The pool-forming mechanism of coalbed methane has its own characteristics.In this paper, based on studies on the typical coal-bearing basins in China,it is pointed out that the reservoir formation of medium- to high-r...The pool-forming mechanism of coalbed methane has its own characteristics.In this paper, based on studies on the typical coal-bearing basins in China,it is pointed out that the reservoir formation of medium- to high-rank coalbed methane has experienced three critical stages：the coalbed methane generation and adsorption stage,the coalbed adsorption capacity enhancement stage,and the coalbed methane desorption-diffusion and preservation stage.The regional tectonic evolution, hydrodynamic conditions and sealing conditions play important roles in the stage of coalbed methane desorption-diffusion and preservation.Medium- to high-rank coalbed methane has three types of enrichment models,that is,the most favorable,the relatively favorable,and the unfavorable enrichment models.展开更多
Based on the pyrolysis products for the Jurassic low-mature coal under programmed temperature,and chemical and carbon isotopic compositions of natural gas from the Kuqa Depression, the genetic origin of natural gas wa...Based on the pyrolysis products for the Jurassic low-mature coal under programmed temperature,and chemical and carbon isotopic compositions of natural gas from the Kuqa Depression, the genetic origin of natural gas was determined,and then a gas filling model was established,in combination with the geological background of the Kuqa Depression.The active energy of CH_4,C_2H_6 and C_3H_8 was gotten after the data of pyrolysis gas products under different heating rates（2℃/h and 20℃/h）were fitted by the Gas Oil Ratio（GOR）Isotope Model soft.When the frequency factor（Af）was chosen as 1×10^（14）,the active energy of CH_4,C_2H_6 and C_3H_8 was 58 kcal/mol,57 kcal/mol and 54 kcal/ mol,respectively.The distributive ranges of theδ^（13）C_1,δ^（13）C_2 andδ^（13）C_3 values for the pyrolysis gas products are-35.9‰to-30.7‰,-26.2‰to-21.3‰and-26.4‰to-22.7‰,respectively.All of the natural gases from the Kuqa Depression are dominated by hydrocarbon gases,with the high gas dryness（C_1/C_（1-4））at the middle and northern parts of the depression and the low values at both east and west sides and the southern part.The carbon isotopes of methane and its homologs as a typical coal-type gas are enriched in ^（13）C,and the distributive range of theδ^（13）C_1,δ^（13）C_2 andδ^（13）C_3 value is-32‰to -38‰,-22‰to-24‰and-20‰to-22‰,respectively,with the carbon isotopes of gaseous alkanes being less negative with the carbon number.With the ethane being enriched in ^（13）C the increasing tendency of the geological reserve of natural gas in the Kuqa Depression is observed.This observed change is consistent with the results of pyrolysate gas yield of coal as a potential gas source in the Kuqa Depression,suggesting natural gas was thermally derived from the humic organic matters and the carbon isotopes of gaseous alkanes would coarsely predict the geological reserve of gas in the Kuqa Depression.Through the simulation of kinetic processes of gas generation for the Jurassic coal in the Kuqa Depression,the gas in the Kela 2 gas field would get the threshold of gas expulsion after 27 Ma,be expelled out of source rocks as＂pulse action＂,and then filled in the gas reservoir.The peak gas-filling history took place during the past 2 Ma.展开更多
The fluid evolution and reservoir formation model of the ultra-deep gas reservoirs in the Permian Qixia Formation of the northwestern Sichuan Basin are investigated by using thin section,cathodoluminescence,inclusion ...The fluid evolution and reservoir formation model of the ultra-deep gas reservoirs in the Permian Qixia Formation of the northwestern Sichuan Basin are investigated by using thin section,cathodoluminescence,inclusion temperature and U-Pb isotopic dating,combined with gas source identification plates and reservoir formation evolution profiles established based on burial history,thermal history,reservoir formation history and diagenetic evolution sequence.The fluid evolution of the marine ultra-deep gas reservoirs in the Qixia Formation has undergone two stages of dolomitization and one phase of hydrothermal action,two stages of oil and gas charging and two stages of associated burial dissolution.The diagenetic fluids include ancient seawater,atmospheric freshwater,deep hydrothermal fluid and hydrocarbon fluids.The two stages of hydrocarbon charging happened in the Late Triassic and Late Jurassic–Early Cretaceous respectively,and the Middle to Late Cretaceous is the period when the crude oil cracked massively into gas.The gas reservoirs in deep marine Permian strata of northwest Sichuan feature multiple source rocks,composite transportation,differential accumulation and late finalization.The natural gas in the Permian is mainly cracked gas from Permian marine mixed hydrocarbon source rocks,with cracked gas from crude oil in the deeper Sinian strata in local parts.The scale development of paleo-hydrocarbon reservoirs and the stable and good preservation conditions are the keys to the forming large-scale gas reservoirs.展开更多
The natural gas components and geochemistry of 38 ultra-deep gas wells(burial depth greater than 6 000 m) in the Sichuan Basin were analyzed to determine the genesis of ultra-deep natural gas in the basin. The ultra-d...The natural gas components and geochemistry of 38 ultra-deep gas wells(burial depth greater than 6 000 m) in the Sichuan Basin were analyzed to determine the genesis of ultra-deep natural gas in the basin. The ultra-deep natural gas components of the basin have the following characteristics: Methane has an absolute advantage, which can be up to 99.56% with an average of 86.6%; ethane is low, with an average of 0.13%; there is nearly no propane and butane. So it is dry gas at over-mature thermal stage. The content of H2 S can be up to 25.21%, with an average of 5.45%. The alkane gas isotopes are: the carbon isotope varies from-32.3‰ to-26.7‰ for methane and from-32.9‰ to-22.1‰ for ethane. There is nearly no carbon isotopic reversal among methane and its homologues. Hydrogen isotope varies from-156‰ to-114‰ for methane, and from-103‰ to-89‰ for some ethane. The carbon isotope of CO_2 varies from-17.2‰ to 1.9‰ and most of them fall within the range of 0±3‰. According to the δ^(13)C_1-δ^(13)C_2-δ^(13)C_3 plot, except some wells, all other ultra-deep gas wells are dominated by coal-derived gas. Based on the CO_2 origin distinguishing plot and δ^(13)C_(CO_2), except some individual wells, most of the ultra-deep CO_2 are of carbonate metamorphic origin. H2 S in the ultra-deep layer of Longgang and Yuanba gas fields belongs to thermochemical sulfate reduction(TSR), while H2 S from Well Shuangtan belongs to thermal decomposition of sulfides(TDS).展开更多
China has made significant progress in the exploration and development of natural gas in the past 70 years,from the gas-poor country to the world’s sixth largest gas production country.In 1949,the annual gas output i...China has made significant progress in the exploration and development of natural gas in the past 70 years,from the gas-poor country to the world’s sixth largest gas production country.In 1949,the annual gas output in China was 1117×104 m3,the proved gas reserves were 3.85×108 m3,and the average annual gas consumption and available reserves of per person were 0.0206 m3 and 0.7107 m3,respectively.By 2018,the average domestic annual gas production per person was 114.8576 m3 and the reserves were 12011.08 m3,and the average domestic annual gas production and reserves per person in the past 70 years increased by 5575 times and 16900 times,respectively.The exploration and development of large gas fields is the main way to rapidly develop the natural gas industry.72 large gas fields have been discovered in China so far,mainly distributed in three basins,Sichuan(25),Ordos(13)and Tarim(10).In 2018,the total gas production of the large gas fields in these three basins was 1039.26×108 m3,accounting for 65%of the total gas production in China.By the end of 2018,the cumulative proved gas reserves of the 72 large gas fields had amounted to 124504×108 m3,accounting for 75%of the total national gas reserves(16.7×1012 m3).New theories of natural gas have promoted the development of China’s natural gas industry faster.Since 1979,the new theory of coal-derived gas has boosted the discovery of gas fields mainly from coal-measure source rocks in China.In 2018,the gas production of large coal-derived gas fields in China accounted for 50.93%and 75.47%of the total national gas production and total gas production of large gas fields,respectively.Guided by shale gas theories,shale gas fields such as Fuling,Changning,Weiyuan and Weirong have been discovered.In 2018,the total proved geological reserves of shale gas were 10455.67×108 m3,and the annual gas production was 108.8×108 m3,demonstrating a good prospect of shale gas in China.展开更多
The original gas reservoirs in different areas and different layers of the Triassic Xujiahe Formation in the central Sichuan Basin are studied to reveal the relationships of iC4/nC4 and iC5/nC5 ratios in coal-derived ...The original gas reservoirs in different areas and different layers of the Triassic Xujiahe Formation in the central Sichuan Basin are studied to reveal the relationships of iC4/nC4 and iC5/nC5 ratios in coal-derived gas components with maturity using conventional natural gas geochemical research methods. The testing results of 73 gas samples from 8 gas fields show that the iC4/nC4 and iC5/nC5 ratios in coal-derived gas have a good positive correlation, and the correlation coefficient is above 0.8. Both the iC4/nC4 and iC5/nC5 ratios become higher with the increase of natural gas dryness coefficient (C1/C1+) and the methane carbon isotope becoming less negative. These parameters are highly correlated. This study not only reveals characteristics of heavy hydrocarbon isomers generated by coal formation, but also puts forward new identification indicators reflecting the maturity of coal-derived gas, the regression between iC4/nC4, iC5/nC5 and Ro, which can provide an important reference for maturity, migration and accumulation of coal-derived gas, and late stage reformation of coal-derived gas reservoirs.展开更多
The evolution of coalbed gas reservoir is characterized by coalbed gas geochemistry and gas content. On the basis of burial history and thermal history, the forming process of coalbed gas reservoir and the gas accumul...The evolution of coalbed gas reservoir is characterized by coalbed gas geochemistry and gas content. On the basis of burial history and thermal history, the forming process of coalbed gas reservoir and the gas accumulative history in the Qinshui Basin are discussed in this paper. The difference of the thermal history, geochemistry characteristic, and gas accumulative history between Yangcheng and Huozhou areas shows that the formation of coalbed gas reservoir in the Qinshui Basin is controlled by the geological process in the critical stage and the critical moment. The components and isotopes of coalbed methane are determined by the stage at which the coal maturation reaches its maximum rank. The coalbed methane accumulative history is related to the temperature and pressure of the coal burial history, because the coalbed gas is mainly in adsorptive state. It is stated that the gas content in the coal seam is controlled by the moment when the coal seam is uplifted to the shallowest position.展开更多
The Ordovician buried hill reservoir in the western Lunnan area, a type of dissolved fracture and cavernous reservoir, is mainly composed of heavy oil. The oil is the mixture sourcing from the Middle-Lower Cambrian an...The Ordovician buried hill reservoir in the western Lunnan area, a type of dissolved fracture and cavernous reservoir, is mainly composed of heavy oil. The oil is the mixture sourcing from the Middle-Lower Cambrian and Middle-Lower Ordovician, with three stages of pool forming process: (1) the destruction and parallel migration/accumulation during the late Caledonian to early Hercynian; (2) the oil and gas accumulation during the late Hercynian characterized by adjustment upward along faults and parallel migration/accumulation; (3) the formation of heavy oil during the latest Hercynian. The Ordovician buried hill reservoir is affected by the diffusion of light oil and gas but had no hydrocarbon charging during the late Yanshan period to Himalayan period, but in this period, formed the association of heavy oil and dissolved gas cracked from crude oil with dry coefficient of 0.91-0.96. The study on accumulation process of the Ordovician buried hill reservoir has important implications for the exploration potential of early oil and gas accumulation in the cratonic area of the Tarim Basin.展开更多
The hydrogeological condition affects the coal-bed gas storage dramatically. In an area of stronger hydrodynamics, the coal has a lower gas content, while a higher gas content exists in an area of weaker hydrodynamics...The hydrogeological condition affects the coal-bed gas storage dramatically. In an area of stronger hydrodynamics, the coal has a lower gas content, while a higher gas content exists in an area of weaker hydrodynamics. Obviously, the flowing groundwater is harmful to coalbed gas preservation. But few researches focus on the mechanism of how the flowing water diminishes the coalbed gas content Based on the phenomenon that the flowing groundwater not only makes coalbed gas content lower, but also fractionates the carbon isotope, this research puts forward an idea that it is the water solution that diminishes the coalbed gas content, rather than the water-driven action or the gas dissipation through cap rocks. Only water-soluble action can both fractionate the carbon isotope and lessen the coalbed gas content, and it is an efficient way to take gas away and affect the gas content.展开更多
Abnormal overpressure occurs in the forelandbasins of Kuqa, South Junggar and West Sichuan in China.The pressure coefficients are high. Overpressure exists inwide areas and various strata. The layers of overpressureha...Abnormal overpressure occurs in the forelandbasins of Kuqa, South Junggar and West Sichuan in China.The pressure coefficients are high. Overpressure exists inwide areas and various strata. The layers of overpressurehave a very close relationship with lithology, and the area ofoverpressure is controlled by the piedmont depression. Themechanisms of overpressure formation in the Kuqa andSouth Junggar Depression include disequilibrium compac-tion and tectonic compression; the importance of these twofactors varies in different basins and in different stages of thesame basin. Different models of gas accumulation are estab-lished to explain the relationship between overpressure dis-tribution and gas pool formation, and the influence of over-pressure on the gas pools. These models include: (i) theviolent tectonic movement leads to the pool formation inoverpressure belt (Kela-2 gas field in Kuqa); (ii) the pres-sure releases at shallow part and the gas pool forms in latetime (Hutubi gas field in southern Junggar Basin); (iii)through the pressure transfer the gas migrates and accumu-lates (Xinchang gas field in Western Sichuan Basin).展开更多
基金supported by the project of China National 973 Program"Basic Research on Enrichment Mechanism and Improving the Exploitation Efficiency of Coalbed Methane Reservoir"(Grant No. 2009CB219600)
文摘The pool-forming mechanism of coalbed methane has its own characteristics.In this paper, based on studies on the typical coal-bearing basins in China,it is pointed out that the reservoir formation of medium- to high-rank coalbed methane has experienced three critical stages：the coalbed methane generation and adsorption stage,the coalbed adsorption capacity enhancement stage,and the coalbed methane desorption-diffusion and preservation stage.The regional tectonic evolution, hydrodynamic conditions and sealing conditions play important roles in the stage of coalbed methane desorption-diffusion and preservation.Medium- to high-rank coalbed methane has three types of enrichment models,that is,the most favorable,the relatively favorable,and the unfavorable enrichment models.
基金financially supported by the Chinese National Natural Science Foundation(Grant No:40802028)the National Key Foundational Research and Development Project (Grant No:2005CB422108)+1 种基金the National Science and Technology Special Project(2008ZX05005-004-004)supported by the Opening Project of Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Accumulation Mechanism,Ministry of Education,China.
文摘Based on the pyrolysis products for the Jurassic low-mature coal under programmed temperature,and chemical and carbon isotopic compositions of natural gas from the Kuqa Depression, the genetic origin of natural gas was determined,and then a gas filling model was established,in combination with the geological background of the Kuqa Depression.The active energy of CH_4,C_2H_6 and C_3H_8 was gotten after the data of pyrolysis gas products under different heating rates（2℃/h and 20℃/h）were fitted by the Gas Oil Ratio（GOR）Isotope Model soft.When the frequency factor（Af）was chosen as 1×10^（14）,the active energy of CH_4,C_2H_6 and C_3H_8 was 58 kcal/mol,57 kcal/mol and 54 kcal/ mol,respectively.The distributive ranges of theδ^（13）C_1,δ^（13）C_2 andδ^（13）C_3 values for the pyrolysis gas products are-35.9‰to-30.7‰,-26.2‰to-21.3‰and-26.4‰to-22.7‰,respectively.All of the natural gases from the Kuqa Depression are dominated by hydrocarbon gases,with the high gas dryness（C_1/C_（1-4））at the middle and northern parts of the depression and the low values at both east and west sides and the southern part.The carbon isotopes of methane and its homologs as a typical coal-type gas are enriched in ^（13）C,and the distributive range of theδ^（13）C_1,δ^（13）C_2 andδ^（13）C_3 value is-32‰to -38‰,-22‰to-24‰and-20‰to-22‰,respectively,with the carbon isotopes of gaseous alkanes being less negative with the carbon number.With the ethane being enriched in ^（13）C the increasing tendency of the geological reserve of natural gas in the Kuqa Depression is observed.This observed change is consistent with the results of pyrolysate gas yield of coal as a potential gas source in the Kuqa Depression,suggesting natural gas was thermally derived from the humic organic matters and the carbon isotopes of gaseous alkanes would coarsely predict the geological reserve of gas in the Kuqa Depression.Through the simulation of kinetic processes of gas generation for the Jurassic coal in the Kuqa Depression,the gas in the Kela 2 gas field would get the threshold of gas expulsion after 27 Ma,be expelled out of source rocks as＂pulse action＂,and then filled in the gas reservoir.The peak gas-filling history took place during the past 2 Ma.
基金Supported by the Special Project of National Key R&D Plan(2017YFC0603106).
文摘The fluid evolution and reservoir formation model of the ultra-deep gas reservoirs in the Permian Qixia Formation of the northwestern Sichuan Basin are investigated by using thin section,cathodoluminescence,inclusion temperature and U-Pb isotopic dating,combined with gas source identification plates and reservoir formation evolution profiles established based on burial history,thermal history,reservoir formation history and diagenetic evolution sequence.The fluid evolution of the marine ultra-deep gas reservoirs in the Qixia Formation has undergone two stages of dolomitization and one phase of hydrothermal action,two stages of oil and gas charging and two stages of associated burial dissolution.The diagenetic fluids include ancient seawater,atmospheric freshwater,deep hydrothermal fluid and hydrocarbon fluids.The two stages of hydrocarbon charging happened in the Late Triassic and Late Jurassic–Early Cretaceous respectively,and the Middle to Late Cretaceous is the period when the crude oil cracked massively into gas.The gas reservoirs in deep marine Permian strata of northwest Sichuan feature multiple source rocks,composite transportation,differential accumulation and late finalization.The natural gas in the Permian is mainly cracked gas from Permian marine mixed hydrocarbon source rocks,with cracked gas from crude oil in the deeper Sinian strata in local parts.The scale development of paleo-hydrocarbon reservoirs and the stable and good preservation conditions are the keys to the forming large-scale gas reservoirs.
基金Supported by the China National Science and Technology Major Project(2016ZX05007-001)
文摘The natural gas components and geochemistry of 38 ultra-deep gas wells(burial depth greater than 6 000 m) in the Sichuan Basin were analyzed to determine the genesis of ultra-deep natural gas in the basin. The ultra-deep natural gas components of the basin have the following characteristics: Methane has an absolute advantage, which can be up to 99.56% with an average of 86.6%; ethane is low, with an average of 0.13%; there is nearly no propane and butane. So it is dry gas at over-mature thermal stage. The content of H2 S can be up to 25.21%, with an average of 5.45%. The alkane gas isotopes are: the carbon isotope varies from-32.3‰ to-26.7‰ for methane and from-32.9‰ to-22.1‰ for ethane. There is nearly no carbon isotopic reversal among methane and its homologues. Hydrogen isotope varies from-156‰ to-114‰ for methane, and from-103‰ to-89‰ for some ethane. The carbon isotope of CO_2 varies from-17.2‰ to 1.9‰ and most of them fall within the range of 0±3‰. According to the δ^(13)C_1-δ^(13)C_2-δ^(13)C_3 plot, except some wells, all other ultra-deep gas wells are dominated by coal-derived gas. Based on the CO_2 origin distinguishing plot and δ^(13)C_(CO_2), except some individual wells, most of the ultra-deep CO_2 are of carbonate metamorphic origin. H2 S in the ultra-deep layer of Longgang and Yuanba gas fields belongs to thermochemical sulfate reduction(TSR), while H2 S from Well Shuangtan belongs to thermal decomposition of sulfides(TDS).
基金Supported by the PetroChina Science and Technology Project(2013B-0601).
文摘China has made significant progress in the exploration and development of natural gas in the past 70 years,from the gas-poor country to the world’s sixth largest gas production country.In 1949,the annual gas output in China was 1117×104 m3,the proved gas reserves were 3.85×108 m3,and the average annual gas consumption and available reserves of per person were 0.0206 m3 and 0.7107 m3,respectively.By 2018,the average domestic annual gas production per person was 114.8576 m3 and the reserves were 12011.08 m3,and the average domestic annual gas production and reserves per person in the past 70 years increased by 5575 times and 16900 times,respectively.The exploration and development of large gas fields is the main way to rapidly develop the natural gas industry.72 large gas fields have been discovered in China so far,mainly distributed in three basins,Sichuan(25),Ordos(13)and Tarim(10).In 2018,the total gas production of the large gas fields in these three basins was 1039.26×108 m3,accounting for 65%of the total gas production in China.By the end of 2018,the cumulative proved gas reserves of the 72 large gas fields had amounted to 124504×108 m3,accounting for 75%of the total national gas reserves(16.7×1012 m3).New theories of natural gas have promoted the development of China’s natural gas industry faster.Since 1979,the new theory of coal-derived gas has boosted the discovery of gas fields mainly from coal-measure source rocks in China.In 2018,the gas production of large coal-derived gas fields in China accounted for 50.93%and 75.47%of the total national gas production and total gas production of large gas fields,respectively.Guided by shale gas theories,shale gas fields such as Fuling,Changning,Weiyuan and Weirong have been discovered.In 2018,the total proved geological reserves of shale gas were 10455.67×108 m3,and the annual gas production was 108.8×108 m3,demonstrating a good prospect of shale gas in China.
基金Supported by the National Natural Science Foundation of China(41872162)
文摘The original gas reservoirs in different areas and different layers of the Triassic Xujiahe Formation in the central Sichuan Basin are studied to reveal the relationships of iC4/nC4 and iC5/nC5 ratios in coal-derived gas components with maturity using conventional natural gas geochemical research methods. The testing results of 73 gas samples from 8 gas fields show that the iC4/nC4 and iC5/nC5 ratios in coal-derived gas have a good positive correlation, and the correlation coefficient is above 0.8. Both the iC4/nC4 and iC5/nC5 ratios become higher with the increase of natural gas dryness coefficient (C1/C1+) and the methane carbon isotope becoming less negative. These parameters are highly correlated. This study not only reveals characteristics of heavy hydrocarbon isomers generated by coal formation, but also puts forward new identification indicators reflecting the maturity of coal-derived gas, the regression between iC4/nC4, iC5/nC5 and Ro, which can provide an important reference for maturity, migration and accumulation of coal-derived gas, and late stage reformation of coal-derived gas reservoirs.
文摘The evolution of coalbed gas reservoir is characterized by coalbed gas geochemistry and gas content. On the basis of burial history and thermal history, the forming process of coalbed gas reservoir and the gas accumulative history in the Qinshui Basin are discussed in this paper. The difference of the thermal history, geochemistry characteristic, and gas accumulative history between Yangcheng and Huozhou areas shows that the formation of coalbed gas reservoir in the Qinshui Basin is controlled by the geological process in the critical stage and the critical moment. The components and isotopes of coalbed methane are determined by the stage at which the coal maturation reaches its maximum rank. The coalbed methane accumulative history is related to the temperature and pressure of the coal burial history, because the coalbed gas is mainly in adsorptive state. It is stated that the gas content in the coal seam is controlled by the moment when the coal seam is uplifted to the shallowest position.
基金"The Fenth Five-Year Plan" of the National Scientific and Technologi-cal Programs of China (Grant No. 2001BA605A-06)
文摘The Ordovician buried hill reservoir in the western Lunnan area, a type of dissolved fracture and cavernous reservoir, is mainly composed of heavy oil. The oil is the mixture sourcing from the Middle-Lower Cambrian and Middle-Lower Ordovician, with three stages of pool forming process: (1) the destruction and parallel migration/accumulation during the late Caledonian to early Hercynian; (2) the oil and gas accumulation during the late Hercynian characterized by adjustment upward along faults and parallel migration/accumulation; (3) the formation of heavy oil during the latest Hercynian. The Ordovician buried hill reservoir is affected by the diffusion of light oil and gas but had no hydrocarbon charging during the late Yanshan period to Himalayan period, but in this period, formed the association of heavy oil and dissolved gas cracked from crude oil with dry coefficient of 0.91-0.96. The study on accumulation process of the Ordovician buried hill reservoir has important implications for the exploration potential of early oil and gas accumulation in the cratonic area of the Tarim Basin.
文摘The hydrogeological condition affects the coal-bed gas storage dramatically. In an area of stronger hydrodynamics, the coal has a lower gas content, while a higher gas content exists in an area of weaker hydrodynamics. Obviously, the flowing groundwater is harmful to coalbed gas preservation. But few researches focus on the mechanism of how the flowing water diminishes the coalbed gas content Based on the phenomenon that the flowing groundwater not only makes coalbed gas content lower, but also fractionates the carbon isotope, this research puts forward an idea that it is the water solution that diminishes the coalbed gas content, rather than the water-driven action or the gas dissipation through cap rocks. Only water-soluble action can both fractionate the carbon isotope and lessen the coalbed gas content, and it is an efficient way to take gas away and affect the gas content.
文摘Abnormal overpressure occurs in the forelandbasins of Kuqa, South Junggar and West Sichuan in China.The pressure coefficients are high. Overpressure exists inwide areas and various strata. The layers of overpressurehave a very close relationship with lithology, and the area ofoverpressure is controlled by the piedmont depression. Themechanisms of overpressure formation in the Kuqa andSouth Junggar Depression include disequilibrium compac-tion and tectonic compression; the importance of these twofactors varies in different basins and in different stages of thesame basin. Different models of gas accumulation are estab-lished to explain the relationship between overpressure dis-tribution and gas pool formation, and the influence of over-pressure on the gas pools. These models include: (i) theviolent tectonic movement leads to the pool formation inoverpressure belt (Kela-2 gas field in Kuqa); (ii) the pres-sure releases at shallow part and the gas pool forms in latetime (Hutubi gas field in southern Junggar Basin); (iii)through the pressure transfer the gas migrates and accumu-lates (Xinchang gas field in Western Sichuan Basin).
