Tectonic Evolution of the Tianhuan Depression and the Western Margin of the Late Triassic Ordos

Abstract:The Ordos Basin is one of the most important oil and gas basins in China. Based on surface outcrop, key exploratory wells and seismic reflection data and by using the technology of “prototype basin recovery”, seismic profile “layer flattening” and “restoration of balanced section”, and other methods, the sedimentary boundary, structure and the evolution history of the Tianhuan depression on the western margin of the Ordos Basin are reestablished. The following results have been obtained. (1) The west boundary of the Late Triassic Ordos Basin was far beyond the scope of the current basin. The basin is connected with the Late Triassic Hexi Corridor Basin, and its western margin did not have tectonic characteristics of a foreland basin. (2) The Tianhuan depression was first formed in the Late Jurassic. At the late stage it was impacted by the late Yanshanian and Himalayan tectonic movement and the depression axis gradually moved eastwards to the present location with a cumulative migration distance of ∼30 km. (3) Eastward migration of the depression axis caused adjustment and even destruction of the originally formed oil and gas reservoirs, so that oil and gas remigrated and aggregated, resulting in secondary structural reservoirs formed at high positions on the western flank of the depression.