Carbon isotope fractionation during methane transport through tight sedimentary rocks:Phenomena,mechanisms,characterization,and implications

The phenomenon of carbon isotopic fractionation,induced by the transport of methane in tight sedimentary rocks through processes primarily involving diffusion and adsorption/desorption,is ubiquitous in nature and plays a significant role in numerous geological and geochemical systems.Consequently,understanding the mechanisms of transport-induced carbon isotopic fractionation both theoretically and experimentally is of considerable scientific importance.However,previous experimental studies have observed carbon isotope fractionation phenomena that are entirely distinct,and even exhibit opposing characteristics.At present,there is a lack of a convincing mechanistic explanation and valid numerical model for this discrepancy.Here,we performed gas transport experiments under different gas pressures(1–5 MPa)and confining pressures(10–20 MPa).The results show that methane carbon isotope fractionation during natural gas transport through shale is controlled by its pore structure and evolves regularly with increasing effective stress.Compared with the carbon isotopic composition of the source gas,the initial effluent methane is predominantly depleted in^(13)C,but occasionally exhibits^(13)C enrichment.The carbon isotopic composition of effluent methane converges to that of the source gas as mass transport reaches a steady state.The evolution patterns of the isotope fractionation curve,transitioning from the initial non-steady state to the final steady state,can be categorized into five distinct types.The combined effect of multi-level transport channels offers the most compelling mechanistic explanation for the observed evolution patterns and their interconversion.Numerical simulation studies demonstrate that existing models,including the Rayleigh model,the diffusion model,and the coupled diffusion-adsorption/desorption model,are unable to describe the observed complex isotope fractionation behavior.In contrast,the multi-scale multi-mechanism coupled model developed herein,incorporating diffusion and adsorption/desorption across multi-level transport channels,effectively reproduces all the observed fractionation patterns and supports the mechanistic rationale for the combined effect.Finally,the potential carbon isotopic fractionation resulting from natural gas transport in/through porous media and its geological implications are discussed in several hypothetical scenarios combining numerical simulations.These findings highlight the limitations of carbon isotopic parameters for determining the origin and maturity of natural gas,and underscore their potential in identifying greenhouse gas leaks and tracing sources.

Simulation Experiments on the Reaction of CH_4-CaSO_4 and Its Carbon Kinetic Isotope Fractionation

Thermochemical sulfate reduction (TSR) in geological deposits can account for the accumulation of H2S in deep sour gas reservoirs. In this paper, thermal simulation experiments on the reaction of CH4-CaSO4 were carried out using an autoclave at high temperatures and high pressures. The products were characterized with analytical methods including carbon isotope analysis. It is found that the reaction can proceed to produce H2S, H2O and CaCO3 as the main products. Based on the experimental results, the carbon kinetic isotope fractionation was investigated, and the value of Ki (kinetic isotope effect) was calculated. The results obtained in this paper can provide useful information to explain the occurrence of H2S in deep carbonate gas reservoirs.

Carbon Isotope Fraction during Subduction Zone Metamorphism

Carbon isotope derived from mantle rocks and diamonds occurring worldwide show a narrow interval of-8‰to-2‰,with a very broad distribution to lower values（;41‰）and higher values（;‰）(Cartigny et al.,2014).

A Study of the Migration and Accumulation Efficiency and the Genesis of Hydrocarbon Natural Gas in the Xujiaweizi Fault Depression

In order to investigate the migration and accumulation efficiency of hydrocarbon natural gas in the Xujiaweizi fault depression, and to provide new evidence for the classification of its genesis, a source rock pyrolysis experiment in a closed system was designed and carried out. Based on this, kinetic models for describing gas generation from organic matter and carbon isotope fractionation during this process were established, calibrated and then extrapolated to geologic conditions by combining the thermal history data of the Xushen-1 Well. The results indicate that the coal measures in the Xujiaweizi fault depression are typical ＂high-efficiency gas sources＂, the natural gas generated from them has a high migration and accumulation efficiency, and consequently a large-scale natural gas accumulation occurred in the area. The highly/over matured coal measures in the Xujiaweizi fault depression generate coaliferous gas with a high δ^13C1 value （〉 -20‰） at the late stage, making the carbon isotope composition of organic alkane gases abnormally heavy. In addition, the mixing and dissipation through the caprock of natural gas can result in the negative carbon isotope sequence （δ^13C1 〉δ^13C2 〉δ^13C3 〉δ^13C4） of organic alkane gases, and the dissipation can also lead to the abnormally heavy carbon isotope composition of organic alkane gases. As for the discovery of inorganic nonhydrocarbon gas reservoirs, it can only serve as an accessorial evidence rather than a direct evidence that the hydrocarbon gas is inorganic. As a result, it needs stronger evidence to classify the hydrocarbon natural gas in the Xujiaweizi fault depression as inorganic gas.

Water Role and Its Influence on Hydrogen Isotopic Composition of Natural Gas during Gas Generation

In order to discuss the role and influence of water during the generation of natural gas,the participation mechanism of water during the evolution of organic matter and its influences were summarized.In addition,we carried out an anhydrous cracking experiment of oil extracted from the Feixianguan Formation source rock in a closed system,which led to the establishment of the kinetic models for describing carbon and hydrogen isotopic fractionation during gas generation from organic matter.The models were calibrated and then applied to the northeastern Sichuan Basin.By combining a series of gas generation experiments from octadecane pyrolysis without water or with distilled water in varying mass proportions,several results were proved:(1) the hydrogen isotopic composition of natural gas becomes lighter with the participation of formation water;(2) we can quantitatively study the hydrogen isotopic fractionation with the kinetic model for describing carbon isotopic fractionation; (3) more abundant and reliable geological information can be obtained through the combined application of carbon and hydrogen isotopic indices.

Kinetic Parameters of Methane Generated from Source Rocks in the Kuqa Depression of Tarim Basin and Their Application

In a thermal simulation experiment of gold tubes of closed-system, calculating with the KINETICS and GOR-ISOTOPE KINETICS software, kinetic parameters of gas generation and methane carbon isotopic fractionation from Triassic-Jurassic hydrocarbon source rocks in the Kuqa depression of Tarim Basin are obtained. The activation energies of methane generated from Jurassic coal, Jurassic mudstone and Triassic mudstone in the Kuqa Depression are 197-268 kJ/mol, 180-260 kJ/mol and 214-289 kJ/mol, respectively, and their frequency factors are 5.265×10^13 s^-1, 9.761×10^11 s^-1 and 2.270×10^14 s^-1. This reflects their differences of hydrocarbon generation behaviors. The kinetic parameters of methane carbon isotopic fractionation are also different in Jurassic coal, Jurassic mudstone and Triassic mudstone, whose average activation energies are 228 kJ/mol, 205 kJ/mol and 231 kJ/mol, respectively. Combined with the geological background, the origin of natural gas in the Yinan-2 gas pool is discussed, and an accumulation model of natural gas is thus established. The Yinan- 2 gas is primarily derived from Jurassic coal-bearing source rocks in the Yangxia Sag. Main gas accumulation time is 5-0 Ma and the corresponding Ro is in the range from 1.25 %-1.95 %. The loss rate of natural gas is 25 %-30 %.

Dissipation of S-metolachlor and butachlor in agricultural soils and responses of bacterial communities:Insights from compound-specific isotope and biomolecular analyses

The soil dissipation of the widely used herbicides S-metolachlor(SM)and butachlor(BUT)was evaluated in laboratory microcosms at two environmentally relevant doses(15 and 150μg/g)and for two agricultural soils(crop and paddy).Over 80%of SM and BUT were dissipated within 60 and 30 days,respectively,except in experiments with crop soil at 150μg/g.Based on compound-specific isotope analysis(CSIA)and observed dissipation,biodegradation was the main process responsible for the observed decrease of SM and BUT in the paddy soil.For SM,biodegradation dominated over other dissipation processes,with changes of carbon isotope ratios(Δδ13C)of up to 6.5‰after 60 days,and concomitant production of ethane sulfonic acid(ESA)and oxanilic acid(OXA)transformation products.In crop soil experiments,biodegradation of SM occurred to a lesser extent than in paddy soil,and sorption was the main driver of apparent BUT dissipation.Sequencing of the 16 S rRNA gene showed that soil type and duration of herbicide exposure were the main determinants of bacterial community variation.In contrast,herbicide identity and spiking dose had no significant effect.In paddy soil experiments,a high(4:1,V/V)ESA to OXA ratio for SM was observed,and phylotypes assigned to anaerobic Clostridiales and sulfur reducers such as Desulfuromonadales and Syntrophobacterales were dominant for both herbicides.Crop soil microcosms,in contrast,were associated with a reverse,low(1:3,V/V)ratio of ESA to OXA for SM,and Alphaproteobacteria,Actinobacteria,and Bacillales dominated regardless of the herbicide.Our results emphasize the variability in the extent and modes of SM and BUT dissipation in agricultural soils,and in associated changes in bacterial communities.

Gas Generation from Coal and Coal-Measure Mudstone Source Rocks of the Xujiahe Formation in the Western Sichuan Depression,Sichuan Basin

The Triassic and Jurassic tight sandstone gas in the western Sichuan depression,Sichuan Basin has attracted much attention in recent years,and the Upper Triassic coal-bearing Xujiahe Formation is believed to be the major source rock.However,there are relatively few studies on the carbon isotopic heterogeneity of methane generated from coal-measure source rocks and the origin of the natural gases in Xinchang Gas Field is still controversial.In this study,one coal-measure mudstone sample and one coal sample of the Xujiahe Formation in western Sichuan Basin were selected for gold tube pyrolysis experiment to determine their gas generation characteristics.Geological extrapolation of gas generation and methane carbon isotope fractionation parameters reveals that the main gas generation stage of Xujiahe Formation ranges from Late Jurassic to Cretaceous in the Xinchang Gas Field.The natural gas in the 5th member of Xujiahe Formation in Xinchang Gas Field is mainly derived from the 5th member of Xujiahe Formation itself,i.e.,self-generation and self-reservoir,however,the gas in the Jurassic gas pools is mainly derived from the source rocks of the 3rd member of Xujiahe Formation rather than the 5th member of Xujiahe Formation.