Restoration and tectonic reworking of the Late Triassic basin in western Ordos Basin

Restoring prototype basin is an important issue for evaluating oil and gas prospect of Upper Triassic in west Ordos Basin. The authors point out that the Qin-Qi Fold Belt and Alashan ancient land controlled the borderline of the western Ordos prototype basin in Later Triassic.The large borderline faults of west Liupanshan and west Helanshan were the southwest and northwest borderlines of the Upper Triassic prototype basin.There is no clear boundary line between the Hexi Corridor and the cadre of the basin.The prototype basin is a large NW-SE asymmetric depressed basin,which is deep in south and shallow in north.The Yinchuan Basin and Liupanshan Basin are part of the prototype basin in the Yanchang stage of Upper Triassic.There are four delta sedimentary systems,which are Ruishuihe-Zhenyuan,Jingtai-Huanxian,Yaoshan-Tianshuipu and Rujigou-Yanding delta sedimentary systems,with different sedimentary characteristics from south to north in western Upper Triassic basin.These delta sedimentary systems were altered in Late Indosinian,Mid-Yanshanian,Late Yanshanian and Himalayan,denudated badly in the hanging wall of the west thrust belt,and conserved integrally in the east of the present thrust belt.The extensive residual Yanchang Formation sedimentary system has good prospecting.