Sedimentary Characteristics And Models Of Microbial Carbonate Rocks In The Upper Ordovician Lianglitage Formation In Tazhong Area

Since the discovery of the Lower Cretaceous microbial carbonate pre-salt reservoirs in the Santos Basin,Brazil in 2014,extensive attention of microbial carbonates from sedimentologists and hydrocarbon exploration geologists had been aroused worldwidely.Research focused mainly on microbial carbonate sedimentology,microbial carbonate hydrocarbon accumulation,and microbial dolomites in recent years,the main research strata were focused on the Precambrian,and Paleozoic which were abundant with microbial carbonates that resuscitated after several mass extinctions.Microbial carbonates were relatively developed in Upper Ordovician Lianglitage Formation in Tazhong Area,Tarim Basin,and the Lianglitage Formation was one of the major reservoirs in the Tazhong oil and gas field.But over time,microbial carbonates were lacking in sufficient knowledge and few specialists were involved in research of them in this area.Therefore,an insightful and intense research of microbial carbonates is necessary in Tazhong Area.Based on the theories of carbonate sedimentology,paleontology,geophysics and geochemistry,combined with core observation and description,microscopic identification of thin sections,seismic and geochemical data,the paper gave analyses of the types,longitudinal development characteristics,lateral distribution characteristics and influence factors of microbial carbonates in NO.1 slope break,Upper Ordovician Lianglitage Formation,and the sedimentary environment and evolution mode were further established in the research area.The main knowledge of the paper is as follows:Microbial carbonates included stromatolite,thrombolite and oncolite predominantly of Lianglitage Formation in Tazhong Area.The stromatolite was divided into dome-shape stromatolite and columnar stromatolite;the thrombolite included reticular thrombolite,columnar thrombolite,pseudo-lamina thrombolite and clotted thrombolite;and the oncolite can be split into thin-lamina type(fully wrapped),normal type(fully wrapped),incompletely wrapped type and transition between thin-lamina type and normal type-compound type(fully wrapped),then adding appearance prefix in the light of outer shape of oncolite.Longitudinally,the thrombolites in the study area were principally developed in the 5th and 4th members of Lianglitage Formation in eastern platform margin,the 4th to 2nd members ifl middle platform margin and 3rd to 1st members in western platforl margin,which associated with grainstones and micrites to mostly form thrombolite-grainstone and micrite-thrombolite shallowing-up sequences;the oncolites were mainly developed in grainstones and biohermal limestones deposited in high-energy environment in 3rd and 2nd members;stromatolites in the area were fairly rare,and occasionally observed only in 4th member of well TZ 12 and TZ 83.Laterally,thrombolites were predominantly distributed in middle and western platform margin,and eastern platform margin were less developed because occupied by the metazoan reefs;oncolites were mainly distributed in the comparatively strong hydrodynamic force zones in the middle and eastern platform margin.A sedimentary differentiation in study area is that reef mounds were primarily developed in eastern platform margin and mud mounds were chiefly distributed in western platform margin.Factors causing the phenomenon are predominantly the differences of platform margin geological structures,sea level eustacy,the actions of equatorial trade winds and ocean currents,and the Great Ordovician Biodiversification Event.The Great Ordovician Biodiversification Event played the most important role in the 4 factors,which is essentially the competition of metazoan reef-builders and microbes for ecospace and the predation of predators.Microbial mud mounds in the western platform margin co-existed with metazoan reef mounds in the eastern platform margin as the remains of Early and Middle Ordovician microbial reefs,which were extremely rare "outliers" amongst the metazoan-dominant reefs in Late Ordovician,formed such unique sedimentary pattern.