The Shales Diagenesis Study Of Wufeng And Longmaxi Formation In Pengshui Area

This study take the shales of Wufeng formation and Longmaxi formation in Pengshui area as the target.Under the premise of the previous research on reservoir characteristics and sedimentary facies of the Wufeng-Longmaxi formation,according to the Wufeng-Longmaxi formation in the studied area on mud shale of field profile and drilling cores observation,combined with the casting thin sections,scanning electron microscope?sem?,nitrogen adsorption method and thermal evolution simulation experiment etc technology.carried out the geological characteristics,diagenesis of mud shale reservoir,and analyzed the influencing factors of mud shale reservoir pore development of Wufeng-Longmaxi formation.According to the research of interval sampling analysis,the lithology mainly black-ash black shale and grey of silty shale of Wufeng-Longmaxi formation,visible a lot of graptolite fossils,belongs to shallow shelf-deep water shelf deposition.From the analysis of rock mineral composition,the shale is mainly composed of quartz and clay minerals,and average content of 36.4%,35.05%respectively,containing a small amount of plagioclase,calcite and pyrite,etc.Clay minerals content is given priority to with illite and illite mixed layer,a small amount of chlorite.Organic matter type is"I",its Total Organic Carbon?TOC?is on average 2.14%.Vitrinite reflectance?Ro?an average of 2.52%,the thermal evolution of organic matter has reached a high-over mature stage.The shale pore types of the Longmaxi Formation are intergranular pore,the average porosity of the Longmaxi formation is 3.23%,the permeability is0.174×10^-3??,overall shows the characteristics of low porosity,low permeability.In the late diagenetic stage of the shale in the study area,the main diagenesis has the compaction,corrosion,hydrocarbon generation and clay mineral transformation.Diagenetic history analysis shows that the study area Wufeng formation-Longmaxi formation precocious rock phase is given priority to with compaction,and the primary pore reduced,began to appear weak corrosion phenomenon,a small amount of secondary pore formation,clay minerals content is given priority to with smectite and illite mixed layer.Middle diagenetic stage is given priority to with organic matter hydrocarbon generation function,compaction disappear gradually reduced to basic,and because the production of organic acid dissolution began to increase,at this point is given priority to with secondary porosity,clay minerals content mainly for illite and illite mixed layer.Of late diagenetic stage,the organic matter in a mature stage,to generate a large number of dry gas,the environment is stable in the pore,clay minerals content is given priority to with illite and chlorite.Analysis of the main factors influencing shale pore structure in the Wufeng-Longmaxi Formation have diagenesis,organic carbon content and mineral content.The shale content of chlorite and feldspar in decreases with increasing porosity,quartz content increases with increasing porosity,reflects the chlorite and feldspar dissolution and quartz production,cause corresponding reservoir space increases,and quartz increased the reservoir compaction resistance,to protect the pore.Pyrite autogenous intergranular pore is also beneficial to the growth of the pore.Sedimentation-compaction Results in lower porosity and shrinkage.Organic matter hydrocarbon generation makes the mud shale of specific surface area and pore volume increased,to improve the adsorption performance of rock mass.Dissolution make unstable components by dissolution and corrosion pore space.Clay minerals content syneresis can increase the clay minerals content of intergranular hole,beneficial to the growth of mudstone shrinkage joint.The porosity of the shale in the study area increased with the increase of organic carbon content,there was a positive correlation,that the increase of the content of organic carbon in favor of the formation of porosity of organic matter,the porosity increases.And the correlation between organic carbon and permeability is bad,may be due to the organic matter of stomatal connectivity is poor,so the weaker influence on permeability.