Palaeoenvironmental Changes,tectonic Sedimentary Evolution Process And Model Of Ordovician In The Awati Sag,Tarim Basin

The Ordovician strata in Awati Sag of the Tarim Basin are well developed,which records the structural evolution and paleoenvironmental changes of the Ordovician.However,the focus and hotspot of oil and gas exploration in the Tarim Basin are still concentrated in several uplift areas in recent years.The study area of Awati Sag not only has little investment in oil exploration,but also has a relatively low level of research and understanding.On the basis of previous research results and outcrops investigation,this paper makes a systematic study of petrography,sedimentology,element geochemistry,inorganic and organic carbon isotope geochemistry of Penglaiba,Yingshan,Dawangou,Saergan,Kanling,Qilang and Yingan Formations of the Ordovician in Awati Sag of the Tarim Basin.And this study fills the research gap in paleoenvironment evolution,sea level change and tectonic-sedimentary evolution process of Ordovician in the study area.The research results and understandings are as follows.(1)Based on outcrop observation,thin section identification,petrological characteristics and sedimentary structure.We analyzed the sedimentary structure,sedimentary sequence and sedimentary facies types and characteristics,and established the vertical evolution model of Ordovician sedimentary facies in Awati Sag.In the Early Ordovician,the Tarim Basin was characterized by the development of large-scale carbonate rocks,which was the most prosperous period of platform growth and development.The sedimentary facies evolution process of the Lower Ordovician Penglaiba and Yingshan Formations is semi-restricted platform to open platform to platform margin.In the Middle Ordovician,the sedimentary system changed rapidly from the carbonate sedimentary system of Dawangou Formation to the black shale sedimentary system of Sargan Formation.The sedimentary facies evolution of Dawangou Formation to Saergan Formation in Middle Ordovician is gentle slope at front of platform to basin.The sedimentary period of the Upper Ordovician Kanling Formation changed back to the carbonate sedimentary system,and changed to black shale again in the sedimentary period of the Yingan Formation in the Late Ordovician.The sedimentary facies evolution process of the Upper Ordovician Kanling Formation to Qilang Formation to Yingan Formation is deep sea shelf to shallow sea shelf to deep sea shelf to basin margin.(2)Based on the element geochemical characteristics and in combination with the research results of sedimentary facies,the volcanic-hydrothermal activity,paleoproductivity,paleoredox,paleoclimatic conditions and their evolution characteristics of the Ordovician in the Awati Sag have been restored.The Ordovician volcano-hydrothermal activity in the study area can be divided into four stages: the weak volcanism stage of the Tremadocian to middle Darriwillian(Penglaiba Formation to Dawangou Formation),the strong volcanic eruption stage of the middle Darriwillian to upper Sandbian(Saergan Formation),the volcanic intermittent eruption stage of the upper Sandbian to middle-lower Katian(Kanling Formation to Qilang Formation)and the slightly enhanced volcanic activity stage of the middle Katian(Yingan Formation);The paleoproductivity conditions can be divided into three stages: the stage of steady increase of paleoproductivity from the upper Tremadocian to Darriwillian(Penglaiba Formation to the upper part of Dawangou Formation),the stage of sharp increase of paleoproductivity from the lower Darriwillian to lower Sandbian(the upper part of Dawangou Formation to the lower part of the Kanling Formation),and the stage of sharp decrease and stabilization from the lower Sandbian to middle Katian(the lower part of the Kanling Formation to Yingan Formation).The paleoredox conditions can be divided into three stages: the oxidation stage in the middle Tremadocian to Darriwillian(Penglaiba Formation to Dawangou Formation),the anoxic stage in the middle Darriwillian to lower Sandbian(Sargan Formation to the lower part of Kanling Formation),and the intermittent anoxic stage in the lower part of Sandbian to the middle Katian(the lower part of Kanling Formation to Yingan Formation).The paleoclimate conditions can be divided into four stages: the dry climate stage in the middle Tremadocian to Darriwillian(the upper part of Penglaiba Formation to Dawangou Formation),the humid climate stage in the upper Darriwillian to the lower Sandbian(Sargan Formation),the climate fluctuation stage in the lower Sandbian to middle-lower Katian(the lower part of Kanling Formation to Yingan Formation)and the humid climate stage in the middle Katian(Yingan Formation).(3)Based on the sedimentary filling sequence of the Ordovician system in the study area,combined with carbon and oxygen isotope geochemistry,the sea level rise and fall process of the Ordovician system in the Awati Sag is reconstructed and compared with the global sea level change.The sea level in Tremadocian to Dapingian(Penglaiba Formation to the lower part of Dawangou Formation)was in a slow rising process,and reached the highest at the upper Floian.During the Darriwillian(the middle-upper part of Sargan Formation to lower part of Dawangou Formation),the sea level changed greatly.In this early stage,the sea level rose rapidly,and reached the maximum flooding surface of this stage quickly.Then,the sea level fell.The sea level in the Sandbian(the upper part of Sargan Formation to the middle-upper part of Qilang Formation)was in a rapid rising process,and the rising time,amplitude and maximum flooding surface were greater than those in Darriwillian.Then,the sea level is in a regression process during Katian(the middleupper part of Qilang Formation to Yingan Formation).The sea level change of Ordovician in the study area is highly similar to that of the global and Bachu Uplift,indicating that tectonic activity is the main reason for the difference of sedimentary sequence between Awati Sag and peripheral uplift(Bachu Uplift,etc.)in the Tarim Basin.(4)Based on the sedimentary filling sequence,paleoenvironmental evolution,sealevel change and the tectonic background of the Tarim Basin,the division of the Ordovician tectonic-sedimentary evolution stages in Awati sag is carried out.Then,the development characteristics and evolution laws of each stage are clarified,and established the tectonic sedimentary evolution process and model.Finally,the petroleum geological significance of tectonic sedimentary evolution is discussed.In the Early Ordovician,the Tarim Basin was in an extensional tectonic background.The Awati Sag and its adjacent areas(Bachu Uplift,Tazhong Uplift,etc.)were unified marine sedimentary basins.At this time,it was in the stage of structural stability(Tremadocian to middle Darriwillian).It was a “tectonic stable-platform system”.In the Middle Ordovician,differential subsidence occurred in the western of the Tarim Basin under the compressive stress from south to north.The strong arc-land collision led the surrounding areas of Awati Sag(Tazhong Uplift,Bachu Uplift and Tangnan area)uplift and exposed to the water and suffered denudation.As a subsidence center,the Awati Sag continued to receive sedimentation,which was the sag stage(middle Darriwillian to early Sandbian).It was a “subduction-basin system”.In the Late Ordovician,the Tarim plate entered the stage of tectonic evolution of Episode II in the Middle Caledonian period.Under the influence of the compressional stress relaxation effect in the north-south direction and the extension of the southern Tianshan ocean,the western part of the Tarim Basin experienced an overall tectonic subsidence,which is the stage of sag redevelopment(lower Sandbian to middle Katian).It was a “tectonic subsidence-shelf-basin margin system”.Finally,it entered the stage of tectonic uplift and denudation(late Katian to Hirnantian).In the Late Ordovician,the Tarim Basin entered the stage of tectonic evolution of Episode II in the Middle Caledonian period.Under the influence of strong compression and collision between the Tarim Craton and the island-arc on the southern margin and subduction of the southern Tianshan ocean in the north,the vast area of the Western Tarim Craton Depression and the Tabei Uplift were denuded.In the Awati Sag,the stratum of the late Katian to Hirnantian was denuded and lost during this period,resulting in the appearance of weathered crust at the top of the Yingan Formation.Finally,with the arrival of the Hirnantian ice age,the global sea level declined,the shallowing of the sea water in the Tarim plate and the emergence of provenance areas in the north and west led to the evolution of the Tarim Basin from the Ordovician marine carbonate platform sedimentary system to the Early Silurian(Late Ordovician)marine clastic rock shore shelf sedimentary system,marking that the evolution of the Tarim Basin has entered a new stage.The Ordovician paleoenvironment and tectonic-sedimentary evolution in the study area have a great influence on the development of source rocks and reservoirs.