Research On Sedimentary Facies Of Penglaiba Formation Of Lower Ordovician In Tarim Basin

The paper is based on the sedimentary petrological theories and carbonatite sedimenttological theories.Core,thin section,well-logging,field profile and geophysical data are synthetically applied to systematically study the characteristics and types of strata,rocks and sedimentary facies and the distribution vertically and horizontally in Penglaiba Formation,Lower Ordovician,Tarim Basin.Meanwhile,combined with regional geological background,logging information of single well and auxiliary seismic data,the paper builds the plan of sedimentary facies in Penglaiba period,Lower Ordovician,Tarim Basin and analyzes the sedimentary facies pattern of Penglaiba Formation.The Ordovician of Tarim Basin is classified into four stratum subarea:Kepin stratum subarea,Takla Makan-Tangguzibasi stratum subarea,Que' erqueke-East Tarim stratum subarea and Luoxi stratum subarea.Penglaiba layers distributed in Kepin stratum subarea,Takla Makan-Tangguzibasi stratum subarea and Luoxi stratum subarea are divided into four lithological members from top to bottom:Peng 1 member,Peng 2 member Peng 3 member and Peng 4 member.Moreover,the Tuershaketa layers distributed in Que'erqueke-East Tarim stratum subarea are heteropical sediments of Penglaiba Formation,which lithology are mainly dark mudstone,micrites and dolomite.The differences of thickness among each areas are huge and it varies from 0 meter to 750 meters,which reflects the facts that layers experience repeatedly transform of tectonic movements in the process of sedimentation or after sedimentation.There are two different rock types in Penglaiba Formation:carbonatite and clasolite.Carbonatite are majorly distributed in Kepin-Takla Makan-Tangguzibasi area and Luoxi area and principal rock types are limestone,dolomite and the transitional type of them.Meanwhile,clasolite are mainly distributed in Que'erqueke-East Tarim area and principal rock types are mudstone and calcareous clay.Two sedimentary facies regions:platform and slope-basin are found in Penglaiba period,Lower Ordovician,Tarim Basin.On the one hand,types of sedimentary facies include semirestricted platform facies,marginal platform facies,slope facies and basin facies.One the other hand,types of subfacies is tidal flat,lagoon,intraplatform shoal,platform-margin shoal,marly hillock,interbank sea and intra-platform depression etc.While interactive sedimentation of marginal platform and interbank sea,intraplatform shoal and lagoon are developed vertically.Sedimentations are principally grain shoal and then interbank sea.The sedimentary facies of Penglaiba Formation varies from platform to platform margin to slope to basin to slope to platform margin and then gradually transit to Luoxi area from west to east.On the contrary,no evident facies changes are found from north to south.The sedimentary facies pattern of Penglaiba period shows that the west of Tarim basin is carbonate platform facies sedimentation and the east of Tarim basin is basin facies sedimentation,which is namely "platform in the west and basin in the east" sedimentary pattern.