Continental sequence stratigraphy and sedimentary evolution of the Triassic strata in the North-Central Tarim Basin, Xinjiang

Apart from a short period of transgression during the Early Triassic, the Tarim Basin was once a typical nonmarine basin during the rest of the Triassic. In this basin, the fan-fan delta-lacustrine sedimentary systems are well developed. The basin is separated by the Xayar uplift in the north and central Tarim erosional area in the central and western parts into four subbasins:Kuqa Basin at the piedmont zone of the South Tianshan, Southwest Tarim Basin at the piedmont zone of the Kunlun Mountains, Southeast Tarim Basin at the piedmont zone of the Altun Mountains and North-Central Tarim Basin in the central part of Tarim Basin. The emphasis will be placed on the last one in this study because of the widespread occurrence of the Triassic strata as the principal target levels in the North-Central Tarim Basin. These Triassic strata consist of three base-level cycles including long-, medium- and short-period cycles. The long-period cycles Ⅰ and Ⅱ are unconformity-bounded cycles. One medium-period cycle (Ⅰ₁) has been identified in the long-period cycle Ⅰ, while the other three medium-period cycles (Ⅱ₁, Ⅱ₂ and Ⅱ₃) are involved in the long-period cycle Ⅱ. The best reservoir facies zones of the Triassic-Jurassic strata in the North-Central Tarim Basin are closely related to the lowstand systems tract of the medium-period cycles. The Ⅲ oil formation (T₂a), Ⅱ oil formation (T₂a) and Ⅰ oil formation (T₃h) correspond to the medium-period cycles Ⅱ₁, Ⅱ₂ and Ⅱ₃, respectively.