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Five model compounds with representative chemical structures were selected for use in simulation experiments of pyrolytic gas production. The gas production and isotopic fractionation characteristics were observed and analyzed. Then, the factors affecting carbon isotope fractionation during natural gas generation were discussed, and a fractionation model was established and calibrated. We concluded that the final hydrocarbon gas (C1–5) yield of octadecane, octadecylamine, octadecanoic acid, decahydronaphthalene, and 9‐phenylanthracene decreased in turn with the effective hydrogen content. Compared with linear alkanes or alkyl compounds, cycloalkanes have higher thermal stability and generate gas later. The variation in the carbon isotopic composition of natural gas is primarily controlled by the following three factors: (a) the thermal evolution of organic matter results in a gradually heavier isotopic composition for the main gas production stage. (b) Gas inherits the isotopic composition of its parent material, and this effect is evident when the chemical structure and gas generation mechanism between parent materials are similar. (c) The structure of organic matter determines the reaction mechanism of gas generation, which has a significant influence on the range and trend of carbon isotope fractionation in the process of methane generation. An improved chemical kinetic model can accurately characterize carbon isotope fractionation during gas generation. Key Points: The factors affecting carbon isotope fractionation during natural gas generation are revealedA chemical kinetic model describing isotope fractionation is establishedThe structure and gas‐generation mechanisms of organic matter controlling isotopic composition of natural gas are explained [ABSTRACT FROM AUTHOR]

Understanding the effects of pore size on shale gas adsorption behavior is necessary for accurate evaluation of adsorbed gas content under geological conditions. Shale is a porous medium, and the pore structure of the shale reservoir is complicated, with a wide distribution of aperture sizes. Critical parameters for investigating pore size effects on shale gas adsorption behavior were determined, using Grand Canonical Monte Carlo (GCMC) simulations, and the shale gas occurrence state in varying sized kerogen pores was documented, by linking GCMC simulations to the experimental pore size distribution. It was found that using the excess adsorption estimation, in terms of per unit surface area (PUSA), which was obtained from the free gas density calculated by using the GCMC method in a bulk simulation cell, and then derived from the free volume probed by the methane, was a reasonable way of demonstrating pore size effects on shale gas adsorption behavior. The distribution profiles of both the gas density and the interaction energy, rather than their average values, could be used to reflect this pore size effect objectively. Gas density in the adsorption phase rose non-monotonically with reducing pore size, under the combined influence of the interactions' overlapping effects and the limited pore space, and the overlapping threshold was determined to be 1.24 nm for the experiments. The gas in the pores that were smaller than the overlapping threshold, which was difficult to desorb under geological pressures, accounted for approximately 40.53% of the total adsorbed gas in the kerogen. The adsorbed gas in the kerogen lay mainly (84.97%) in smaller pores (<5 nm), while the free gas was mainly located (77.70%) in larger pores (>5 nm). • Overlapping threshold (OT) is 1.8 nm for simulations and 1.24 nm for experiments. • In small pores (OT), pore size no longer affects the adsorption of shale gas. • Adsorbed gas mainly lies in pores with the size smaller than 5 nm. [ABSTRACT FROM AUTHOR]

Abstract Understanding the adsorption mechanisms of different gas molecules on various minerals is crucial for accurately modelling shale gas adsorption behaviors and for objectively evaluating adsorbed gas contents under geological conditions. We simulated the adsorption behaviors of CH 4 , CO 2 and N 2 on both organic matter and inorganic minerals at 60 °C and 90 °C over a range of pressures up to 50 MPa by using the Grand Canonical Monte Carlo (GCMC) method. It has been found that both the comprehensive effect of the adsorption sites with differential adsorption capacity and the distribution density of the adsorption sites on the organic matter and inorganic mineral surfaces control the adsorption capacity in terms of per unit surface area of minerals. For individual minerals with a certain adsorption capacity in terms of per unit surface area, the specific surface area of individual minerals is the critical factor that determines the adsorption capacity in terms of per unit mass of the minerals. The interaction among gas molecules also affects the adsorption behavior slightly. We further compared the adsorption capacity among various gas molecules on both organic matter and inorganic minerals by inspecting the strength and distribution density of the adsorption sites on mineral surfaces, the specific surface area of the minerals and the interaction strength among gas molecules. These investigations allowed us to identify the key factors controlling shale gas adsorption mechanisms on different minerals, which provide some helpful insights for both of the exploration and the development of shale gas. Highlights • Both strength and density of adsorption sites control adsorption capacity of mineral surface. • Strengths of adsorption sites on clay surface are different, but similar on kerogen surface. • Specific surface area is one of the main factors controls adsorption behaviors. • Density of adsorption sites and specific surface area of kerogen are larger than that of clay. • Interactions among gas molecules also affect the adsorption behaviors. [ABSTRACT FROM AUTHOR]