Characteristics Of Different Types Of Shale Pores And Their Relationship With Shale Gas Occurrence State In Wufeng-Longmaxi Formation,Changning Area,Southern Sichuan Basin

Marine shale of Wufeng-Longmaxi Formations in Sichuan Basin is the main shale gas exploration and development interval in China.At present,the daily output of shale gas in Changning area in southern Sichuan is the first in China,which lays a solid foundation for the commercial development of shale gas in China.The Marine shale of Wufeng-Longmaxi Formations in Changning area is generally characterized by high TOC,high porosity,high gas content,bedding and microfracture development,and is a high-quality interval for exploration and development.Shale reservoir pores can be divided into organic pores and inorganic pores in genesis.Organic pores are formed in the process of shale hydrocarbon generation and are affected by the abundance,type and maturity of organic matter,while the preservation of organic pores is controlled by maturity,mineral composition,diagenesis,depth,pressure,fracture and structure.Similarly,inorganic pores are formed during shale diagenesis,and their formation and preservation are influenced and controlled by sedimentary environment,mineral composition,diagenesis,depth,pressure,fracture and structure.Quantitative characterization of different types of pores is the key to shale gas reservoir evaluation,while shale gas bearing capacity is an important index for shale gas resource potential evaluation,exploration favorable area selection,resource amount calculation,and also a key parameter for later development planning,gas reservoir description,and reserve calculation.Therefore,it is of great significance to find out the relationship between pore type and occurrence state of shale gas to improve the occurrence mechanism of shale gas and guide the evaluation of shale gas content.Taking the Wufeng-Longmaxi Formation shales in Changning area,southern Sichuan Basin as the research object,the lithofacies classification and characteristics,organic geochemical characteristics,microscopic characteristics of pores and their development degree of shales were investigated by means of thin section observation,X-ray diffraction,TOC determination,microscopic examination,nuclear magnetic resonance,scanning electron microscopy and field analysis.The porosity of organic and inorganic pores was quantitatively characterized,and the influence of adsorbed gas and free gas in different types of pores was discussed.The experimental results show that the target shale in the study area is mainly developed in siliceous shale facies,a small amount of calcareous siliceous shale facies and clayey siliceous shale facies,mainly developed in type I kerogen,which is Marine over-mature organic-rich shale with good hydrocarbon generation potential.In addition,a large number of circular,round-like honeycomb and spongy organic pores are developed in shale samples.At the same time,many intergranular pores,dissolution pores,pyrite intergranular pores and interclay micropores are also developed in shale,and the pore morphology is diverse and widely distributed.In order to reduce the influence of factors such as structural conditions and depositional environment,the Wufeng Formation-Longmaxi Formation shale in two adjacent wells(Well X1 and Well X2)in the study area were selected for NMR experiments to characterize the pore characteristics and organic porosity and inorganic porosity of the samples.At the same time,the desorbed gas,lost gas and residual gas content of the sample were obtained according to the on-site analytical experiment,and the free gas and adsorbed gas content of the sample were obtained by combining the relevant formulas.Combined with data and tests,the occurrence state of different types of pores and shale gas was identified: Among the organic pores in the shale samples from X1 and X2 Wells,macro pores occupy an absolute advantage in terms of the number and area of pores.The increase of organic pore porosity is dominated by the increase of pore size,that is,with the increase of organic pore porosity,adsorbed gas content decreases instead.At the same time,in well X1,free gas content is positively correlated with organic pore porosity,while in well X2,there is no correlation,that is,organic pore porosity is one of the controlling factors of free gas content,and other influencing factors include water saturation,mineral composition and retention conditions discussed by previous studies.In shale inorganic hole,the occurrence of the adsorbed gas are mainly affected by the influence of the pores of the clay,the form of clay minerals content is rich and diverse structure in the reservoir space of inorganic hole provide a large number of adsorbed gas adsorption,the X1 and X2 well adsorbed gas content in samples with clay minerals content increasing first increase then reduce,the rest of the minerals is less or no correlation with adsorbed gas correlation,At the same time,the porosity of inorganic pores increases first and then decreases with the increase of clay mineral content,that is,clay minerals control the storage of adsorbed gas in inorganic pores,so the adsorbed gas content increases first and then decreases with the increase of porosity of inorganic pores.Inorganic pores are widely distributed in the shale in the study area,and are the main storage space for free gas.The content of free gas in X1 and X2 Wells increases with the increase of the porosity of inorganic pores.In the Marine shale of Wufeng-Longmaxi Formation in the study area,the increase of porosity of organic pores is not conducive to the occurrence of adsorbed gas.The storage of free gas in organic pores is restricted by water saturation,mineral composition and retention conditions,and the porosity of organic pores is one of the controlling factors of free gas content.In inorganic pores,the occurrence of adsorbed gas is mainly affected by the content of clay minerals,and the content of free gas increases with the increase of the porosity of inorganic pores.