的。结果表明，扩散和吸附/解吸均导致显着的同位素分馏，而压力驱动的渗流的贡献是有限的。干酪根溶解气体的扩散对运输后期的同位素分馏具有重要意义，尽管它对气体生产的贡献很小。纯扩散过程中同位素分馏的控制因素顺序如下：扩散系数比（d ^⁎ / d）>初始压力（P ₀）>他人，其中的值d ^⁎ / d主要取决于岩石的平均孔径。吸附/解吸引起的同位素分馏与沉积岩的Langmuir参数密切相关。它实质上是吸附气体传输过程中的动态非平衡分馏，而不是吸附气体和自由气体之间的热力学平衡分馏。本文开发的模型确定了每种单一作用对表观同位素分馏的贡献，并提供了一种新方法来获取原位岩石的Langmuir参数地质条件。在可变边界条件下，该模型可用于评价非常规油藏的天然气资源潜力和可采储量，并显示出监测页岩气井生产状况的巨大潜力。