Ordos Western Margin Of The Ordovician Sedimentary Evolution And Under The Control Of Oil And Gas Geological Conditions

The western margin of Ordos basin is situated in the northern part of South-North trending structural zone which lies between different tectonic realm of east and west china.It is surrounded by different tectonic units of the north china craton,alxa block,the qinling orogen and qilian orogen. The western margin of Ordos basin has experienced complicated tectonic evolution and the ordovician is the most important stages in its evolutive history.The ordovician of the western margin of ordos basin has two different sedimentary systems,one belongs to the early -mid age of early odovician which is characteristic of carbonate deposits,and the paleogeography of this time is uniform;the other belongs to the late age of early ordovician -middle and late ordovician,which features by sedimentary series of sandstone ,mudstone and carbonate .The two series differ from each other in sedimentary feature and biofacies.The interface of the two corresponds to the one between Guniutan Formation and Dawan Formation in southwestern area of china.Through research ,conclusion is arrived that the interface represents a significant dividing surface of two different prototype basin.In early ordovician ,the western margin of ordos basin is situated in extending setting,the Helan area is in the late evolutive stage of aulacogen and Xiangshan area is the passive margin inclining to the Qilian ocean.The deposits are mainly carbonate that deposit in shallow water ,source rock do not develop and the reservoir are mainly grain band facies dolomites.In the late age of early ordovician to mid-late ordovician ,the qilian ocean crust begin to subduct northwardly and a typical trench-arc- basin system come into being.The Helan area become an extensional faulted depression influenced by the regional compressive stress and the Xiangshan area is the back arc basin of the north Qilian island arc.During this period , dark mudstone and shale of slope and basin facies are good source rocks because of their high content of organism and good organic type;In addition, reginal tectonic movement provides basic facilitating mechanism and gradient condition for the deep water gravity slow,which consists of turbidite clastic sandstones and carbonate debris flow deposits ,the two kinds of deposites can be favorable resevior for oil and gas.