Structural Characteristics And Controlling Gas Of Ordovician Middle Lower Assemblages In Eastern And Middle Ordos Basin

In the middle east of Ordos basin,the lower Paleozoic Ordovician is one of the important strata in natural gas exploration.Previous studies have focused on the control factor analysis of hydrocarbon source rock conditions and reservoir conditions on natural gas accumulation,while the study on the control role of natural gas in Ordovician system from tectonic characteristics is relatively rare.Therefore,based on the previous studies,this paper studies the structural characteristics of mid-lower assemblages of Ordovician in the eastern part of the basin and the evolution characteristics of each time period in combination with well logging,drilling,logging,seismic and analytical text.And combined the features of natural gas distribution,we discusses the control of natural gas distribution in the present structural features and the history of structural evolution.The following conclusions are finally drawn: First,the top surface of the middle and lower assemblages in Ordovician was successively experienced five large construction stages,which finally evolved into the current structural features of low southwest and high northeast.These tectonic movements directly or indirectly affect the development of the middle and lower composite reservoirs and migration channels in Ordovician.The most important one is the Caledonian movement in the late Ordovician period and the YanShan tectonic movement in the late cretaceous.The former caused the basin to be subjected to the weathering and denuation of 150 million years,which led to the direct contact relation between the upper Paleozoic coal source rock strata and the middle and lower composite strata in the Ordovician Majiagou formation,which formed a good relationship of source storage and configuration;The latter which is the structural inversion not only makes the dense limestone of regional facies change in the direction of structure,but also forms an effective lithologic trap for the dolomite reservoir on the western side.Meanwhile,it also changes the original source storage contact relation,which is converted from the original source to the lateral contact relation,and even in the local area,the contact relation has become the reservoir under the source,which is beneficial to the migration of the coal type gas in the upper Paleozoic to the reservoir of dolomite in the middle and lower assemblages of the Majiagou formation of Ordovician;Second,the middle and lower assemblages of Ordovician Majiagou Formation in the study area is mainly composed of lithologic trap gas reservoirs,stratigraphic-lithologic trap gas reservoirs,and fault-lithologic trap gas reservoirs.Due to the influence of lithology and physical conditions,there are certain differences in gas reservoir types in the western and middle eastern regions of the study area.In the western part of the study area which nears the paleo-up-lift,lithologic trap gas reservoirs and stratigraphiclithologic trap gas reservoirs are mainly developed,while in the central and eastern regions of the study area,fault-lithologic trap gas reservoirs are developed around the faults.Third,based on previous research,this paper mainly discusses the control effects of structural features on the distribution of natural gas through three aspects: local structural features,structural evolution features,and faults.Finally,it is believed that the structure plays an important role in controlling the transport and accumulation of natural gas...