| The"Three Gas"resources survey project of the deep coal measures in Yushe Wuxiang block has made a major breakthrough.It has been preliminarily demonstrated by the experts of the Ministry of land and resources that its unconventional natural gas resources are over 5000×10~8m~3,which are super large gas fields and have the resources conditions for building large natural gas industrial bases.Among them,Taiyuan Formation coal seam,dark shale and tight sandstone are well developed,and have good"Three Gas"geological conditions.It is an important favorable gas producing section of"Three Gas"in coal measures.In this paper,the unnormal conventional natural gas reservoir in the Taiyuan formation of Yushe-Wuxiang block is taken as the research object.Through the data collection,experimental test and theoretical analysis,the core problems of the"Three Gas"reservoir forming conditions and the accumulation mechanism of the Taiyuan formation in the study area are systematically studied.The results showed that:The Taiyuan formation in the study area is formed in the depositional environment of the sea and land.Through the discrimination of the sequence interface of the study area and the key sequence of the sea surface,the sequence stratigraphic framework of the late Paleozoic Benxi period Taiyuan period is established,and 2three sequence,4 individual tract and 9 quasi sequence are divided.The study shows that the total thickness of Taiyuan group shows the distribution characteristics of"Northwest large and Southeast small"on the whole,the burial depth of Taiyuan formation in the study area is between 400~1800m,and the depth of buried depth is gradually increased from the South East to the North West,and the formation tendency is NW direction.The"Three Gases"material base and the geochemical characteristics of organic matter abundance,type and maturity of Taiyuan formation in the study area are systematically discussed.The mineral composition of dark shale in Taiyuan formation is mainly quartz or clay minerals.The clay minerals including kaolinite,illite,chlorite and illite mixed layer etc.The coal and rock micro coal and rock fractions of Taiyuan formation are dominated by vitrinite,and have some inert mass and few mineral impurities.The detrital components of sandstone reservoirs in Taiyuan formation are mainly quartz,followed by feldspar and lithic components.The organic matter type evaluation methods include kerogen element analysis,rock pyrolysis identification and maceral microscopic identification,and the organic matter type of dark shale is type III.Dark shale has a large range of organic matter abundance,with an average of2.91%.From the Ro value,shale of Taiyuan formation in the study area is at a high maturity stage.Generally speaking,Taiyuan coal is low moisture,medium and high ash content,medium and low volatile matter,medium and high fixed carbon coal,and coal rank is lean coal anthracite.The pore fracture and permeability characteristics of"three gases"and different lithologic reservoirs in Taiyuan formation are mainly analyzed,and the stress sensibility of different types of reservoir permeability is discussed.The high-pressure mercury test shows that the dominant pore diameter of coal reservoirs is basically distributed in the range of 2~100nm and>10000nm.The main micropores and large pores of the shale are developed.The pore size distribution of tight sandstone reservoir is 10~10000nm,and the pore development is relatively uniform.Low temperature liquid nitrogen experiment shows that little pore development in coal and rock reservoirs.The micro-little pore of shale samples is more developed.The pore size of tight sandstone samples is wide,and it is more developed in the range of5~100nm.It indicates that the pore and micropore development in tight sandstone reservoir is within the scope of characterization of liquid nitrogen adsorption.The occurrence characteristics and gas bearing property of"Three Gas"in coal measures are analyzed.According to the characteristics of vertical lithologic overlay and measured gas content data,the source rock reservoir caprock combination type of Taiyuan formation is divided into four types:mudstone-sandstone-mudstone combination,limestone-coal-mudstone-sandstone-mudstone-limestone combination,mudstone-coal-mudstone combination,mudstone,coal,sandstone and mudstone.The advantage combination type of"three gas"superposition in Taiyuan formation is put forward.The simulation of hydrocarbon generation history in the study area revealed that the Taiyuan formation experienced two stages of hydrocarbon generation:the hydrocarbon process in the first life was late Paleozoic to the end of the Triassic,and the second hydrocarbon generation occurred in the early Middle Cretaceous.Based on the"Three Gas"reservoir combination and spatial distribution characteristics and the evolution characteristics of hydrocarbon accumulation history in Taiyuan formation,the"Three Gas"superposition process in Taiyuan formation is divided into five parts:late Carboniferous-early Triassic three,early Triassic early,Triassic,early Middle Jurassic,late Jurassic-Early Cretaceous,late Cretaceous-Cenozoic.The mechanism of reservoir formation in Taiyuan is the early continuous subsidence,forming gas reservoirs.Hydrocarbon generation and transformation in later stage,such as regional uplift and tectonic thermal events.Several sets of natural gas accumulation have been constructed on favorable source reservoir cap combinations.Because of the large difference of lithologic combination in each section of Taiyuan formation,there are also great differences in their accumulation models.Jinci is Shale gas-coalbed methane reservoir formation model.Xinshan is combined formation mode of shale gas-coalbed gas-tight sandstone gas reservoir.Shangou is gas accumulation model of shale gas-tight sandstone gas.The research results of this paper not only have certain theoretical significance,but also have practical significance for the co exploration and co mining of"Three Gas"in the coal measure. |
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