High resolution sequence stratigraphy and sedimentary characteristics of the Middle-Lower Cambrian in Bachu Uplift,the Tarim Basin

The Cambrian is of large burial depth and few exploration wells in Tarim Basin,The specific lithologic assemblage consisting of intebedded evaporites,carbonate rocks and bronzing mudstones in the moddle-lower Camberan is analyzed using sequence stratigraphy and sedimentology theories.We suggest that clay contents of GR within the depositional system can not reflect the relative sea level change,and that fine sedimentary facies division is key to base-level cycle identification.The Cambrian is divided into 3 supersequences and 7 third-order sequences,among which 4 third-order sequences occur in the Middle-Lower Cambrian and 3 in the Upper Cambrian.In addition,the features of each third-order sequence are analyzed and an isochronous stratigraphic framework is built for the Cambrian.The sedimentary environments of the Middle-Lower Cambrian in Bachu Uplift are evaporate platform and restricted platform,where 2 sedimentary facies,5 subfacies and 19 microfacies are identified.The Lower Cambrian is mainly of restricted platform deposits dominated by lime flat,limestone-dolomite flat,gypsum-dolomite flat and dolomite flat deposition.In Bachu Uplift,the gypsum-dolomite flat deposits dominate in the west and dolomite flat deposits in the east.In contrast,the major sedimentary facies of the Middle Cambrian is evaporate platform dominated by intertidal flat and saline lacustrine deposits.Taking the Ba3-Ba1 wells as the line of demarcation,the Middle Cambrian is mainly of supertidal flat deposits to its west,and is dominated by intertidal flat and salt lake deposits to its east.