| The lower-middle Ordovician carbonate reservoirs in Tarim Basin,as the key strata of deep marine carbonate exploration in China,have made significant breakthroughs in oil and gas exploration in recent years.The database of this study contains exploration and production data,including drilling well data,seismic data,drilling cores,and outcrops description,and lab-testing petrology and geochemical data.The sequence stratigraphic framework was firstly established.Then the characteristics of sedimentary facies were analyzed.On the basis of detailed diagenesis analysis,this study discussed the control of various sequences and their boundaries and interfaces,sequence exposure(unconformity),and sequence fracture on diagenesis.Distribution of penecontemporaneous and burial-epigenetic diagenetic facies within sequence framework was clearly defined.This study is aimed to reveal the geological conditions for the development of the high quality carbonate reservoirs.On the basis of identifying 5 key sequence boundaries of 2 periods,the lower-middle Ordovician carbonate strata are divided into 3 3rd sequences,including SQ1(Penglaiba formation),SQ2(lower Yingshan Formation),and SQ3(upper Yingshan Formation).The carbonate platform mainly developed two typical sedimentary sequences: tidal flat type and subtidal type.Petrographic and geochemical study implies that the dolomitization mechanism including seepage-reflux dolomitization related to mesosaline to penesaline seawater,burial dolomitization related to compaction and recrystallization,and hydrothermal dolomitization related to crustal magmatic hydrothermal fluids.SQ1 is characterized by oxidizing environment during early transgressive system and late highstand system,while it is characterized by reduction environment during late transgressive system and early highstand system.SQ2 is characterized by seawater diagenetic environment influenced by meteoric water during short periods.SQ3 is characterized by oxidizing environment during transgressive system and reduction environment during highstand system.Parasequence,developing in Tidal flat sedimentary environment,is characterized by mixing water cementation within regressive sequences,and marine micritization and cementation within progressive sequences.In subtidal sedimentary environment,parasequence developed syngenetic meteoric dissolution in early transgression and late highstand,and marine diagenesis in late transgression and early highstand.In the3 rd sequence,transgression is characterized by dissolution controlled by parasequences and apparently increasing cementation in the downward direction.Weak karst developed in tidal-flat during the late highstand.Seawater cementation is widely developed near marine flooding surface.The 3rd sequence boundary controlled the development of large scale of karst fractures and holes.High-angle faulting,which appeared as strike-slip faulting initiated along basement-rooted faults,likely acted as the important pathway that channeled the hydrothermal fluids from the depths.Initially,hydrothermal fluids with elevated pressure,high temperature,and low fluid/rock conditions were likely responsible for hydrofracturing and brecciation.When distal fluids of cooler temperatures flowed into permeable formations,the open fluid-dominated system with higher fluid/rock conditions facilitated the occurrence of dissolution vugs and precipitation of Saddle dolomite.Diagenetic facies distribution in early diagenetic stage is obviously controlled by sedimentary environment,while the diagenetic facies distribution in the burial and epigenetic stage is controlled by the sequence boundary and fractures.The constructive diagenesis responsible for the reservoir development include penecontemporaneous dissolution controlled by sea level change and sedimentary environment,karst related to long period sea level decline of 3rd sequence boundary,epigenetic karst controlled by 2nd sequence exposure,fault related hydrothermal alteration,and dolomitization. |
PDF Full Text Request