Organic Matter Pore Characteristics And Influencing Factors Of Lower Cambrian Niutitang Formation Shales In Northern Guizhou

Black shale of Niutitang Formation in northern Guizhou is a set of source rocks with the greatest development potential in Guizhou,but it is controlled by the thermal evolution process of organic matter.Therefore,the identification of organic matter components and quantitative characterization of organic matter pores in Niutitang Formation in northern Guizhou are insufficient,which leads to certain difficulties in the evaluation of shale gas exploration and development in northern Guizhou.In this study,the Niutitang Formation shale in northern Guizhou with potential development was studied.The basic mineral composition and organic geochemical characteristics of the shale were analyzed,and the lithofacies types were classified.The occurrence state and pore structure of organic matter were qualitatively evaluated by field emission scanning electron microscopy（FE-SEM）and atomic force microscopy（AFM）.The pore structure of shale and kerogen was quantitatively characterized by small Angle scattering and low temperature gas（CO₂/N₂）adsorption experiments,and the contribution rate of organic pore to surface area（CRA）and organic to pore volume（CRV）were proposed.The organic pore development characteristics and pore structure of over-mature Marine shales are studied and analyzed by combining traditional and new techniques.The main understanding is as follows:（1）Quartz and clay minerals are the main mineral components of the Niutitang Formation shale in the study area.The quartz content is higher at the bottom of the Niutitang Formation,but decreases upward.The content of feldspar and clay minerals is low at the bottom of Niutitang Formation,but increases upward.It shows that the sedimentary water of Niutitang Formation changed from deep to shallow and the biological or hydrothermal deposition was gradually influenced by terrigenous clastic deposition.Sapropeli formation is the main maceral of kerogen in Niutitang Formation,and the organic matter types are mainly type I and type II kerogen,which has good hydrocarbon generation ability.The organic carbon content is generally more than 2%,which is an important set of organic-rich shale series.The maturity of organic matter is high,and it is in the high-over mature stage.（2）Organic matter in Niutitang Formation in Guizhou can be divided into four occurrence types:banded organic matter,massive organic matter,filled organic matter and mutually wrapped organic matter.Organic pore structure of different occurrence types is obvious.The banded organic matter is controlled by compression,and the pores are generally not developed.The development of massives organic and mutually wrapped pores is not uniform,and the development of filled organic matter pores and mutually wrapped organic matter pores are not uneven.According to the AFM,the surface roughness of organic pores are different,and the R_skare positive and negative,indicating that gas escapes and produces bubble pores when organic matter are developed.SEM and AFM experiments showed the heterogeneity of organic pore development.（3）It is clear that organic pores are the main contributors to the pore system of shale.In the low pressure gas adsorption experiment and small angle scattering experiment,the relative adsorption capacity,pore volume and specific surface area of organic matter are higher than that of shale,and organic matter has more abundant pores.The fractal dimensions of D₁and D₂based on the FHH model show that the complexity of the pore system of black shale is slightly higher than that of organic.CRV and CRA can quantitatively characterize the contribution rate of organic matter pore volume and pore surface area.The CRV of organic pores ranges from 40.87%to 59.39%,and the CRA ranges from 30.44%to43.33%,indicating that organic matter pores are an important part of shale system.（4）Organic matter type,total organic carbon content（TOC）,vitrinite reflectance（R_o）and minerals composition are the influencing factors of organic pore development.Organic matter type has direct influence on organic pore development,type I and type II kerogen organic pore development are good.During diagenetic process,mineral types can affect the development characteristics of organic pores in different pore sizes.The micropores increase and macropores decrease with the increase of felsic minerals,and there is no correlation with mesoporous pores.The content of micropores and mesoporous pores decreased with the increase of pyrite,but had no correlation with macropores.With the increase of clay minerals,micropores decrease and mesoporous and macropores increase.During the thermal evolution of organic-rich shales,TOC and R_oalso affect the development of organic pores.With the increase of TOC content,micropores and mesoporous pores first decrease and then increase,while macropores first increase and then decrease.The increase of R_odecreases the number of micropores and mesoporous pores,and has no correlation with macropores.The relationship between carbonate minerals and organic pore development is not significant.Organic matter pore development is not controlled by a single factor,but by many factors during diagenesis and thermal evolution.