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The Ordos Basin is a craton basin developed based on the Precambrian rift system during Permian. It has experienced two stages of evolution including the Early Paleozoic craton marginal depression and Late Paleozoic intra-craton depression. The former can be further divided into four periods, including the Zhangxia period of Early to Middle Cambrian, the Sanshanzi period of Late Cambrian to the Liangjiashan period of Early Ordovician, the Majiagou period of Middle Ordovician, and the Pingliang-Beiguoshan period of Late Ordovician. The latter can be further divided into two periods, including Late Carboniferous to Early Permian and Middle to Late Permian. Controlled by the transformation of surrounding tectonic system of North-China blocks, the Paleozoic Ordos Basin evolved from passive continental margin extension in the Early Cambrian and Middle Ordovician to active continental convergence in Late Ordovician and Permian. The internal tectonic sedimentary pattern gradually evolved from a "L-shaped" extensional rift in the southwest of the basin during Cambrian, to "depression-slope-uplift-plateau" druing the Middle-Late Cambrian, to "depression-slope-uplift-depression" during Ordovician, and to "one uplift and two depressions" during[JP] Carboniferous to Permian. During the evolution of Paleozoic prototype basin and the change of tectonic-sedimentary pattern, several sets of source rocks and reservoirs were derived. In the continuously subsiding region on the western and southern margin of the basin, coal-measure source rocks deposited in Cambrian Suyukou-Xinji formations, Xuzhuang Formation, [JP]Ordovician Pingliang Formation, Carboniferous and Permian. In the subsiding region inside the basin, coal-measure source rocks deposited in Ordovician Xuzhuang Formation, Majigou Formation, Carboniferous and Permian. Effective reservoirs, such as high-energy oolitic beach of Zhangxia Formation, bioreef-granule beach of Krimori Formation and granulate beach of Majiagou Formation, were developed in the western and eastern paleoslope areas of the central paleouplift of the Paleozoic Ordos Basin. |
