Study On Reservoir Characteristics And Shale Gas Enrichment Of Wufeng-longmaxi Formation Shale In Southeast Chongqing

Taking the organic matter-rich shale of Wufeng-Longmaxi Formation in Southeast Chongqing as the research object,this dissertation studied the core scientific problem of the reservoir characteristics and enrichment of shale gas by adopting research approaches such as literature survey,field investigation,experimental test,mathematical simulation and theoretical analysis.Besides,this dissertation illuminated the evolution characteristics of pores in the shale reservoir and the accumulation process of shale gas in the studied area,revealed factors controlling shale gas enrichment and selected the “dessert zone” of shale gas through systematically collecting and collating the basic geological data of the studied area,carrying out the field investigation work and comprehensively studying the reservoir characteristics of shale gas as well as the “three histories” of shale,namely,construction-burial-hydrocarbon generation.The results are shown as follows.The first section of Wufeng-Longmaxi Formation belongs to deep-water shelf deposit while its upper part belongs to shallow-water shelf deposit,and the depositional environment gradually evolves from deep-water shelf facies to shallow-water shelf facies from the Qiyueshan fault to the central Guizhou old land.In Wufeng-Longmaxi Formation,shale mainly consists of type I kerogen which has reached the mature evolution stage;the content of organic matter is between 0.20% and 6.70% with an average of 2.85%;the area of high TOC values is located in the deep-water shelf environment.Shale is composed of complex and diversified minerals with strong heterogeneity.In this dissertation,the lithofacies is divided into five types,namely,siliceous shale lithofacies,calcareous siliceous shale lithofacies,mixed siliceous shale lithofacies,clay-rich siliceous shale lithofacies and calcareous/ siliceous mixed shale lithofacies by using the diagram of the three end members of siliceous minerals,carbonate minerals and clay minerals.In addition,shale has a high brittleness index,thus showing a good fracturing property.There are two kinds of macro fractures in the reservoir,namely,high angle fractures and horizontal fractures and four kinds of micro pores,namely,organic matter pores,intragranular pores,intergranular pores and microfractures,among which organic matter pores have the smallest average pore size and intergranular pores have the largest average pore size.TOC and the content of quartz and clay minerals are determined to be the main factors influencing the pore structure at the top and bottom of shale respectively via a combination of mercury penetration test,liquid nitrogen test and carbon dioxide adsorption test,as well as a full scale characterization of shale pore structure.Although the permeability of shale is generally low,the permeability of shall parallel to the bedding plane is relatively high with obvious anisotropy due to the development of bedding plane.The contribution of slip flow,Knudsen diffusion and surface diffusion to the gas transmission wanes and waxes,and the contribution is controlled by the pore scale and environment.Based on the percolation theory and the characteristics of fracture development,the critical fracture length for the occurrence of shale gas percolation is 6m/m2.Through the calculation of errors of different adsorption models,it is found that D-A model has the biggest error,followed by Freundlich model and D-R model,and Langmuir model has the smallest error.Moreover,statistics show that the adsorption capacity of shale is positively correlated with organic carbon content,micro pore volume,total specific surface area and pressure.According to the microstructure observation,the rock structure analysis and the fracture development characteristics,the tectonic evolution of the studies area is divided into four stages: the uplift into land during the early period of the Indosinian movement,the strong compression in SE-NW direction between the later period of the Indosinian Movement and the early period of the Yanshan Movement,the formation of a trough-like deformation band between the later period of the Yanshan Movement and the early period of the Himalayan Movement and the tectonic superposition during the later period of the Himalayan Movement.Besides,based on the restoration of the burial and hydrocarbon generation history of shale and the configuration of the “three histories”,shale gas accumulation in the studied area is divided into four stages: the deposition period of source rock,the initial accumulation period,the main accumulation period and the preservation period.Periods after the middle and later period of the Yanshan Movement is critical for the migration,dissipation and accumulation of shale gas.The evolution law of shale pore structure is studied and an evolution model of shale pores is established through the simulation of maturity shale and shale of low maturity,as well as the process of tectonic uplift.Based on the study of main factors controlling shale gas enrichment in the studied area,the law of shale gas enrichment in southeast Chongqing is found as follows: The material basis for the accumulation of shale gas is the organic matter-rich shale controlled by the deep-water shelf depositional environment;the decisive condition for the enrichment is the occurrence of shale gas under the control of reservoir physical properties;and the key guarantee for the enrichment is shale gas preservation under the combined effect of multiple factors.This law reveals that the enrichment of shale gas is jointly affected by multiple factors among which depositional environment,reservoir characteristics and preservation conditions are the main controlling factors.Ten shale gas “Dessert zones” with a total area of 2100.14 km2 can be selected from Wufeng-Longmaxi Formation in the studied area on the basis of the above law,auxiliary evaluation parameters,superposition of multiple factors and comprehensive analysis.The reservoir characteristics and the shale gas enrichment laws of Wufeng-Longmaxi Formation in southeast Chongqing revealed systematically in this dissertation are of great significance for guiding the high efficiency exploration of shale gas in this area.