The Study On Coupling Accumulation Conditions Of Coal Measure Gases In Juhugeng Mining Area Of Qinghai Province, Northwest China

Coal measure gases exploration is a leading and hot research topic in the field of unconventional energy in recent years,which has attracted widespread attention from scholars at home and abroad.Coal-bearing unconventional gases were characterized by self-generatedandself-storedorshort-distancemigration,similar hydrocarbon-forming materials,ordering of hydrocarbon accumulation time,and spatial correlation of storage,showing close internal connection and a unified geodynamic background.However,China’s coal-bearing basins have generally experienced multiple-staged tectonic and thermal evolution events with late reformation of coal measures,which has complicated and diversified the enrichment laws of coal measure gases in China.The negative effects of multi-period tectonic actions are manifested in the destruction of the original integrity and continuity of coal-bearing strata.Therefore,coal measure gases coupling and accumulation mechanisms in complex structural regions has become an important issue for the study of coal measure gases in China.The Juhugeng mining area in the Muli coalfield is a typical area where coal measure gases accumulation occurs.It is greatly influenced by the later tectonic transformation,and the coal measure gases enrichment conditions are complex,and the analysis of coal-bearing gas accumulation conditions in the coal-mining areas can further deepen the theoretical understanding of coal measure gases exploration and development in the regions with complex structures in China.Therefore,in this paper,the coal-bearing rock series in the Jurassic of Juhugeng mining area is taken as the research object,and the complex structure is used as the entry point to systematically discuss the occurrence patterns of coal measure gases.With the the main line of gas generation,migration,storage and preservation,field geological survey and sampling,indoor testing of samples,analysis and processing of experimental data,compilation of maps,comprehensive application of structural geology,coal geology,unconventional oil and gas geology to discuss the development of coal series and coal measure gases coupling types in the Juhugeng mining area.The hydrocarbon generation and coal-bearing gas migration process of coal-derived source rocks under structural control are discussed,and combined with the coal-bearing reservoirs and preservation conditions,coal mesure gases coupling models under structure control are established.（1）Through field surveys,drilled specimen observations,and geochemical experiments of source rocks,there are three kinds of source rocks in the Jurassic coal-bearing series in the study area:coal,dark mudstone,and oil shale.Among them,the coal seam is distributed in the upper section of the Muli Formation with a thickness of 1.80m to 83.65m in all area of Juhugeng mining area,and the giant coal seam has become the carrier of the coalbed methane;the dark mudstone is also distributed throughout the Middle Jurassic coal system with a thickness range of 0.79.m²45.70m;while the oil shale is mainly distributed in the upper section of the Jiangcang Formation and is distributed only in the central and southern parts of the mining area,with a thickness ranging from 3.33m to 362.00m.The organic matter of the dark mudstone sourced in the upper part of the Jiangcang Formation is the best,with the highest grade,followed by the lower section of the Jiangcang Formation,with moderate organic matter abundance,and the worst being the upper part of the Muli Formation.Dark mudstone and oil shale in the coal-bearing system are mainly concentrated between type II₁ and type II₂,and almost all coal seams are type III kerogen,and all source rocks are mainly in the mature stage.According to the selecting criteria for coal bearing source rocks,the grades of the dark mudstone and oil shale in the upper section of the Jiangcang Formation are good,fit-good,and the coal seam in the upper section of the Muli Formation is good-very good,and dark mudstones in the upper part of the Muli Formation and the dark mudstones in the lower part of the Jiangcang Formation are poor.（2）Based on the hydrocarbon generation capacity and lithologic combination features of coal and mud/shale,the source-reservoir-caprock association of coal,mud/shale and tight sandstone in the study area were recognized,and three major categories and six sub-groups of coal measure gases in the study area were identified.The three major categories are mainly coalbed meathane and shale gas（Type I）,coalbed methane and shale gas and tight sandstone gas（Type II）,shale gas and tight sandstone gas（Type III）.Then,according to the hydrocarbon-generating ability of mud/shale,two smaller subcategories are further subdivided for each major category.Each coal measure gases combination type has different distribution horizons in the longitudinal direction:Type I and II coupling types are mainly distributed in the upper part of the Muli Formation,while the type III coupling is mainly distributed in the upper and lower sections of the Jiangcang Formation.The low-level system field in each sequence is often a stratum occupied by self-generated and self-reserving coal measure gases combination types.On the other hand,there are few well-developed hydrocarbon source rocks in the high-level system domain of the sequence in the study area,and it is easy to form a good tight sandstone reservoir.The effect of the later transformation of coal measures on the coal measure gases coupling types is mainly the erosion of coal-bearing rock series,the thickening and thinning of the stratum,mainly caused by the cutting of the coal measures by the thrust faults,making the transformation of coal measure gases coupling types accoured in repetitive strata,that is,the adding or missing strata maskes the changes of coal mesure gases coupling types.（3）The tectonic-thermal evolution history since the Mesozoic was recovered with computer simulation and apatite fission track experiment results in the study area,and to the history of hydrocarbon generation（gas）and migration history in coal measures were revealed.It was concluded that the study area received stable deposits from the Middle Jurassic to the Early Cretaceous and precipitated in the Early Cretaceous,and coal measures subsided with a depth of over 2000m in the early Cretaceous period,and the paleotemperature was within the range of 100°C to 130°C.The main source rocks in coal measures had transited from low maturation to maturation and generated a lot of gases.From the late Cretaceous to the end of the Paleogene,the study area was in the tectonic uplift phase,the temperature dropped below 60°C,and source rocks of coal measures stopped producing hydrocarbons.In the period of the Paleogene,it was still in a slowly rising stage with a compression state.In the early Paleogene,there was a rapid sedimentation in the study area.A set of purple-red or brown-red mudstones and sandstones were deposited in this peroid.Most of the coal-derived source rocks entered the mature stage,and part of the source rocks regenerated hydrocarbons.However,the hydrocarbon generation intensity is low,and the hydrocarbon generation stops again in the late Paleogene due to tectonic uplift.In the late Miocene,the study area was again uplifted,and the age of the Triassic apatite fission track was about 13Ma,confirming the uplift of this period,and the coal-bearing rock series began to suffer a lot of damage.At this time,the gas migrated due to the cutting coal system of the thrust fault.The fluid inclusion experiment showed that both the upper and lower plates of the fault had inclusions,and they were shown as one-stage inclusions.During the Pliocene to Pleistocene period,the Qilianshan region continued to rise until the Quaternary,and the tectonic activity in the Qilianshan region weakened,and the current structure of the Muli basin is dominated by the thrust nappe structure.（4）Through the microscopic identification,scanning electron microscopy observation,mercury injection experiment,cryogenic liquid nitrogen experiment,and isothermal adsorption experiment,the reservoir characteristics were finely characterized and the depositional environment and tectonic function controlling effects on reservoirs were revealed.It was found that the coal pores in the study area were mainly primary pores,including intracellular and interdigital pores.The pore types were mainly one-side closed and opened pores.Some samples also have some bottleneck pores.The porosity,specific surface area and pore volume in each layering depth of thick coal seams are different.The permeability distribution ranges from0.005×10^-3μm² to 5.523×10^-3μm²,and the vertical heterogeneity is relatively strong.From the point of view of adsorption,the Langmuir volume of the coal reservoir in the study area is relatively high,which is favorable to the accumulation of coalbed methane.Inaddition,thematchingrelationshipbetweenthe water-transgression/regressive and the reservoir parameters of giant coal reservoirs was discussed.It was found that the water-transgression cycle reduced the content of mesopores,and also increased the content of micropores,specific surface area and pore volume,and water-regressive cycle shows the opposite rules.This is because water-transgression/regressive controls the proportion of the microscopic components of coals,and the microscopic components of coal have a direct relationship with the coal reservoir parameters.The pore type of mud/shale is mainly composed of inter-grain pores,the total pore volume of mud/shale in Jiangcang Formation changes greatly,and that in Muli Formation changes less.The specific surface area of mud/shale in the Jiangcang Formation has a greater change than that in the Muli Formation with a higher average value.The pore size distribution of the coal/mud shale in the study area is microporosity>pore>mesopores=macropores,and the permeability is less than0.3mD,which can be counted as ultra-low permeability reservoir,and the Langmuir volume of mud/shale is not It is particularly large,but it can still reach a maximum of4.95 m3/t,which can be considered as a strong adsorption capacity.After investigating the effect of deformation characteristics on pore structure and permeability,it is found that brittle deformation has little effect on the deformation of shale pores,while ductile deformation has great influence on pores,and pores with sizes in the range of100-1000 nm are most strongly compressed,and the deformation amount of pores with pore diameters between 100-500 nm are the most.According to the porosity and permeability tests under different pressure conditions,the content of clay minerals is proportional to the change of pores or permeability,and the content of quartz minerals is inversely proportional to the change of porosity or permeability.The content of brittle minerals in tight sandstones of coal measures is the largest,followed by clay minerals,while the carbonate minerals are the least.The pore types are mainly intergranular pores.The porosity of sandstone in the upper Jiangcang Formation,the lower Jiangcang Formation,and the upper Muli Formation has an increasing trend from top to bottom.The pores mainly consist of transition pores and micropores less than 100 nm,and they have a high degree of pore connectivity.The permeability distribution of tight sandstones is mainly concentrated in the range of less than 0.05mD,the permeability is extremely low,and the pores of 16nm⁶3nm in size contribute the most to the permeability.After calculating the fractal dimension of pores,it is found that the higher the quartz content is,the greater the pore structure complexity of macropores tends to increase,while the pore structure complexity of micropores and transition pores decrease.Along with the clay mineral content,the complexity of pore structure of micro and transition increases.（5）The preservation of coal measure gases in the study area was mainly controlled by structural,surrounding rock,permafrost,and hydrogeological conditions:coal measures in the southwest wing thrust of synclines were damaged to a high degree,and the faults were the most serious to the shallow coal measures.The damage to the north wing of synclines is relatively lower.Coal measures were buried deep in the core region of synclines,and the fault cutting effect is weak.The integrity of the coal system in the imbricate fan region has benn greatly damaged.However,many"structural stress traps"have also been formed in this area.The vertical lithological sealing strength of wall rocks in the preservation of coal-bearing gases is the upper section of the Jiangcang Formation>the lower section of the Jiangcang Formation>the upper section of the Muli Formation.From the plane,the sequence of the lithological sealing strength is Sanlutian>Yijingtian>Sanjingtian>Erlutian>Sijingtian>Erjingtian>Yilutian.The continuous distribution of permafrost in the study area effectively prevented the coal measure gases from escaping and could serve as a good regional coal measure gases cap in the entire region.The Jurassic coal measures in the study area is a water-bearing formation with weak water-richness,and the lithology of the aquifer is mainly sandstone,and the permafrost and the Xia 1 and the Xia 2 coal seams constitute the main water-retaining layers in coal measures.（6）It is summarized with three typical coal measure gases accumulation models in the Juhugeng mining area area,namely thrust-and hydraulic sealing,fault-ramp and lithologic sealing,and imbricate fan and permafrost sealing:（1）coal measure gases are favorably formed in the core of syncline and the top of the coal measures in the thrust-and hydraulic sealing model;（2）In the fault-ramp and lithologic saling model,the thickness of the coal measures is increased in the longitudinal direction,making the coal measure gases has good sealing conditions,and the core region of syncline is often a groundwater detention zone where groundwater can stop for a long time,which is conducive to the coal measure gases accumulation;（3）in the imbricate fan and permafrost sealing model,the region between two thrust faults and the top of Jiangcang Formation is susceptible to the formation of coal measure gases.