The Lower Paleozoic shale in south China has very high maturity and experienced a strong tectonic reformation,which is quite different from the North American shale.This study,based on a comparison on geological and geochemical characteristics between the Lower Paleozoic and American shales as well as between the Lower Cambrian and Lower Silurian shales in south China,made a comprehensive investigation of the reservoir property,preservation condition,gas content and other related data of the Lower Paleozoic shale,and discussed the evaluation criteria for shale gas prospective areas.It is deemed that shale with an equivalent vitrinite reflectance(EqRO)>3.5% will have a high risk for shale gas exploration.In the range of 2.0%-3.5% of EqRO,the preservation condition becomes a main controlling factor on shale gas.The evaluation criteria of the Lower Silurian and Lower Cambrian shales are proposed for the uplifted/folded area(Sichuan Basin)as well as the folded\|faulted area(outside the Sichuan Basin).It is believed that the Lower Paleozoic shale with a burial depth <1 500m has no industrial shale gas potential.Within the Sichuan Basin,the Lower Paleozoic shale system presents an overpressure,and the Lower Cambrian shale with TOC>1.5% and the Lower Silurian shale with TOC>2.0% have gas potential.In the faulted\|folded area,it is predicted that the deeply-burial shale(Lower Silurian >2 000m and Lower Cambrian >2 500-3 000m)reaches the limit value of gas content with economic potential,and will become the focus for further exploration and development in this type of area.
The Yangtze platform is relatively stable in the geological history,so it is one of the most possible regions for the accumulation of shale gas.Because of large lateral span of Yangtze platform,the tectonic settings between the Upper Yangtze platform on the west and the Lower Yangtze on the east are rather different,which is of obvious significance for the formation and preservation of the shale gas.The comparative analysis on the Paleozoic sedimentary-tectonic evolution of the Upper and Lower Yangtze platforms,combined with the recent reservoir properties,indicate that sedimentary-tectonic evolution progresses between the two platforms are similar,and have good conditions for shale gas formation.The comparation on the tectonic activities since the Paleozoic show that the Upper Yangtze platform has been in a background of compression since the Mesozoic,and is conducive to the preservation of shale gas,while the Lower Yangtze platform has been in compression since Indosinian,and tectonic inversion took place in Yanshanian and was in an extensional environment,which is destructive for the reservoirs of shale gas.So,the Upper Yangtze platform has greater potential for shale gas exploration and development than the Lower Yangtze region.
The composition and structure of shale gas reservoirs show remarkable heterogeneity from macro-scale to nano-scale.By comprehensively analyzing various experimental data,the pore structure,micro-heterogeneity and the affecting factors,along with pore evolvement rule of both the Qiongzhusi Formation and Longmaxi Formation in the Middle-Upper Yangtze were characterized.Both formations are organic enriched with shale thickness more than 100m and 50m,respectively.However,the porosity,pore volume,specific surface area and other parameters of the shales differ prominently and the Longmaxi Formation represents a better pore structure parameter.The micropore and transitional pore are the dominant pore type in the Longmaxi Formation,which is unlike the Qiongzhusi Formation,whose pore volume of samples is quite centralized in certain interval with poor connectivity.The variation coefficient of pore structure and the NMR relaxation time (T2) indicate that the Qiongzhusi shale is of higher heterogeneity,and the micropore volume,which is the key space for gas occurrence,differs between samples in the Qiongzhusi shale and represents a weak connectivity.These two shales experienced similar external factors,such as multiphase tectonic stress field deformation,and formed tensional and shear fissure network,which increased the reserving volume,connectivity,and permeability.Composition and the diagenesis modification are the main internal factors and the nature of pore evolvement,and the pores and fissures are mainly influenced by the organic matter,quartz,and other compositions.TOC and quartz content represent a positive liner correlation with pore volume and specific surface area,while clay content is negative.High maturity is beneficial in forming effective pore network because the dissolution of organic acid.Compaction leads to the volume decrease of inter-and intrapartical pores,and the draining of interlayer water as well as mechanical deformation of clay impacts on the pore structure,too.
Based on the data from drilling and core samples,the characteristics of organic matter,porosity,microscopic pore and pore structure from the Lower Paleozoic Qiongzhusi and Longmaxi formations marine shale gas reservoirs in the south of Sichuan basin were studied,by using a lot of experimental methods such as organic geochemistry,organic petrology,reservoir pore analysis.The results show that the Lower Paleozoic marine shale in the south of Sichuan basin is characterized by high organic carbon content(most TOC>2.0%),high thermal maturity(ROm=2.3%-3.8%),and low porosity(1.16%-6.87%).The organic carbon content and thermal maturity of the Qiongzhusi Formation shale is higher than those of the Longmaxi Formation shale,while the porosity of the Qiongzhusi Formation shale is lower than that of the Longmaxi Formation shale.There are inter-granular pore,dissolved pore,crystal particle pore,particle edge pore and organic pore in the Lower Paleozoic Qiongzhusi Formation and Longmaxi Formation shale samples.More micro-nano scale pores are developed in the Longmaxi Formation shales than those in the Qiongzhusi Formation shales.The inter-granular pore,dissolved pore and organic pore are most common,which are main storage space for shale gas.The microscopic pores in the Lower Paleozoic shale are mainly composed of micropore and mesopore,and a little micropore.The micropore and mesopore in the Qiongzhusi Formation shale account for 83.92% of the total pore volume.The micropore and mesopore in the Longmaxi Formation shale account for 78.17% of the total pore volume.Thus,micropore and mesopore are the main contribution of microscopic pores in the Lower Paleozoic shale gas reservoir in the south of Sichuan Basin.
Based on the analysis of previous researches and drilling data,this paper interpreted the impact of gypsolith on the preservation and exploration prospect of shale gas in Sichuan Basin.Results show that the halite layer of Lower Cambrian and Triassic plays an important controlling role in tectonic deformation,resulting in different structural styles in Sichuan Basin.The formation under gypsolith tends to be overpressure because of the halite layer.Gypsolith in Sichuan Basin is one of the most important factors which controls the shale gas exploration of Longmaxi Formantion.Based on the distribution of salt layers,depth and thickness of shale,and organic geochemical characteristics,we suggest that Zigong-Yibin,Chongqing-Yongchuan,and Fuling are the favorite shale gas exploration regions in Sichuan Basin.
Southeastern Chongqing is an important part of upper Yangtze plate.It's also a key area of shale gas exploration and development of China in the future.Hydrocarbon generation potential of Lower Cambrian Niutitang shale is discussed according to the analysis on organic matter abundance,type and source material.Niutitang shale is characterized by high organic content,with an average of 7%.The organic matter is dominated by type I,with high oil generation potential.Bitumen A,hydrocarbon generation potential(S1+S2)and hydrogen index(IH)are very low,indicating that liquid hydrocarbon which was generated during oil-generation peak period may be used up at high-over mature stage.Remaining gas was extracted by vacuum crushing analytical method,mainly containing methane.Carbon isotope distribution pattern was positive.δ13C1averages at-36.1‰,showing typical heat causes.δ13C2ranges between-39.6‰ and-18.6‰.Majority of studied shale samples possess δ13C2close to δ13Corg,or even heavier.Analysis of gas geochemical characteristics of hydrocarbon gas shale further confirmed that Niutitang shale gas is mainly from liquid hydrocarbon cracking under high temperature conditions.
High shale gas flow of 20.3×104m3 was tested in Longmaxi Formation in Nov.28,2012,which declared the discovery of Fuling shale gas field.On July 10,2014 the proved geological reserves of the first shale gas field in China was submitted to National Reserves Committee.The practices of exploration and development prove that the reservoirs in Fuling shale gas field belong to quality shales deposited in deep shelf,with great thickness,stable distribution,no interlayers.The shale gas field is characterized by high production well,high pressure reservoirs,good ingredients of gas and good effects on testing production,belonging to mid-deep quality natural gas reservoirs with high pressure.According to the comprehensive studies on regional sedimentary background,lithology,micro-pore structures,geophysical properties,gas sources,features of gas reservoirs,logging responding features and producing features of gas wells,it shows that(1)Longmaxi Formation in Fuling shale gas field belongs to deep shelf environment where organic-rich shales developed well;(2)Thermal evolution of shales of Longmaxi Formation is moderate and Nanometer-level pores develop;(3)The shale gas sources came from kerogens of Longmaxi Formation itself;(4)The shale gas reservoirs of Fuling Longmaxi Formation is similar to the typical ones in north America in geological features and producing rules.So all above proves that the shale gas produced from Longmaxi Formation in Fuling is the typical in-situ detained,self-generated and self-stored shale gas.
Methane adsorption-diffusion property in coalbed/shale is one of the key factors for coalbed methane/shale gas resource evaluation.With No.3 coal seam of Permian Shanxi Formation in Sihe coalfield Jincheng mine area and shale of Longmaxi Formation in Lower Silurian of Paleozoic in south China as study objects,the nano-pore system determination using low temperature liquid nitrogen adsorption method and methane adsorption-diffusion experiments of manometer method was conducted,methane adsorption-diffusion properties of high-maturity organic-rich shale samples and high-rank coal sample were elucidated,and then a mathematical model of methane adsorption-diffusion in coal/shale was established,finally the difference of methane adsorption-diffusion in coal and shale and its controlling mechanism were analyzed.It turns out that,meso-pores and macro-pores contribute in a major way to the pore volume of high-maturity organic-rich shale and coal samples and micro-pores and meso-pores contribute in a major way to the specific surface area;the nano-pore system is more developed in shale samples than that in coal sample and the specific surface area and pore volume in shale samples are bigger than that in coal samples;the methane adsorption-diffusion in coal and shale samples can be governed by Langmuir Equation,high-maturity organic-rich shale has a much lower methane adsorption capacity than high-rank coal and the methane adsorption capacity of shale increases with the increase of total organic carbon;open pores dominate in high-maturity organic-rich shale samples while semi-sealed pores dominate in high-rank coal sample;for semi-sealed pores,the dissolution of strong adsorbate into adsorbent will alter the macromolecular structure of adsorbent and result in the irreversibility of adsorption and desorption,that is,desorption hysteresis;therefore the diffusion properties during methane desorption-diffusion are different from that during methane diffusion-adsorption in high-maturity organic-rich shale and coal.
Baseds on the studies of samples from Lower Cambrian Qiongzhusi Formation and Lower Silurian Longmaxi Formation in the Upper Yangtze area,the organic pore structure of the target formation is quantitatively characterized.To achieve the goal,comprehensive analysis using stereoscope,CLSM,SEM were employed and further discussed associated with the organic precursor.The carbonated fossils in Qiongzhusi Formation were mainly originated from algae and preserved in 3D stereo.Owing to the influence of mineralization,the fossils are of high porosity but the surrounding is tight.The Longmaxi Formation is enriched with carbonate graptolite fossils that distributes along the bedding,leading to the unification of high TOC content layers which is ideal gas occurrence section and the mechanical weak plane which is important during fracturing.The 3D model of graptolite denoted that the multilayers formed certain pores,and the bitumen in the fossils is of positive effect in sealing and gas preservation.
To investigate pore characteristics and methane sorption capacity of Permian shale reservoir in Southeast Anhui,Lower Yangtze region,eight core samples were selected for test including Ar-ion milling-field emission scanning electron microscope,N2 adsorption-desorption and methane sorption experiments and related geochemical analysis.The results show that the shales contain multi-types of pores including inter-granular pore,intra-granular pore,intra-crystalline pore,dissolved pore,organic pore and micro-cracks.The total porosity of our samples ranges from 2.15% to 6.10% and average permeability is 0.002 57×10-3μm2.The nanoscale pores mainly consist of meso-pores and small parts of micro-and macro-pores.The pore diameters mostly range from 2 to 50nm.The pore volume counted by BJH equation ranges from 3.30-11.23mm3/g and the total surface area calculated by BET equation ranges from 3.91-20.84m2/g.The micropore surface calculated by T-method ranges from 1.015-4.053m2/g.Meso-and micro-pores mainly contribute to the surface areas.These namopores and large surface area in the Permian organic-rich shales could provide good conditions for shale gas storage.The methane adsorption capacity is up to 2.3-3.2m3/t at 27MPa.This indicates that the Permian shales have relatively favorable storage condition for shale gas.
Thermal properties of rocks are the important parameters for studying the present-day geothermal regime and tectono-thermal evolution of the sedimentary basins,which would also provide fundamental data for the assessment and exploration of oil and gas resources in this basin.The Lower Yangtze area of the South China is considered as one of the preferred areas for shale gas exploration and development in China.The thermal properties of shales are of great significance for the exploration of shale gas resources in this area.However,the measured properties data of shales in this area are still rare,limiting our understanding of the geothermal field and resource assessment.In this study,thermal properties of 18 mudstone and shale samples from the Lower Yangtze area are measured,including the thermal conductivity,density and radiogenic heat production as well.Our results show that the values of thermal conductivity of shales generally range from 1.500W/(m·K) to 3.000W/(m·K)with a mean of 2.174±0.682W/(m·K);while the values of radiogenic heat production basically vary from 1.00μW/m3 to 2.50μW/m3 with a mean of 2.04±0.86μW/m3.Compared with other sedimentary rocks,the shale has unique thermal properties,and is characterized by a lower thermal conductivity and a higher radiogenic heat production.The thermal properties of the samples with same or similar lithology are somewhat different,indicative of their unavailability for distinguishing lithological types alone.Additionally,the estimated present-day subsurface formation temperature distribution pattern of the whole Yangtze area shows that the temperature gradually decreases from the Jiang-Zhe-Wan area(the Lower Yangtze area in the east)to the Upper Yangtze area in the west,but the temperatures in the southeastern Sichuan and the Chuxiong basins are relatively higher,which is the combination of local tectonics and variation in thermal properties.Generally,the shale bed is just like a heat insulation layer,owing to its low thermal conductivity,and this thermal blanket effect can result in anomalous high temperature in the underlying formation but anomalous low temperature in the formation above.Anomalous temperature caused by such unique thermal properties of shale rocks would change the distribution of temperature field in the deep formation,thereby affecting the source rock thermal maturation evolution.These unique thermal properties of shales and associated thermal effects must be taken into account in the assessment of shale gas resources in China.
We measured density,porosity and permeability of typical mudstone and shale samples from the Lower Cambrian Hetang Formation,Lower Ordovician Ningguo Formation,Middle Ordovician Hule Formation,Lower Silurian Longmaxi Formation,and Upper Permian Changqiao Formation in the Yangtze block.Our mudstone and shale samples have very low porosity between 0.73%-2.95%,and the effective porosity increases with pressure.Under the effective pressure of 10MPa,the matrix permeability of sample ranges from 1.25×10-22m2 to 7.15×10-22m2.The occurrence of microcracks can remarkably enhance the permeability of samples.Permeability of mudstone and shale decreases exponentially with increasing effective pressure,which can be attributed to the gradual closure of microcracks and pores with pressure.Hysteresis of effective porosity and permeability were observed during the pressurization-depressurization cycle.Below 20MPa,permeability derived from pore pressure oscillation method is significantly lower than that from the pressure pulse decay method.
This study illustrated that the organic matter-hosted pores vary significantly in different organic matter matrix. Three types of organic matter-hosted pores were identified in the shales of Longmaxi Formation in Sichuan Basin by combining field emission scanning electron microscopy(FE-SEM)images,X-ray diffraction and energy dispersive spectrum analysis. Pores of TypeⅠorganic matter are observed in migrated pyrobitumen. Pores of TypeⅡorganic matter are aligned parallel to the mineral edges. Pores of Type Ⅲ organic matter develop in residual kerogen. Pores of TypeⅠorganic matter are formed from the exsolution of gaseous hydrocarbons during the secondary thermal cracking of retained oil. The mechanism of typeⅡorganic matter pores remains unclear,and pores of type Ⅲ organic matter are related to the residual kerogen after hydrocarbon generation. Pores of TypeⅠorganic matter can provide more extensive connected pathways than other isolated organic matter pores as in kerogen.
Based on a large number of thin section identification,scanning electron microscopy analysis,X-ray diffraction tests and some other experiments,the reservoir diagenesis of Silurian Longmaxi Formation in Sichuan Basin were studied.Seven diagenetic processes were identified,including compaction,cementation,clay mineral transformation,replacement,dissolution,organic matter thermal maturation and tectonic disruption.Three kinds of cements (quartz,carbonate and sulfide) were recognized,while the source material of quartz cements and the main factor of forming abandant carbonate cements were summed up.According to single well analysis of N3,it shows that the best,suboptimal and none shale reservoir sections were subjected to different diagenetic transformations.As to best shale reservoir,except for compaction,all the main inorganic diagenesis was significantly related to organic matter maturation.Through comprehensive analysis of diagentic indicators,it is indicated that the reservoir currently has already been in period B of middle diagenetic stage to late diagenetic stage.The inorganic diagenesis has a significant impact on shale reservoir,because it controls not only the conservation,development and evolution of porosity,but also the mechanical property and the adsorption capacity of rocks.However,the organic diagenesis is the source material of shale gas,and it generates a large number of nanoporosity in organic matter,which increase the total porosity and the adsorption capacity of the reservoir.
The water uptake and main controlling factors on the organic-rich mudstone of Longmaxi Formation are investigated by water absorption experiment,N2 absorption and TOC and clay mineral contents.The results show that TOC and clay mineral contents are key factors that affect water absorption on mudstone.Water uptake in mudstone has a positive correlation with BET surface area,and the water uptaking in mudstone absorption is spontaneous and reversible.Based on the linear correlation between saturated water uptake of the same sample with different sample size and volume,the calculated effective porosity is 6.23%.Water absorption experiment provides a new method to measure the connected and effective porosity.
The Paleozoic marine shale in the southern Sichuan basin is one of the most favorable fields for current shale gas exploration in China.The paleo-heat flow history of 5 wells in the southern Sichuan basin were reconstructed by using the vitrinite reflectance (RO) data.The heat flow in the Paleozoic was relatively low,but a heat flow peak occurred during the Permian,with a maximum value of 110mW/m2 in the southwestern area and 70-90mW/m2 in the southeastern area.After the Triassic,the heat flow gradually reduced to the present 55-65mW/m2.Based on the thermal history results,the thermal evolution of the Lower Cambrian Qiongzhusi shale were modeled.Due to the difference in burial history and heat flow history,there are differences in the thermal evolution of the Qiongzhusi shale in different regions in the southern Sichuan Basin.In the southeast area,the Qiongzhusi shale began to generate oil during the Silurian,then it experienced a rapid thermal evolution stage and became overmature during the Permian.However,in the southwest area,the Qiongzhusi shale became mature in the Carboniferous period,and it evolved rapidly in the Permian,during which time the RO reached 2.5%.The burial depth of the Qiongzhusi shale in the Leshan-Longnvsi paleo-uplift area was relatively small during the Caledonian period,so the source rocks remained immature during that time.These source rocks didn't begin to generate oil until the Middle-Late Permian,then they began to produce gas during the Triassic and became overmature in the Jurassic.
In order to evaluate the marine-terrestrial shale gas reservoir in coal measures,the morphologic development characteristics and structure of microscopic pores in shale samples from the Permian Longtan Formation in the southern Sichuan Basin were studied,and their major influencing factors of shale pores are analyzed in this paper,by using scanning electron microscope(SEM),high-pressure mercury injection,low-temperature liquid nitrogen absoption and carbon dioxide absoption.The results show that there are many kinds of reservoir spaces,such as intergranular pores,dissolution pores and organic pores.The main pores are in circular,oval,triangular,and irregular shapes.These microscopic pores provide reservoir space for shale gas.The nano-scale pores in the Longtan Formation shale samples are dominated by micropores and mesopores.Both micropores and mesopores provide with 56.2% volume and over 80% specific surface area of total pores,and they are favorable for shale gas accumulation.There is a positive correlation between pore volume and specific surface area,and the relation between BJH volume,DFT volume and specific surface area is better.The pore structure is mainly with plate slit type,cylindricity,and mixed types.The pore aperture is mainly distributed in 0.4-1nm,3-30nm,and averaged at 4.66nm.All the organic carbon content,maturity(RO) of organic matter and clay minerals are certain favorable for the development of reservoir pores,while the content of brittle minerals has the opposite relation to it.
By field inspection,laboratory test and observation under microscope,the following characteristics of Longmaxi Formation shales in Nanchuan region,Chongqing were summed up: The scope of microcrack width,the dominant orientation of microcrack and the manifestation in the graph of the amount of mercury injected by stages in experiment.It was analyzed that the effects upon the microcrack development of shale were caused by mineral composition,TOC,ROvalue,porosity and tectonic deformation,and so on.The relation between both of the indexes “>100nm pore volume”,“total amount of mercury injection” and microcrack development degree was studied.By using the experimental data such as mineral composition content,TOC,RO value and porosity,combined with the structure factor,quantitative indexes of the influential factors about microcrack development of shale were built.The quantitative relationship between the influential factors of microcrack and the degree of microcrack development was researched by using BP Neural Network(BPNN) model.The forecast for the degree of microcrack development was studied by using BPNN at the same time.It is concluded that the microcracks with width between 100nm to 100μm are developed in Longmaxi Formation shales in the research area.These microcracks constitute a network in different ranks.The graph of the amount of mercury injected by stages indicate the development degree of ultra-microcrack of shale to a certain degree.The width of microcracks in micron-level focuses the range of under 100μm and the microcrack in micron-level is related to tectonic deformation tightly.The weight matrix in BPNN model indicates the quantitative relationship between the influential factors of microcrack and the degree of microcrack development.Therefore,the degree of microcrack development can be forecasted quantitatively by using BPNN model.
The pore types and structures are critical to gas bearing capacity of shale reservoirs.Conventional scanning electron microscope,argon ion polishing,high resolution power scanning electron microscope are used to describe pore system of Lower Cambrian Niutitang Formation shale in Southeast Chongqing and discuss the relationship between pore characteristics and gas bearing capacity.There are four categories and nine classes matrix pores and three categories and four classes natural fractures in Lower Cambrian Niutitang Formation shale.Intragranular pore,dissolved pore,high angle shear fracture and low angle decollement fracture constitute the pore system of Lower Cambrian Niutitang Formation shale jointly.The pore system has the characteristics of various types,small pore diameter,poor connectivity and low permeability.Pore volume is mainly provided by the nanoscale pores.No organic pores,poor pore connectivity and regional developed decollement fractures maybe the reason why the Niutitang Formation shale has poor gas-bearing capacity.
By using high pressure mercury,low temperature nitrogen adsorption,argon ion polishing-scanning electron microscope observation,the reservoir of Longmaxi Formation shale samples has been [HJ2.1mm]characterized.Combined with the molecular potential energy theory,the relationship between the Longmaxi Formation porosity and shale gas occurrence has been revealed.The research shows that:(1)Pores smaller than 100nm are most developed in the Longmaxi Formation shale reservoir,and the specific surface area is mainly provided by pore radius smaller than 8nm,and nano-pores are mainly organic matter pores.Those pores are not smooth,and have fractal characteristics.The surface of bigger pores has superimposed many small pores.(2)When the distance between CH4 and pore wall is shorter than 2nm,there are strong interactions between the two and when the distance is longer than 2nm,the interactions can be ignored,and the CH4 is in a free state.In pores larger than 2nm,adsorption capacity is proportional to the specific surface area,but in pores smaller than 2nm,average concentration has a great relationship with the aperture.(3)In the reservoir of Longmaxi shale,the volume of adsorbed gas decreases with increasing pore size,and when the radius is larger than 30nm,the volume of adsorbed gas can be ignored.Volume of free gas first increased and then decreased with increasing pore size.It reached a peak in the vicinity of 45nm.In the vicinity of the 10nm,free gas volume became more than adsorbed gas volume;In pores with radius smaller than 75nm,adsorption region accounts for 66%-71% of the total.
Gas geochemical analysis has been conducted for the shale gas from Longmaxi Formation in Weiyuan-Changning areas,Sichuan Basin,China.Chemical composition,carbon,hydrogen and helium isotopic compositions were measured using an integrated method of gas chromatography combined with mass spectrometry.The results show that Longmaxi shale gases after are dominated by methane (94.0%-98.6%) with low wetness (0.3%-0.6%),minor non-hydrocarbon gases,which are mainly CO2 and N2,and trace amount of He.δ13CCO2=-2.5‰--6.0‰,3He/4He=0.01-0.03Ra.The shale gases in Weiyuan and Changning areas have reversed carbon isotope between methane and ethane (δ13C1>δ13C2) and distinct carbon isotopic compositions.Shale gas from the Weiyuan pilot has heavier carbon isotopic compositions for methane (δ13C1:-34.5‰--36.8‰),ethane (δ13C2:-37.6‰--41.9‰) and CO2 (δ13CCO2:-4.5‰--6.0‰) than those in Changning pilot (δ13C1:-27.2‰--27.3‰,δ13C2:-33.7‰--34.1‰,δ13CCO2:-2.5‰--4.6‰).The Longmaxi shale was at thermally high or over mature stage of organic matter with good sealing condition.The shale gas after hydraulic fracturing could be originated from the thermal decomposition of kerogen and the secondary cracking of liquid hydrocarbons which caused the reversal pattern of carbon isotopes.Some CO2 could be derived from decomposition of carbonate.The differences in carbon isotopes of gases between Weiyuan and Changning areas could be derived from different mixing proportion of gas from the secondary cracking of liquid hydrocarbons caused by the specific geological and geochemical conditions.
Through geochemical analysis of core and outcrop source rocks from the Upper Permian Longtan Formation in Sichuan Basin,organic matter type and redox depositional conditions were discussed through organic carbon and biomarker characteristics.The results show that the content of TOC and TOS has fluctuation in the profile,showing obvious heterogeneity.The n-alkanes in coal and carbonaceous mudstone of Upper Permian Longtan Formation are mainly unimodally distributed with relatively high abundance of high carbon number n-alkanes.However,the n-alkanes in shale of Upper Permian Longtan Formation are mainly bimodally distributed with phytanic advantage,and the main peak of the front mode was at nC17,in contrast,the main peak of the back mode was at nC25 or nC27.These results indicate that the organic matter in Longtan shale is mainly of aquatic and continental organic matter input,whereas the organic matter in Longtan coal and mudstone are terrestrial organic matter input.
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