| As a demonstration and pilot area of marine shale gas exploration and development in China,Sichuan Basin has realized the commercial exploitation of shale gas for the first time,which has set off an energy revolution in searching for unconventional natural gas nationwide.Pore structure and methane adsorption capacity of shale are important geological parameters for evaluating gas potential and oil-gas occurrence in a Basin,and are also one of the geological problems of great concern in the process of shale gas exploration and development.At present,the related research mainly focuses on large sedimentary basins,such as Sichuan Basin and Ordos Basin,but the research on small basins is not deep enough.Shale gas resources in some small and medium-sized marine sedimentary basins in China have great potential,which is expected to become an important research direction of oil and gas exploration in the future.In order to study the pore and adsorption characteristics of the lower Cambrian marine shale in Xiuwu Basin,the organic rich shale of the lower Cambrian Hetang Formation in the Basin was selected as the research object.The pore development and adsorption characteristics of the marine shale were systematically analyzed through the experiments of porosity was measured by helium,low temperature N2adsorption,CO2adsorption,CH4isothermal adsorption and scanning electron microscopy.Combined with organic geochemical analysis and petrology,the main controlling factors of pore development and adsorption are discussed.The study shows that:according to the geological parameters of the main gas producing shale in North America and China,the Hetang Formation shale has small burial depth(421.20?527.73m),large thickness(83.23m),high organic matter abundance(TOC average is3.03%),low maturity(ROaverage is 1.44%),low porosity(average is 1.97%)and good gas bearing property(Langmuir volume average is 1.44%),with high content of brittle minerals(average 63.7%),which is a favorable shale gas exploration horizon;the spatial structure of micro pores in shale reservoir is very complex,no organic pores are developed,and inorganic pores and micro fractures are the main reservoir space.Inorganic pores include interlayer pores of clay minerals,intergranular pores,dissolution pores of brittle minerals,intergranular pores of strawberry pyrite,etc.;the main types of microfractures are diagenetic shrinkage fractures and structural fractures.The pore morphology mainly includes open cylindrical pore,layered slit pore and thin necked wide body ink bottle pore;the total pore volume of shale is(4.3?20.1)×10-3cm3/g,with an average of 9.07×10-3cm3/g,in which micropores and mesopores are the main contribution,with an average proportion of 43.24%and 41.91%respectively;the total specific surface area is 6.94?34.94 m2/g,with an average of 16.25 m2/g,in which the specific surface area of micropores is the main one.TOC and quartz are the main factors affecting the pore development of shale,while pore structure,brittle minerals,TOC and chlorite content are the main factors affecting CH4adsorption capacity.The specific relationship is as follows:1.TOC has a significant positive correlation with micropore volume,DFT and BET+DFT specific surface area;quartz has a medium positive correlation with micropore volume,DFT and BET+DFT.2.There is a significant positive correlation between the Langmuir volume and the total specific surface area and micropore volume,which first increases and then decreases with the total pore volume,and first decrease and then increase with the brittle minerals;there is a medium positive correlation with TOC;and there is a medium negative correlation with chlorite.In addition,the influencing factors of porosity are not clear,and only weakly negative correlation with quartz content is found. |
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