Stratum And Sedimentary Characteristics Of Middle Ordovician Yijianfang Formation In Middle Tarim Basin

Whether middle Ordovician Yijianfang Formation in north slope of middle Tarim basin is exist,together with its unknown horizontal distribution and sedimentary characteristics,are important and basic geological questions about oil-gas exploration.This research adopt methods of Palaeontology Stratigraphy?Sedimentology?Logging Geology and Seismology to systematically analyze the conodont in north slope of middle Tarim basin,and confirm the distribution of Yijianfang Formation.Based on that,research systematically analyzed the core sedimentary facies?common logging and image logging characteristics,combined with seismic tracing,basically confirmed the distribution and sedimentary characteristics of Yijianfang Formation in north slope of middle Tarim basin and laid the foundation for oil-gas exploration in this area.Research choosed typical drilling core especially for debris which possibly existed Yijianfang Formation to get samples to identify conodont,and got homologous Yijianfang Formation conodont one fossil sequence compared with outcrop of west Tarim basin for the first time.Conodont zone include Amorphognathus variabilis?Eoplacognathus suecicus and Pygodus serrus,proved that middle Ordovician Yijianfang Formation is distributed in wider area in north slope of middle Tarim basin.Combined with Yijianfang logging characteristics,research re-divided and correlated Yijianfang Formation in north slope of middle Tarim basin.By observing all the Yijianfang Formation drilling cores and typical casting lamellas,research indicated that Yijianfang Formation in north slope of middle Tarim basin mainly shown as open carbonate platform grain beach facies arenite limestones on the edge of the area and mainly shown as combined sedimentary subfacies of grain beach facies and algal mat facies;in specific region,Yijianfang Formation mainly shown as low energy storm-zone micrite and micritic limestone.By studying image logging data and seismic section tracing in north slope of middle Tarim basin,research basically divided the distribution range of Yijianfang Formation.Research indicated that Yijianfang Formation mainly distributed along the area of Zhonggu-3 to Tazhong-85 to Zhonggu-20 to Tazhong-88 to Tazhong-86 to Zhonggu-17 to Zhonggu-19 in the north slope of middle Tarim basin.In the platform,Yijianfang Formation mainly preserved in the north area of the line of Zhonggu-29 to Zhonggu-27 to Zhonggu-24 and in the west area of the line of Tazhong-63 to Zhonggu-15 to Zhonggu-14.In general,Yijianfang Formation distributed generally in the III area of middle Tarim basin and along the edge of the platform in the II area while generally missing in the I area of middle Tarim basin.