Origin Of Middle Permian Dolomite And Diagenetic Fluid Tracing In Central Sichuan

The dolomite reservoir in the Qixia formation and the Maokou formation of middle Permian is long regarded as the replacement layer of natural gas exploration in sichuan basin.But the formation of this high-quality dolomite reservoir has been controversial for a long time.Penecontemporaneous dolostone and circumfluence dolomite is challenged with the lack of the circumscribed environment,and there is no report of evaporite in the whole basin,so early dolomization associated with evaporation causes a lack of evidence.The dolomitization of mixed water has long been questioned.As the burial diagenesis and Permian tectonic thermal events are taken seriously in recent years,related to buried and hydrothermal origin has become the mainstream of Permian in sichuan basin,dolomite origin mechanism,repeatedly reported.Based on these conditions.This selection includes abundant drilling data in Qixia formation and Maokou formation,hoping to know more about dolomite.It based on the drilling information and the upper region's rich siltstones and coring data,combined with the regional structure,stratigraphic and sedimentary background,on the basis of dolomite and its surrounding rock samples,by cathodoluminescence,package the experiment on package temperature measurement,element geochemical analysis,obtained the target strata in the study area petrology characteristics and geochemical characteristics of dolomite,rely on knowledge of carbonate sedimentology and geochemistry,the study area Qixia formation Maokou formation and the cause of dolomite re-recognition of the system.The Permian dolomite formation in the study area was formed between late Permian and early Triassic,mainly formed in late Permian.Qixia formation after the exposure period,formed the pack-half a pack of karst system,the late Maokou stage and longtan stage active volcanic activity deep rich magnesium fluid,heating the deep fault that hydrothermal along the dongwu movement formed through the upper part of the karst system in Qixia formation inside,then the floodplains replacement with the first deep hole,karst system internal fillings and the surrounding rock,the scale of dolomite diagenesis.At the bottom of the Maokou formation,the sediments of micrite and argillaceous limestone have played a part in a cap rock role for the lower hydrothermal fluid.Qixia formation after the exposure period formed the karst system in filling or half filling,the late Maokou period and Longtan period active volcanic activity heat deeply rich magnesium fluid,hydrothermal along the fracture system in Dongwu movement inject Qixia formation inside,then flowing filling and taking metasomatic of karst system formed the dolomitization with scale.Maokou formation after the deposition of,taked massive karstification,accepting the Early diagenetic karst in the Longtan formation,the faults relatively undeveloped,which led to mantle-derived fluid and massive marine-derived fluid with insufficient communication entered into karst system,mixed with sealed crust-derived fluid at varying degrees,promoting dolomitization in karst system and bedrock.During late Longtan period,the thicken overlain sedimentation sealed the epigenetic karst system,insufficient thermal fluid in deep formations mixed with existent fluid in the early karst system,jointly reforming dolomite with good pore permeability in previous karst system,it is cause that the geochemical characteristics are changed.in karst dolomite.Plentiful intercrystalline pores are retained in the karst system by hydrothermal dolomization,and dolomite is more automorphic in the karst system,because of inside of karst system have more free space.By contrast,matrix surrounding of karst systems has been densification in the early burial stage,which form dolomite with the poor physical property after dolomization.In the study area,the dolomitization is only provided the ability of the reservoir to resist compaction and pressure solution,but it is not significantly enhance the storage space of the reservoir.In fact,the main reservoir space of this type of karst dolomite is inherited from the unfilled pore formed in the karst system,and the original intergranular pores that are retained in the early stage of diagenesis.Therefore,we should not ignore the prediction of grain beach and the restoration of karst landform in the study area.