| This thesis focuses on coal measure gas of TaiYuan-ShanXi formation in YuShe-WuXiang area,aiming at elucidating the mechanism and model of superposed accumulation of coal measure gas in the study area.By using field investigation,experimental test,numerical simulation,and systematic theory analysis,following conclusions have been drawn:(1)According to the correlation theory of sequence stratigraphy,the key stratum identification index is established and the key stratum spatial distribution rule is defined.Restricted by sequence stratigraphic framework,the indexes of sedimentary assemblage,cap sealing,groundwater dynamic conditions,reservoir pressure characteristics,vertical gas-bearing distribution and gas isotope distribution of various gas reservoirs are extracted.The difference in vertical distribution of coal measure gas and its geological control factors are revealed,and then,the superimposed independent gas systems are divided.The results show that the spatial distribution of the target strata in the study area is mainly controlled by monocline structure,with a few faults and folds.The buried depth of the reservoir is between 600-2500 m,and the overall depth is above 1000 m,which belongs to the category of deep unconventional natural gas.The target strata is divided into three gas-bearing systems and several subsystems.No.3 coal in gas-bearing system III and No.15 coal in gas-bearing system I have stable development and large thickness in the whole region,which is favorable for the formation of coal-bed gas reservoirs.The development of shale and sandstone in each gas-bearing system is different,and they have the characteristics of small thickness of single layer and large accumulative thickness.Mud shale and sandstone also have certain gas bearing properties.Under the limitation of gas bearing system,they present multiple types of superimposed gas reservoirs.(2)Based on the combined method of multiple testing technologies,the geochemical characteristics and physical characteristics of coal measure gas source-reservoir rocks were comprehensively studied.Then,the hydrocarbon generation potential of source rocks and reservoir system were deeply understood.The results show that the type of organic matter in coal measure gas source rocks is mainly type III,and the overall thermal evolution is in the stage of high-over mature,with good potential for gas generation.As aggregates of organic matter,the abundance of organic matter of coal is significant(> 60%).As another important hydrocarbon source rock in coal measures,mud shale(dispersed organic matter)has a wide range of organic matter abundance(overall,it is in the range of 1%-3%).The organic matter abundance of shale has a certain tendency in the vertical direction,that is,the closer to coal seam,the higher the content of organic matter,indicating that the organic matter abundance(hydrocarbon generation potential)of shale is significantly controlled by the sedimentary environment.In terms of inorganic minerals,clay minerals and quartz are dominant in mud shale and sandstone,with clay mineral content ranging from 25% to 63.4%,with an average of 46.6%.Quartz content ranged from 26.2% to 47.1%,with an average of 35.7%.The quartz mineral content of sandstone reservoir is up to 81.4% and 62.4% on average.There are stomata,residual plant tissue pore,secondary mineral pore,intercrystalline pore and primary intergranular pore in coal.Micropores are well developed in coal,and some microfractures are developed.Mud shale is mainly composed of pores of organic matter,intergranular pores and intragemical pores,with mesopores and micropores developed and poor pore connectivity.The tight sandstone is mainly composed of dissolution pores and intercrystalline pores,with relatively developed macropores and microfractures and good pore connectivity.In terms of permeability,coal measure gas reservoirs are all low permeability reservoirs.Compared coal measure gas reservoir characteristics of different gas-bearing system,reservoir in gas-bearing system ? has great brittleness index,the porosity and permeability,while pore volume and specific surface area are relatively large as well,indicating that reservoir in gas-bearing system ? has a larger reservoir space and favorable conditions of gas migration.(3)By means of gas logging,field desorption and experimental testing,the in-situ gas-bearing capacity and the maximum gas-bearing capacity of coal measure gas reservoirs are comprehensively studied,and the controlling effects of different geological factors on gas-bearing capacity of various reservoirs are further explored.The results show that the gas content of coalbed methane is very significant,and the in-situ gas content and the maximum gas content are much higher than those of sandstone and shale reservoirs.At the same time,the sandstone and shale reservoirs have certain gas-bearing capacities.The distribution of gas-bearing properties of various reservoirs shows a similar change trend,indicating that external geological factors have a tendency to control the coal measure gas.On the contrary,the difference of gas-bearing capacity among different types of gas reservoirs is the manifestation of reservoir difference under the control of intrinsic organic-inorganic fabric.The main controlling factors of gas-bearing property of coal measure gas reservoirs are further analyzed,and relevant gas-bearing capacity models are established: The coal-bed methane is mainly adsorbed gas,when it is buried in the depths of 0~800m,with the increase of buried depth,the gas content increases rapidly.The gas content decreases totally when the buried depth is more than 800m;the gas content of shale is low as a whole,which is dominated by adsorbed gas as well.The gas content increases rapidly with the buried depth in the range of 0~1000m.When the buried depth is more than 1000 m,the gas content tends to be stable;when the buried depth of tight sandstone is in the range of 0~2500m,the gas content increases gradually as the buried depth is deeper,and when the buried depth is more than 2500 m,the gas content decreases gradually.From the perspective of exploration and development of combined gas reservoirs,the depth between 1000 ~ 3500 m is more advantagous.(4)Based on the hydrocarbon generation potential of coal measure gas source rocks and reservoir systems,the evolution history of superimposed accumulation of coal measure gas is studied.The key period of accumulation of coal measure gas,hydrocarbon generation evolution of source rocks and dynamic process of reservoir energy adjustment are further analyzed.The mechanism of coal measure gas co-generation reservoir is revealed.The results show that the formation process of reservoir of coal measure gas in the target stratum in the study area includes five stages: source rock-reservoir superimposed combination formation stage,rapid subsidence-primary hydrocarbon generation stage,buried depth fluctuation-gas escape stage,secondary hydrocarbon generation-filling stage,sustained uplift-gas adjustment stage;In secondary hydrocarbon generation-filling stage,a large number of hydrocarbon gases generated by source rocks cause reservoir overpressure.By means of caprock breakthrough,interlayer seepage and diffusion,the adjacent reservoirs are recharged;during the period of continuous uplift-gas adjustment,the energy balance of various types of gas reservoirs is destroyed,and the energy systems of different types of gas reservoirs are dynamically adjusted,accompanied by gas exchange among gas reservoirs,forming the present superimposed reservoir.Based on the analysis of the coupling relationship of gas content,buried depth,reservoir thickness,reservoir physical properties and structural characteristics,it can conclude that the key controlling factors for superimposed reservoirs of coal measure gas are the thickness of the combined reservoir and the development of structure.Specifically,lage thickness of reservoir assemblage and the syncline-anticline turning area are favorable for the enrichment of coal measure gas;in addition,the structural characteristics,groundwater conditions and biological action can also form enriched coal measure gas reservoirs locally;the main formation model of the superimposed reservoir of coal measure gas in the study area is the symbiotic sedimentary assemblage with a certain thickness distributed in the transition zone of wide and gentle folds.The traditional coalbed methane accumulation model(symbiotic deposits with a certain thickness distributed in the core of syncline)and special symbiotic gas reservoir model(the combination of geological conditions and biological factors)are typical examples of the superimposed accumulation model in the study area. |
PDF Full Text Request