Distribution and significance of the Ordovician Qrebake and Yijianfang formations in the Central Uplift of the Tarim Basin

The Qrebake Formation of the Upper Ordovician and the Yijianfang Formation of the Middle Ordovician have been intersected in ten wells in the Central Uplift of the Tarim Basin.They belong to two stratigraphic sequences: the Lianglitage/Qrebake/Yijianfang/Yingshan formations, and the Queerqueke/Qrebake/Yijianfang/Yingshan formations.The Qrebake Formation is composed of purplish red, brown and grey micritic limestones, marls and calcareous mudstones representative of slope facies.This formation is about 17-25 m thick, and has a Pygodus anserinus zone at the bottom.The Yijianfang Formation is composed of grey micritic sand-clastic limestones, grained limestones and bioclastic limestones representive of open platform and platform margin facies.This formation is about 80-120 m thick, and has a Pygodus serrus fossil zone at the top.The Qrebake and Yijianfang formations are mainly distributed in the lower parts of the uplift edge and the transition area from the uplift to the depression, while a sedimentary gap for the two formations is observed in the Central Uplift.The Ordovician of the Central Uplift is divided into the uplift zone, the transition zone (A, B), and the depression zone.The western boundary of the transition zone should be at least 100 km to the west compared to previous studies.During the Middle and Late Ordovician, the study area was an open platform to platform margin and slope facies, and developed no hydrocarbon source rocks of the "Salgan" type.Two sets of favorable reservoir and cap assemblages have been discovered in the transition zone, showing a good potential for hydrocarbon exploration.