Fine Characterization Of Pore Structure Of Continental Shale

The potential of shale gas resources in China is huge.At present,the exploration and development of shale gas are mostly concentrated in the marine shale gas in the Sichuan Basin.Due to its strong heterogeneity and complex pore structure,continental shale is difficult to explore and develop,and its production is low.As an important part of shale gas,it is necessary to further study continental shale.The continental shale in the Ordos Basin is developed,among which the continental shale in the Chang 7 member of the Yanchang Formation is widely developed and has a large thickness,which is an important shale oil and gas development horizon.In order to further clarify the characteristics of continental shale in the study area,the strata,shale distribution,sourcereservoir contact relationship and characteristics are finely divided by logging,drilling,mud logging and various analytical laboratory data.The microscopic morphological characteristics of pores were observed and analyzed by scanning electron microscopy.The micropores,mesopores and macropores of continental shale were characterized by gas adsorption method and high pressure mercury intrusion method respectively.The pore types and structural characteristics of different source-reservoir types were clarified,and the pore influencing factors of continental shale were analyzed.Combined with the thermal evolution experiment of shale,the evolution law of pore development of shale was analyzed,and the pore evolution model of shale was established.The results show that the thickness of shale in the study area is large,generally 70 m ～ 80 m,the abundance of organic matter is high(average TOC4.90 %),the organic matter is in the mature stage(average Ro0.90 %),and the organic matter pores,inorganic pores and micro cracks are mainly developed.Shale can be divided into three categories : thick shale with thick sandstone(type I),shale and lamina interbedded(type II),shale with thin sandstone(type III).The pore size of the micropores developed in type I is 0.53 nm ～ 0.62 nm,and the mesopores develop open pores and semi-closed pores.The pore size is mostly 2 ～ 7nm,and the specific surface area is mainly provided by 2 ～ 40 nm pores.The pores have good physical properties,and the maximum pore throat reaches 1.5 ～ 1.6.The pore size of the micropores developed in type II is 0.49 nm ～ 0.57 nm.The mesopores develop inclined plate-like pores with a pore size of 2 ～ 50 nm.The specific surface area is mainly provided by pores with a pore size of 2 ～ 50 nm.The maximum pore throat reaches 0.73 ～ 0.76,with poor physical properties and wide pore size distribution.The pore size of the micropores developed in type III is 0.52 nm ～ 0.63 nm,and the mesopores develop ink-bottle-like pores.The pore size is between 2 ～ 20 nm,and the specific surface area is mainly provided by pores with a pore size of 2 ～ 30 nm.The pore development of shale in the study area is mainly affected by organic matter content and mineral composition.It is positively correlated with TOC content,negatively correlated with clay mineral content,and negatively correlated with brittle mineral content.With the increase of buried depth and maturity,the pore decline rate of shale decreases gradually until it tends to be gentle.Buried depth and maturity are the main controlling factors affecting the evolution law of shale.