Continental Shale Gas Formation Mechanism And Occurrence State Of The 5^th Sector Of The Xujiahe Formation In Western Sichuan Depression

Continental shale sequences are very developed in the Sichuan basin,which has broad prospects for exploration.But the characteristics of complex petrology and lithology combination,strong heterogeneity,misunderstanding reservoir property,the weak research of shale gas-bearing characteristics and no research on the proportion of shale gas occurrence and its main control factors are seriously restricted the gas exploration and development process.In this paper,the 5th Sector of the Xujiahe Formation continental shale in western Sichuan depression is selected as the research object,and its macroscopic and microscopic characteristics,such as the stratum,the sedimentation,reservoirs,the geochemistry,and gas-bearing features are analyzed through the field outcrop investigation,core analysis and other methods,such as drilling and logging data analysis,geochemical experiments,field gas containing tests.Besides,various experimental methods are utilized including field desorption,isothermal adsorption,TOC pyrolysis and FESEM,which illustrate the formation mechanism and occurrence state of the natural gas.Moreover,main controlling factors of the gas-bearing characteristics are explored from the perspective of adsorbed gas and free gas.The research shows that the depositional system of the 5th Sector of the Xujiahe Formation is distal delta front-shore shallow lake-swamp,and the lithology is dark gray and gray shale,fine-grained sandstone interbedded,the thin coal lines are discontinuously in it.The reservoir physical property of mudstone is better than siltstone and fined sandstone.The reservoirs in T3X5 have a small amount of primary and intergranular dissolution pore,which was caused by strong diagenesis and the sedimentary environment from distant part of the delta to shore shallow lake with low hydrodynamic energy.The large quantitative of flake intergranular pores,organic pore,and micro-fractures in shale make it being main contribution to the reservoir property.The gas of the 5th Sector of the Xujiahe Formation continental shale has the features of the low methane content,low heavy hydrocarbon content,low dry coefficient and wide distribution of carbon isotope,and the gas would be pyrolytic origin coal-type gas at medium-high maturity.Experimental analysis data and log interpretation results showed that the features of high methane content,low heavy hydrocarbon content,low dry coefficient and wide distribution of carbon isotope is caused by its early formation and long duration process.Moreover,the continental shale gas also has the features of a combination of two kinds of gases,adsorption gas and free gas,however,the adsorption gas is the major part,occupying more than 70% of the total gas content.With the increase of organic carbon content,the clay mineral content and specific surface area for the adsorption of natural gas increased,thereby the adsorption ability of shale gas is increasing,so the adsorption volume is also increasing.The mainly reservoir space for free gas is micro-pore between clay minerals,inter-particle dissolution pore,intra-granular dissolved pore and micro-fracture,and the free-gas volume and effective porosity is obviously positive correlation.