The total organic carbon （TOC） in the marine source rock of the Ordos Basin mostly ranges from 0.2% to 0.5%. The industrial standard commonly states that the TOC value has to be no less than 0.5% （0.4% for high mat...The total organic carbon （TOC） in the marine source rock of the Ordos Basin mostly ranges from 0.2% to 0.5%. The industrial standard commonly states that the TOC value has to be no less than 0.5% （0.4% for high mature or over-mature source rock） to form large petroleum reservoirs. However, gas source correlation indicates that the natural gas in the Jingbian gas field does receive contribution from marine source rocks. In order to determine the effect of Carboxylate salts （or called as organic acid salts） on TOC in highly mature source rocks with low TOC value, we sampled the Ordovician marine source rock and the Permian transitional facies source rock in one drilled well in the southern Ordos Basin and performed infrared and GC-MS analysis. It is found that both kerogen-derived organic acids and carboxylate salt-conversed organic acids exist in both marine and transitional facies source rocks. The carboxylate salt-conversed organic acids mainly come from the complete acidification of carboxylate salts, which confirms the presence of carboxylate salts in the marine source rocks. Although the C16：o peak is the main peak for the organic acids both before and after acidification, the carboxylate salt-conversed organic acids have much less relative abundance ahead of C^6：o compared with that of the kerogen-based and free organic acids. This observation suggests that the kerogen-based and free organic acids mainly decarboxylate to form lower carboxylic acids, whereas the carboxylate salt-conversed organic acids mainly break down into paraffins. By using calcium hexadecanoate as the reference to quantify the kerogen-derived and carboxylate salt-conversed organic acids, the high TOC （〉2.0%） marine source rocks have low carboxylate salt content and the low TOC （0.2%-0.5%） marine source rocks contain high content of carboxylate salt. Therefore, for the marine source rocks with 0.2%-0.5% TOC, the carboxylate salts may be a potential gas source at high maturity stage.展开更多
In-situ upgrading by heating is feasible for low-maturity shale oil,where the pore space dynamically evolves.We characterize this response for a heated substrate concurrently imaged by SEM.We systematically follow the...In-situ upgrading by heating is feasible for low-maturity shale oil,where the pore space dynamically evolves.We characterize this response for a heated substrate concurrently imaged by SEM.We systematically follow the evolution of pore quantity,size(length,width and cross-sectional area),orientation,shape(aspect ratio,roundness and solidity)and their anisotropy—interpreted by machine learning.Results indicate that heating generates new pores in both organic matter and inorganic minerals.However,the newly formed pores are smaller than the original pores and thus reduce average lengths and widths of the bedding-parallel pore system.Conversely,the average pore lengths and widths are increased in the bedding-perpendicular direction.Besides,heating increases the cross-sectional area of pores in low-maturity oil shales,where this growth tendency fluctuates at<300℃ but becomes steady at>300℃.In addition,the orientation and shape of the newly-formed heating-induced pores follow the habit of the original pores and follow the initial probability distributions of pore orientation and shape.Herein,limited anisotropy is detected in pore direction and shape,indicating similar modes of evolution both bedding-parallel and bedding-normal.We propose a straightforward but robust model to describe evolution of pore system in low-maturity oil shales during heating.展开更多
Based on the current research status of shale oil exploration and development at home and abroad,through field observations,dissection of typical shale oil regions,analysis and testing of organic-rich shale samples,et...Based on the current research status of shale oil exploration and development at home and abroad,through field observations,dissection of typical shale oil regions,analysis and testing of organic-rich shale samples,etc.,we compare the differences in geological and engineering characteristics of shale oil reservoirs in marine and continental basins in China and the United States,put forward several issues worthy of attention in the exploration and development of lacustrine shale oil in typical basins of China,including the concept of tight oil and shale oil,vertical permeability and horizontal permeability,differences between continental and marine shale oil reservoirs,medium-low maturity and medium-high maturity,source-reservoir and source-caprock,geology and engineering,selection criteria of favorable areas and“sweet spots”,basic scientific research and application research.By comparing and analyzing organic-rich shales in the Triassic Yanchang Formation of the Ordos Basin,the Permian Lucaogou Formation in the Jimsar Sag of the Junggar Basin,the Permian Fengcheng Formation in the Mahu Sag,the Cretaceous Qingshankou&Nenjiang Formation in the Songliao Basin and the Paleogene Kongdian&Shahejie Formation in the Bohai Bay Basin,we believe that three key scientific issues must be studied in-depth from shale oil exploration to development in the future:(1)the physical,chemical and biological processes during the deposition of terrestrial fine-grained sediments and the formation mechanism of terrestrial organic-rich shale;(2)the dynamic evolution of diagenesis-hydrocarbon generation-reservoir formation,and the mechanisms of hydrocarbon formation and accumulation;(3)the fracturing mechanisms of terrestrial shale layers in different diagenetic stages and the multi-phase and multi-scale flow mechanism of shale oil in shale layers of different maturities.In addition,we should clarify the main controlling factors of shale oil reservoir characterization,oil-bearing properties,compressibility and fluidity of shale oil with展开更多
: Four typical distribution patterns of pentacyclic triterpenoid hydrocarbons (types A-D) are distinguished in the low-mature source rocks from eastern China. Type A has a relatively high content of pentacyclic triter...: Four typical distribution patterns of pentacyclic triterpenoid hydrocarbons (types A-D) are distinguished in the low-mature source rocks from eastern China. Type A has a relatively high content of pentacyclic triterpenes. It exists in immature sediments and the distribution and abundance of triterpenes vary with the maturity of the sediments. An unknown C30 triterpene (UCT2) has also been detected in very shallow sediments. This compound is very unstable and disappears rapidly with the increase of depth. Type B is characterized by a relatively high amount of 17α(H), 21β(H)-30-homohopane. This kind of distribution pattern is common in coals and terrestrial sediments of low maturity. Type C has a relatively high content of diahopane and neohopane series. The analysis shows that this distribution pattern may have an indirect relationship with the input of higher plants despite its microbial source. There are C30—unconfirmed triterpane (UCT2) and a relatively high content of C35 hopane in type D. The distribution patterns of pentacyclic triterpenoids are varied under influences of biological sources, diagentic and sedimentary environments, thermal maturity etc. Therefore, they can provide useful information about the source rocks. Based on geochemical characteristics of the source rocks, the authors postulated that the type B and type C patterns are mainly related to suboxic-anoxic environments. Type D indicates the reducing to anoxic environment, in which C30-UCT3 may be related to some specific biological source in saline environments. The precursors and thermal evolution of some triterpenoid hydrocarbons are also discussed herein.展开更多
Coal-bearing strata are widespread in the western Great Khingan Mountains. Abundant coal resources have been found in the Jurassic Alatanheli Groups, the Cretaceous Bayanhua Groups, the Damoguaihe Formation and the Yi...Coal-bearing strata are widespread in the western Great Khingan Mountains. Abundant coal resources have been found in the Jurassic Alatanheli Groups, the Cretaceous Bayanhua Groups, the Damoguaihe Formation and the Yimin Formation. The organic geochemical characteristics were analyzed in combination with hydrocarbon source rock evaluation and molecular organic geochemistry experiments, and the coal gas potential of coal seams was evaluated. The source rock evaluation results indicated that the Mesozoic coal samples have the characteristics of high organic matter abundance(TOC>30%), low maturity(Ro values of approximately 0.6%), and type Ⅲ composition. The hydrocarbon generation potentials of the Alatanheli Groups and Bayanhua Groups are high, while the generation potentials of the Damoguaihe Formation and the Yimin Formation are low. The results of geochemistry show that the depositional environment of the coal seam was a lacustrine, oxidizing environment with a low salinity, and the source of the organic matter was mainly higher plants. Affected by weak degradation, the coal seams mainly formed low-maturity gas of thermal catalytic origin. The Cretaceous coal seams contain a large amount of phytoplankton groups deposited in a low-stability environment affected by a transgression event, and the potential range varied widely. For the Jurassic coal seams, the depositional environment was more stable, and the coal seams feature a higher coal-forming gas potential.展开更多
Temperature-triaxial pressure permeability testing at the axial pressure of 8 MPa and confining pressure of 10 MPa,closed shale system pyrolysis experiment by electrical heating and scanning electron microscopy analys...Temperature-triaxial pressure permeability testing at the axial pressure of 8 MPa and confining pressure of 10 MPa,closed shale system pyrolysis experiment by electrical heating and scanning electron microscopy analysis are used to study the evolution mechanism of in-situ permeability in the direction parallel to bedding of low-maturity shale from Member 2(K_(2)n_(2))of Cretaceous Nenjiang Formation in northern Songliao Basin with mainly Type I kerogen under the effect of temperature.With the increasing temperature,the in-situ permeability presents a peak-valley-peak tendency.The lowest value of in-situ permeability occurs at 375℃.Under the same temperature,the in-situ permeability decreases with the increase of pore pressure.The in-situ permeability evolution of low-maturity shale can be divided into 5 stages:(1)From 25℃to 300℃,thermal cracking and dehydration of clay minerals improve the permeability.However,the value of permeability is less than 0.01×10^(-3)μm^(2).(2)From 300℃to 350℃,organic matter pyrolysis and hydrocarbon expulsion result in mineral intergranular pores and micron pore-fractures,these pores and fractures form an interconnected pore network at limited scale,improving the permeability.But the liquid hydrocarbon,with high content of viscous asphaltene,is more difficult to move under stress and more likely to retain in pores,causing slow rise of the permeability.(3)From 350℃to 375℃,pores are formed by organic matter pyrolysis,but the adsorption swelling of liquid hydrocarbon and additional expansion thermal stress constrained by surrounding stress compress the pore-fracture space,making liquid hydrocarbon difficult to expel and permeability reduce rapidly.(4)From 375℃to 450℃,the interconnected pore network between different mineral particles after organic matter conversion,enlarged pores and transformation of clay minerals promote the permeability to increase constantly even under stress constraints.(5)From 450℃to 500℃,the stable pore system and crossed fracture system in 展开更多
The origin of the Jurassic low maturity natural gas in the Turpan-Hami Basin has attracted scientists’attention for some time,and it is known that radiogenesis may have played a role.However,little has been done on t...The origin of the Jurassic low maturity natural gas in the Turpan-Hami Basin has attracted scientists’attention for some time,and it is known that radiogenesis may have played a role.However,little has been done on the uranium-rich background of the whole basin.Based on plentiful logging and geological data for the Jurassic strata in the Turpan-Hami Basin,this research examined the features and factors controlling the distribution of rocks with high gamma reading.The results show that 70%-100%of the rocks with high gamma readings correspond to mudstones in the prodelta subfacies rather than those in semideep-deep lacustrine subfacies rich in mudstones.Therefore,we propose that the distribution of rocks with high gamma readings is mainly controlled by sedimentary facies rather than by lithology.Further analysis of the gamma spectrometry logging data shows that high gamma values are more strongly correlated with U content than with Th or K content.By comparing the U and Th contents of felsic rocks in peripheral provenances,we find that the Jueluotage Mountain and Harlik Mountain were the dominant uranium sources for the Jurassic Turpan-Hami Basin.Radiolysis due to high-level uranium in the prodelta subfacies can make the low maturity source rocks generate H2 and CH4,thus contributing to the production of low maturity natural gas in the Turpan-Hami Basin.展开更多
Low maturity coal samples were taken from the Ordos Basin to conduct gold tube thermal simulation experiment in a closed system,and the characteristics of the products were analyzed to find out the fractionation mecha...Low maturity coal samples were taken from the Ordos Basin to conduct gold tube thermal simulation experiment in a closed system,and the characteristics of the products were analyzed to find out the fractionation mechanism of carbon isotopes and the causes of abnormal carbon isotopic compositions of natural gas.At the heating rates of 2℃/h(slow)and 20℃/h(rapid),the low maturity coal samples of the Ordos Basin had the maximum yields of alkane gas of 302.74 mL/g and 230.16 mL/g,theδ13C1 ranges of-34.8‰to-23.6‰and-35.5‰to-24.0‰;δ13C2 ranges of-28.0‰to-9.0‰and-28.9‰to-8.3‰;andδ13C3 ranges of-25.8‰to-14.7‰and-26.4‰to-13.2‰,respectively.Alkane gas in the thermal simulation products of rapid temperature rise process showed obvious partial reversal of carbon isotope series at 550℃,and at other temperatures showed positive carbon isotope series.In the two heating processes,theδ13C1 turned lighter first and then heavier,and the non-monotonic variation of theδ13C1 values is because the early CH4 is from different parent materials resulted from heterogeneity of organic matter or the carbon isotope fractionation formed by activation energy difference of early enriched 12CH4 and late enriched 13CH4.The reversal of carbon isotope values of heavy hydrocarbon gas can occur not only in high to over mature shale gas(oil-type gas),but also in coal-derived gas.Through thermal simulation experiment of toluene,it is confirmed that the carbon isotope value of heavy hydrocarbon gas can be reversed and inversed at high to over mature stage.The isotope fractionation effect caused by demethylation and methyl linkage of aromatic hydrocarbons may be an important reason for carbon isotope inversion and reversal of alkane gas at the high to over mature stage.展开更多
基金supported by National Natural Science Foundation of China(Grant Nos.41173035&41322016)National Key Foundational Research and Development Project(Grant No.2012CB214800)the National Science & Technology Special Project(Grant No.2011ZX05005-004-004)
文摘The total organic carbon （TOC） in the marine source rock of the Ordos Basin mostly ranges from 0.2% to 0.5%. The industrial standard commonly states that the TOC value has to be no less than 0.5% （0.4% for high mature or over-mature source rock） to form large petroleum reservoirs. However, gas source correlation indicates that the natural gas in the Jingbian gas field does receive contribution from marine source rocks. In order to determine the effect of Carboxylate salts （or called as organic acid salts） on TOC in highly mature source rocks with low TOC value, we sampled the Ordovician marine source rock and the Permian transitional facies source rock in one drilled well in the southern Ordos Basin and performed infrared and GC-MS analysis. It is found that both kerogen-derived organic acids and carboxylate salt-conversed organic acids exist in both marine and transitional facies source rocks. The carboxylate salt-conversed organic acids mainly come from the complete acidification of carboxylate salts, which confirms the presence of carboxylate salts in the marine source rocks. Although the C16：o peak is the main peak for the organic acids both before and after acidification, the carboxylate salt-conversed organic acids have much less relative abundance ahead of C^6：o compared with that of the kerogen-based and free organic acids. This observation suggests that the kerogen-based and free organic acids mainly decarboxylate to form lower carboxylic acids, whereas the carboxylate salt-conversed organic acids mainly break down into paraffins. By using calcium hexadecanoate as the reference to quantify the kerogen-derived and carboxylate salt-conversed organic acids, the high TOC （〉2.0%） marine source rocks have low carboxylate salt content and the low TOC （0.2%-0.5%） marine source rocks contain high content of carboxylate salt. Therefore, for the marine source rocks with 0.2%-0.5% TOC, the carboxylate salts may be a potential gas source at high maturity stage.
基金financially supported by the National Key Research and Development Program of China(Grant No.2022YFE0129800)the National Natural Science Foundation of China(Grant No.42202204)。
文摘In-situ upgrading by heating is feasible for low-maturity shale oil,where the pore space dynamically evolves.We characterize this response for a heated substrate concurrently imaged by SEM.We systematically follow the evolution of pore quantity,size(length,width and cross-sectional area),orientation,shape(aspect ratio,roundness and solidity)and their anisotropy—interpreted by machine learning.Results indicate that heating generates new pores in both organic matter and inorganic minerals.However,the newly formed pores are smaller than the original pores and thus reduce average lengths and widths of the bedding-parallel pore system.Conversely,the average pore lengths and widths are increased in the bedding-perpendicular direction.Besides,heating increases the cross-sectional area of pores in low-maturity oil shales,where this growth tendency fluctuates at<300℃ but becomes steady at>300℃.In addition,the orientation and shape of the newly-formed heating-induced pores follow the habit of the original pores and follow the initial probability distributions of pore orientation and shape.Herein,limited anisotropy is detected in pore direction and shape,indicating similar modes of evolution both bedding-parallel and bedding-normal.We propose a straightforward but robust model to describe evolution of pore system in low-maturity oil shales during heating.
文摘Based on the current research status of shale oil exploration and development at home and abroad,through field observations,dissection of typical shale oil regions,analysis and testing of organic-rich shale samples,etc.,we compare the differences in geological and engineering characteristics of shale oil reservoirs in marine and continental basins in China and the United States,put forward several issues worthy of attention in the exploration and development of lacustrine shale oil in typical basins of China,including the concept of tight oil and shale oil,vertical permeability and horizontal permeability,differences between continental and marine shale oil reservoirs,medium-low maturity and medium-high maturity,source-reservoir and source-caprock,geology and engineering,selection criteria of favorable areas and“sweet spots”,basic scientific research and application research.By comparing and analyzing organic-rich shales in the Triassic Yanchang Formation of the Ordos Basin,the Permian Lucaogou Formation in the Jimsar Sag of the Junggar Basin,the Permian Fengcheng Formation in the Mahu Sag,the Cretaceous Qingshankou&Nenjiang Formation in the Songliao Basin and the Paleogene Kongdian&Shahejie Formation in the Bohai Bay Basin,we believe that three key scientific issues must be studied in-depth from shale oil exploration to development in the future:(1)the physical,chemical and biological processes during the deposition of terrestrial fine-grained sediments and the formation mechanism of terrestrial organic-rich shale;(2)the dynamic evolution of diagenesis-hydrocarbon generation-reservoir formation,and the mechanisms of hydrocarbon formation and accumulation;(3)the fracturing mechanisms of terrestrial shale layers in different diagenetic stages and the multi-phase and multi-scale flow mechanism of shale oil in shale layers of different maturities.In addition,we should clarify the main controlling factors of shale oil reservoir characterization,oil-bearing properties,compressibility and fluidity of shale oil with
文摘: Four typical distribution patterns of pentacyclic triterpenoid hydrocarbons (types A-D) are distinguished in the low-mature source rocks from eastern China. Type A has a relatively high content of pentacyclic triterpenes. It exists in immature sediments and the distribution and abundance of triterpenes vary with the maturity of the sediments. An unknown C30 triterpene (UCT2) has also been detected in very shallow sediments. This compound is very unstable and disappears rapidly with the increase of depth. Type B is characterized by a relatively high amount of 17α(H), 21β(H)-30-homohopane. This kind of distribution pattern is common in coals and terrestrial sediments of low maturity. Type C has a relatively high content of diahopane and neohopane series. The analysis shows that this distribution pattern may have an indirect relationship with the input of higher plants despite its microbial source. There are C30—unconfirmed triterpane (UCT2) and a relatively high content of C35 hopane in type D. The distribution patterns of pentacyclic triterpenoids are varied under influences of biological sources, diagentic and sedimentary environments, thermal maturity etc. Therefore, they can provide useful information about the source rocks. Based on geochemical characteristics of the source rocks, the authors postulated that the type B and type C patterns are mainly related to suboxic-anoxic environments. Type D indicates the reducing to anoxic environment, in which C30-UCT3 may be related to some specific biological source in saline environments. The precursors and thermal evolution of some triterpenoid hydrocarbons are also discussed herein.
基金the National Natural Science Foundation of China(Grant No.41430322)The anonymous journal reviewers and editors are acknowledged for their valuable comments.
文摘Coal-bearing strata are widespread in the western Great Khingan Mountains. Abundant coal resources have been found in the Jurassic Alatanheli Groups, the Cretaceous Bayanhua Groups, the Damoguaihe Formation and the Yimin Formation. The organic geochemical characteristics were analyzed in combination with hydrocarbon source rock evaluation and molecular organic geochemistry experiments, and the coal gas potential of coal seams was evaluated. The source rock evaluation results indicated that the Mesozoic coal samples have the characteristics of high organic matter abundance(TOC>30%), low maturity(Ro values of approximately 0.6%), and type Ⅲ composition. The hydrocarbon generation potentials of the Alatanheli Groups and Bayanhua Groups are high, while the generation potentials of the Damoguaihe Formation and the Yimin Formation are low. The results of geochemistry show that the depositional environment of the coal seam was a lacustrine, oxidizing environment with a low salinity, and the source of the organic matter was mainly higher plants. Affected by weak degradation, the coal seams mainly formed low-maturity gas of thermal catalytic origin. The Cretaceous coal seams contain a large amount of phytoplankton groups deposited in a low-stability environment affected by a transgression event, and the potential range varied widely. For the Jurassic coal seams, the depositional environment was more stable, and the coal seams feature a higher coal-forming gas potential.
基金Supported by the Special Major Scientific Research Project of PetroChina(2021ZZ10-01)。
文摘Temperature-triaxial pressure permeability testing at the axial pressure of 8 MPa and confining pressure of 10 MPa,closed shale system pyrolysis experiment by electrical heating and scanning electron microscopy analysis are used to study the evolution mechanism of in-situ permeability in the direction parallel to bedding of low-maturity shale from Member 2(K_(2)n_(2))of Cretaceous Nenjiang Formation in northern Songliao Basin with mainly Type I kerogen under the effect of temperature.With the increasing temperature,the in-situ permeability presents a peak-valley-peak tendency.The lowest value of in-situ permeability occurs at 375℃.Under the same temperature,the in-situ permeability decreases with the increase of pore pressure.The in-situ permeability evolution of low-maturity shale can be divided into 5 stages:(1)From 25℃to 300℃,thermal cracking and dehydration of clay minerals improve the permeability.However,the value of permeability is less than 0.01×10^(-3)μm^(2).(2)From 300℃to 350℃,organic matter pyrolysis and hydrocarbon expulsion result in mineral intergranular pores and micron pore-fractures,these pores and fractures form an interconnected pore network at limited scale,improving the permeability.But the liquid hydrocarbon,with high content of viscous asphaltene,is more difficult to move under stress and more likely to retain in pores,causing slow rise of the permeability.(3)From 350℃to 375℃,pores are formed by organic matter pyrolysis,but the adsorption swelling of liquid hydrocarbon and additional expansion thermal stress constrained by surrounding stress compress the pore-fracture space,making liquid hydrocarbon difficult to expel and permeability reduce rapidly.(4)From 375℃to 450℃,the interconnected pore network between different mineral particles after organic matter conversion,enlarged pores and transformation of clay minerals promote the permeability to increase constantly even under stress constraints.(5)From 450℃to 500℃,the stable pore system and crossed fracture system in
基金supported by the National Natural Science Foundation of China(grant numbers 41330315,41402093)。
文摘The origin of the Jurassic low maturity natural gas in the Turpan-Hami Basin has attracted scientists’attention for some time,and it is known that radiogenesis may have played a role.However,little has been done on the uranium-rich background of the whole basin.Based on plentiful logging and geological data for the Jurassic strata in the Turpan-Hami Basin,this research examined the features and factors controlling the distribution of rocks with high gamma reading.The results show that 70%-100%of the rocks with high gamma readings correspond to mudstones in the prodelta subfacies rather than those in semideep-deep lacustrine subfacies rich in mudstones.Therefore,we propose that the distribution of rocks with high gamma readings is mainly controlled by sedimentary facies rather than by lithology.Further analysis of the gamma spectrometry logging data shows that high gamma values are more strongly correlated with U content than with Th or K content.By comparing the U and Th contents of felsic rocks in peripheral provenances,we find that the Jueluotage Mountain and Harlik Mountain were the dominant uranium sources for the Jurassic Turpan-Hami Basin.Radiolysis due to high-level uranium in the prodelta subfacies can make the low maturity source rocks generate H2 and CH4,thus contributing to the production of low maturity natural gas in the Turpan-Hami Basin.
基金Supported by the National Natural Science Foundation of China(41902160,41625009)the China Postdoctoral Science Foundation(2019M650967,2020T130721)the China National Science and Technology Major Project(2016ZX05007-001)
文摘Low maturity coal samples were taken from the Ordos Basin to conduct gold tube thermal simulation experiment in a closed system,and the characteristics of the products were analyzed to find out the fractionation mechanism of carbon isotopes and the causes of abnormal carbon isotopic compositions of natural gas.At the heating rates of 2℃/h(slow)and 20℃/h(rapid),the low maturity coal samples of the Ordos Basin had the maximum yields of alkane gas of 302.74 mL/g and 230.16 mL/g,theδ13C1 ranges of-34.8‰to-23.6‰and-35.5‰to-24.0‰;δ13C2 ranges of-28.0‰to-9.0‰and-28.9‰to-8.3‰;andδ13C3 ranges of-25.8‰to-14.7‰and-26.4‰to-13.2‰,respectively.Alkane gas in the thermal simulation products of rapid temperature rise process showed obvious partial reversal of carbon isotope series at 550℃,and at other temperatures showed positive carbon isotope series.In the two heating processes,theδ13C1 turned lighter first and then heavier,and the non-monotonic variation of theδ13C1 values is because the early CH4 is from different parent materials resulted from heterogeneity of organic matter or the carbon isotope fractionation formed by activation energy difference of early enriched 12CH4 and late enriched 13CH4.The reversal of carbon isotope values of heavy hydrocarbon gas can occur not only in high to over mature shale gas(oil-type gas),but also in coal-derived gas.Through thermal simulation experiment of toluene,it is confirmed that the carbon isotope value of heavy hydrocarbon gas can be reversed and inversed at high to over mature stage.The isotope fractionation effect caused by demethylation and methyl linkage of aromatic hydrocarbons may be an important reason for carbon isotope inversion and reversal of alkane gas at the high to over mature stage.
