Early tectonic evolution and sedimentary response of Ordos basin： A case study of Interval 8～Interval 6 oil layers of Yanchang Formation in Jiyuan area

Abstract：Based on an analysis of rock component characteristics, sedimentary facies characteristics, development regularity of seismites and tuff in Interval 8～Interval 6 oil layers of Yanchang Formation, the authors put forward a method for investigating the early structural evolution and the process of sedimentary response. Researches indicate that Qinling was uplifted at the early depositional stage of Interval 8 whereas Liupanshan was uplifted at the late depositional stage of Interval 7. The depositional period from Interval 8 to Interval 6 of Yanchang Formation is regarded as the early formation period of Ordos Basin in the Mesozoic. The tectofacies of Interval 8 is relatively stable, and there exist no seismites and carbonate debris in the sandstone, only with the development of shallow water delta in the study area. The compression of the western margin of Ordos Basin at the Interval 7 depositional stage led to intense depression of the lake basin, and carbonate debris started to emerge in sandstone while many seismites were associated with widely-existent tuff. The study area is characterized by semi-deep lake that developed the subwater fan at the depositional stage. The volcanic action of Interval 6 depositional stage became weak, but the tectonic action of faulted zone in the west of Ordos Basin was intense. Seismites can commonly seen in Interval 6, with the development of the lake delta in the study area. Paleogeographic evolution from Interval 8 to Interval 6 of Yanchang Formation in Jiyuan area indicates that the evolution of Ordos Basin was mainly controlled by tectonic actions of the south margin before Interval 7 depositional stage, and was subsequently controlled by tectonic actions of the west margin and affected by tectonic actions of the south margin too.