Research On The Influence Of Organic Matter-Clay Minerals On The Occurrence Of Shale Gas In Northern Guizhou

Shale gas resources are abundant in Guizhou Province,with a geological resource of 1.048 billion cubic metres,ranking fourth in China.Northern Guizhou is the most realistic and favorable shale gas exploration and development area in this province.The Lower Cambrian Niutitang Formation,Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation are the main target formations.Although previous scholars have generated many research achievements in terms of sedimentary environment,organic geochemical characteristics,reservoir spatial characteristics,the influence of geological structure on reservoir formation,and the main controlling factors of gas-bearing,there is few in depth and systematic researches about relationship between organic matter and clay minerals in reservoirs,diagenetic evolution of clay minerals and hydrocarbon generation of organic matter,andeffect of organic matter and clay minerals on gas content,especially clay-organic complex.In this paper,the black shales of Lower Cambrian Niutitang Formation,Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in northern Guizhou were the research objects,studies of regional geological background,mineral composition,sedimentary environment,organic geochemical characteristics,mutual characteristics of organic matter and clay minerals,reservoir space characteristics,the evolution characteristics and relations of organic matter and clay minerals,clay minerals were performed.The adsorption characteristics of shale gas by clay-organic complexes,as well as the influence of organic matter and clay minerals on shale gas enrichment were investigated.A theoretical system based on the influence of organic matter-clay minerals on shale gas accumulation was established,which is used to identify the favorable sections from the aspects of potential of hydrocarbon generation and storage characteristics.The results are valuable to exploration and development of shale gas in northern Guizhou in both theoretical and practical aspects.Major achievements of this research include:1.Microscopic thin section identification,X-ray diffraction analysis(XRD),scanning electron microscopy(SEM-EDAX)and electron probe analysis(EPMA)were conducted.It is found that the black shale of Niutitang Formation and Wufeng Formation-Longmaxi Formation in northern Guizhou are mainly composed of quartz and clay,and also contain albite,potash feldspar,pyrite,dolomite and calcite.The clay mineral composition in the black shale of Niutitang Formation is relatively simple and illite is the basic type.The clay minerals in Wufeng Formation and Longmaxi Formation are mainly illite.In addition,they also contain chlorite and illite mixed layers.The content of quartz and pyrite in Niutitang Formation is higher than that in Wufeng Formation-Longmaxi Formation,while the content of clay minerals is lower than that in Wufeng Formation-Longmaxi Formation.2.The analysis of trace elements,rare earth elements and carbon isotope shows that the sedimentary environment of Longmaxi Formation was a weak oxidation environment,while Niutitang Formation and Wufeng Formation were deposited in an anoxic environment,which was more favorable for preservation of organic matter.The black shale in the study area was mainly formed in the estuarine deposit environment near the paleocontinental margin,and its material source was controlled by terrigenous,mantle and deep sources.There is a growth-decline relationship between organic matter and ?REE in black shale in the study area,and a positive correlation between ?REE and clay mineral content.3.Organic geochemical analysis shows that the TOC of shale in Niutitang Formation,Wufeng Formation and Lower Longmaxi Formation is more than 2%.The organic carbon content in Longmaxi Formation decreases from bottom to top,and overall content is lower;kerogen type of Niutitang Formation,Wufeng Formation and Longmaxi Formation is mainly type I;the organic matter maturity of black shale is high,and it is in the stage of cracking and generating dry gas.The results show that Niutitang Formation and Wufeng Formation-Lower Longmaxi Formation in northern Guizhou have a promising potential of hydrocarbon generation.4.Organic matter in black shale in northern Guizhou mainly exists in the form of clay-organic complexes.In the process of diagenesis of clay minerals,interlayer water in clay-organic complexes in shale were discharged twice.First,part of the interlayer water was discharged,and then interlayer organic matter was discharged.After that,stagnant interlayer water affected by interlayer adsorbed organic matter was discharged.The diagenetic evolution of clay minerals,hydrocarbon generation evolution of organic matter and catalytic activity of clay have corresponding relations.During the period of rapid transformation from smectite to illite,clay had the strongest catalytic activity for the removal of interlayer water and is the most favorable period for hydrocarbon generation of organic matter.Wufeng FormationLower Longmaxi Formation in northern Guizhou is at the end of this period,generating considerable shale gas,and is the most favorable section for shale gas exploration and development.5.The adsorption characteristics of shale gas by black shale samples,clay-organic complexes and standard clay minerals in the study area conformed to the Langmuir adsorption model and was considered as type I isothermal adsorption.The experimental results show that the order of methane adsorption capacity of clay minerals was: montmorillonite > illite > chlorite > illite,and the simulation results were montmorillonite > illite > chlorite.Their orders are generally consistent,and the conclusions are the difference was due to different objects.The simulation results also show that the clay mineral kerogen composite structure and the clay mineral adsorption of methane had the same order: montmorillonite kerogen composite structure > illite kerogen composite structure > chlorite kerogen composite structure.The comparative study on the adsorption properties of clay mineral kerogen composite structure and clay mineral shows that the adsorption capacity of montmorillonite kerogen composite structure was higher than that of montmorillonite,illite kerogen composite structure was higher than illite,and chlorite kerogen composite structure was less than chlorite.6.In addition to intergranular pore,intergranular pore,mineral casting pore and secondary dissolution pore,organic matter pore and clay mineral interlamellar pore were mainly developed in shale reservoirs in the study area.Shale reservoir space in the study area is dominated by micropore-mesopore,and the proportion of macropore is relatively small.A large number of micropores and mesopores were developed in the evolution of hydrocarbon generation of organic matter.A large number of nano-scale pores were developed in organic matter,making the specific surface area for shale gas adsorption larger.At a certain temperature and pressure,the ability of shale to absorb gas mainly depends on TOC,the specific surface area of nano-scale micropores and pore volume in reservoir.The main types of shale reservoir fractures in the study area are structural fractures,diagenetic shrinkage fractures,interlayer shales and fissures,and micro-fractures surrounding mineral particles and mineral aggregates.Organic matter and clay minerals have a certain contribution to crack formation.7.In the study area,the main influencing factors of adsorbed shale gas volume are TOC,thermal evolution degree of organic matter and diagenetic evolution degree of clay minerals;organic matter and clay minerals have a certain influence on free shale gas volume.8.The clay-organic complexes have a higher adsorbability than shale.Shale gas in northern Guizhou is dominated by adsorbed gas.The proportion of adsorbed gas in Niutitang formation of Well TM1 is 58%-63%,that in Niutitang formation of Well FC1 is 56%-58%,and that in Wufeng-Longmaxi formation of Well BZ1 is 73%-79%.The amount of adsorbed gas calculated based on clay-organic complexes is much larger than that of shale rock sample.The content of adsorbed gas in the local enrichment area of claorganic complexes is much higher than that in other areas,which is the “sweet spots” of shale gdevelopment.