Research On Pore Structure Characteristics Of Wufeng-Longmaxi Formation By N₂ Adsorption,Sichuan Basin

Pore structure characteristics for 22 fresh outcrop shale samples collected from Shizhu County and 19 core samples collected from the Qianqian 1 core well of the Wufeng Formation and Longmaxi Formation in the east of the Sichuan Basin.The pore size distribution and pore type of shale in two sections were investigated by N₂adsorption experiment at 77K.Based on the correlation analysis of total organic carbon(TOC),mineralogy content and pore structure parameters,the main factors affecting the pore development of shale were studied by argon ion milling-high resolution scanning electron microscopy(SEM).The effects of tectonic uplift,weathering,denudation and leaching on the shale porosity were discussed.The results show that:(1)The pores of the shale of the Wufeng Formation and Longmaoxi Formation are mainly cylindrical and parallel plate-like open pores with large specific surface area and pore volume.The specific surface area is mainly provided by micropores and mesopores with pore diameters less than 50 nm.The specific surface area and pore volume of organic matter shale in the Wufeng Formation and Longmaixi Formation showed good positive correlation,indicating that the organic matter had an important contribution to the porosity of the Longmaixi Formation shale reservoir.(2)The pore volume has a positive correlation with the quartz content,but it has a weak negative correlation with the clay mineral content.With the clay mineral content increased,the pore volume decreased,clay minerals are not conducive to the development of the pores.However,with the strong compaction ability of quartz,the preservation of the intergranular pores is favorable,and it is also beneficial to the preservation of organic pores.(3)SEM images show that the outcrop section and the drilling core section of the shale samples organic matter hosted-pores are honeycomb evenly distributed oval,round and angular distribution of the harbor-shaped irregular shape.The amount of authigenic pyrite present in the form of strawberry and dispersed crystal particles is high,and the mixture of organic matter and clay minerals distributed in triangular or strip shape is developed.(4)However,due to the weathering effect of the outcrop shale samples,such as weathering,denudation and leaching,the dissolution effect is more intense,resulting in larger pore size,isolated dissolution pore development.Late weathering,only a certain transformation of inorganic pores,organic pores almost no effect.