Genetic Mechanism Of Glutenite Diagenetic Traps In Es4s Of Yanjia Area

Reservoirs of glutenite diagenetic traps in the nearshore subaqueous fan in the steep slope of faulted lacustrine basins have been significant targets for subtle trap exploration in eastern China.However,genetic mechanism of glutenite diagenetic traps is unclear,which has severely restricted further exploration on this type of reservoirs.Combined with the methods of core observation,thin section identification,scanning electron microscope(SEM)observation,fluid-inclusion analysis,an analysis of core properties and displacement pressure analysis,a case study of the nearshore subaqueous fan lying in the upper part of the forth member of the Shahejie Formation(Es4s)of Yanjia Area in the Dongying Depression was carried out.It is concluded that the types of glutenite textures in the upper part of the forth member of the Shahejie Formation(Es4s)of Yanjia Area can be divided into three categories and six subcategories.Generally,particle size turns fine,sorting becomes better,gravel contents decrease and vertical degradation degree increases at first then decreases from the fault boundary by the near and distant in source direction.Reservoirs of glutenite diagenetic traps in the nearshore subaqueous fan in the steep slope of faulted lacustrine basins have been significant targets for subtle trap exploration in eastern China.However,their genetic mechanism is unclear,which has severely restricted further exploration on this type of reservoirs.A case study of the nearshore subaqueous fan lying in the upper part of the forth member of the Shahejie Formation(Es4s)in the north zone of the Minfeng Sag in the Dongying Depression,a combination of core observation,thin section identification,scanning electron microscope(SEM)observation,fluid-inclusion analysis and an analysis of core properties,with research achievements on evolution of diagenetic environment,based on the study of the diageneses characteristics and their evolution sequence in different sub/micro-facies,the genetic mechanism of diagenetic traps in the nearshore subaqueous fan has been discussed.The sandy-conglomerate body in Es4 s of the Minfeng Sag in the Dongying Depression experienced diagenetic evolution of “alkalinity-acid-alkalinity-acid-weak alkalinity” and had a characteristic of diagenetic evolution sequence which is “compaction/early pyrite cementation/siderite cementation/gypsum cementation → feldspar dissolution/quartz overgrowth → quartz dissolution/carbonate cementation/lime-mud matrix recrystallization/feldspar overgrowth → carbonate dissolution/feldspar dissolution/quartz overgrowth/pyrite cementation”.Among different sub/micro-facies in nearshore subaqueous fan their sedimentary difference controls the difference of their diagenetic characteristics.With the burial diageneses of conglomerate in root fan are mainly compaction and lime-mud matrix recrystallization,with weak dissolution and the physical properties keep reducing,and when at a middle-deep burial the recrystallization of lime-mud matrix rapidly bring down the physical properties.In the reservoirs far away from mudstone strata in the braided channels of middle fan,a great number of primary pores and secondary pores are developed because two-stage of acid dissolution reconstructs the reservoirs and the hydrocarbon filling inhibits cementation,and physical properties of the reservoirs are better,which enables them to be effective reservoirs of hydrocarbon.A decrease of physical properties in root fan as well as an increase of property difference between root fan and mid fan both enhance the ability for root fan to plug oil and gas,and it can be some blocking layers.Because reservoirs adjacent to mudstone strata in the braided channels of middle fan as well as ones in the interdistributary area of channels in middle fan are intensively cemented,dense crust is formed at both the top and bottom of sand bodies,which can be some preferable cap rocks.Therefore,glutenite of nearshore subaqueous fan has formed diagenetic traps which have vertical or lateral plugging in root fan,dense carbonate crust near the edge of mud strata in braided channels of middle fan as well as lacustrine mudstones among different conglomerate-sandstone bodies in middle fan as covers,and glutenite far from mud strata in braided channels of middle fan as reservoirs.In time,from 32 Ma before present(B.P.)to 24.6Ma B.P.,in root fan lime-mud matrix of conglomerate was recrystallized to agglutination,and in braided channels of middle fan carbonate-cementation took place adjacent to mudstone strata,which worsened physical properties of the reservoirs.This is when the diagenetic trap was formed,while after 24 Ma B.P.,the diagenetic trap has a better capacity of.hydrocarbon plugging In depth,with the increase of burial depth,the ability of hydrocarbon plugging in root fan increases and diagenetic traps beneath 3200 m have strong power of hydrocarbon plugging.