Research On Diagenesis In Sequence Stratigraphic Framework Of Ordovician In Tarim Basin

By making full use of outcrop,drilling,logging,seismic,laboratory analysis and other data,sequence stratigraphy,depositional systems and diagenesis of Ordovician strata in Tarim basin are studied deeply and systematically.On the basis of these works,the diagenesis characteristics in sequence stratigraphic framework and relationship between the diagenesis development and sequence stratigraphy of Ordovician carbonate rocks in Tarim basin are researched,eventually,diagenenesis model in sequence stratigraphic framework of Ordovician carbonate rocks in Tarim basin are established.By identifying sequence boundary,the Ordovician is divided into 5 second-order sequences,including 13 third-order sequences.Each third-order sequence is mainly composed of transgressive systems tract(TST)and highstand system tract(HST).The main types of sedimentary facies developed in Ordovician strata are:tidal flat platform facies,limited platform facies,open platform facies,platform edge facies,slope facies,shelf facies and basin facies.Carbonate platform facies are distributed mainly in the mid-western of Tarim basin.Among these facies,the tidal flat platform facies and restricted platform facies with a wide range of planar distribution mainly developed in the lower Ordovician.Compared with restricted platform facies,the distribution of open platform facies is confined relatively in Ordovician strata,which is distributed as "horseshoe-shaped","beak-like" and "V-shaped" in the upper Ordovician in Tabei,Tazhong and TangNan region respectively.Through the observation of core and thin sections under microscope,the main types of diagenesis in Ordovician carbonate strata are identified as:dissolution,filling,cementation,newborn deformation,metasomatism,compaction,pressure solution,fracture,etc.Diagenesis developed in different diagenetic environments exhibit different characteristics.Based on these studies,the diagenesis in sequence stratigraphic framework of Ordovician carbonate rocks in Tarim basin are analysed via Lianglitager Formation in upper Ordovician and Yingshan Formation in lower-middle Ordovician.According to the research,diagenesis characteristics are distinct in different system tracts,facies and levels of sequence boundary.The frequency of meteoric dissolution developed in platform edge facies is high,in open platform facies is occasional,in slope facies and shelf facies is almost none.It is more likely to distribute near the third-order sequence boundary or the top of high frequency sequence.Burial dissolution is more likely to develop in the TST of lower-middle Ordovician near sequence boundary.Dolomitization is generally developed in the limited platform facies.Submarine compaction is well developed in Lagoon subfacies on limited platform,intraplateform subfacies on open platform,slope facies and shelf facies.Submarine cementation is generally developed in intraplateform subfacies on limited platform,intraplateform subfacies on open platform and platform edge facies.