Structural Characteristics, Evolution And Hydrocarbon Occurrence Of The South-central Segment Of The Western Margin Of The Ordos Basin

The western margin of Ordos Basin is located at the intersection of several tectonic belts,and the geotectonic position is very special and the structural characteristics are complex,among which the middle southern segment of the western margin is the most typical.Mesozoic structural oil reservoir such as Majiatan and Dashuikeng have been put into production in this area.With the deepening of exploration in recent years,high-yielding wells have been drilled in multiple horizons such as Ordovician and Permian,which shows that it has great exploration potential and prospect.Structure characteristics in the region,however,is extremely complex,drilling shows that the reservoir is controlled by the faults.Because of insufficient seismic and well data,the structural characteristics and evolution,controlling factors of petroleum accumulation is still not very clear.The middle-south segments of western Ordos basin is the main research area of this paper.By collecting a large number of previous research results,interpreting newly acquired or reprocessed seismic exploration data and well data,investigating outcrops and using a variety of interpretation analysis methods,the characteristics of main faults and structures in this area have been systematically studied,and the Weizhou-Shigouyi stripping and gliding nappe structure has been identified.There are four main faults from east to west respectively:Qingtongxia-Guyuan fault,Qinglongshan-Pingliang fault,Weizhou-Anguo fault,Huianbu-Shajingzi fault.Thrust faults along the Carboniferous-Permian are developed in Majiatan and other areas east of Huianbu-Shajingzi fault,in addition,there are several groups of minor faults related to the tectonic evolution of the westren Ordos basin in the west of Tianhuan syncline.The study area can be divided into three sections: Shigouyi-Majiatan,Huianbu and Shajingzi,presenting a tectonic pattern of high in the south and low in the north.The sedimentary and structural characteristics of the Weizhou-Shigouyi syncline are obviously inconsistent with the surrounding strata.It is considered that the syncline is formed by southward slip,overlaying and superposing on the already formed overthrust nappe structure.The slippage surface are consist of Huianbu-Shajingzi fault,Carboniferous-Permian and Triassic Liujiagou formation,the stripping and gliding nappe structure occurred during late Jurassic to early Cretaceous.On the basis of the above results,combined with growth stratigraphic analysis,balanced section,low temperature thermal chronology data statistics,it is concluded that the evolution of the study area has experienced five main stages,and the present tectonic framework was mainly formed since the Yanshan Movement.From the Late Jurassic to the Early Cretaceous,there was a strong thrusting and compaction from west to east in the middle and southern segments of the western margin,and due to the difference of compaction intensity,a pattern of high in the south and low in the north was formed,which resulted in the thrusting and nappe structure in the western margin and the tripping and gliding nappe structure superimposed above it.Afterwards,the study area and adjacent area experienced Early Cretaceous extension,Late Cretaceous regional compression and uplift,Eocene extensional fault depression,and late Miocene differential uplift,which further complicated the fault structural pattern.Multiple sets of source rocks are found in the study area,and the oil and gas reservoirs are accumulated in multiple strata,including Mesozoic structural reservoirs,Upper Paleozoic structure-lithologic reservoirs,Carboniferous Yanghugou Formation tight gas reservoirs,Middle and Upper Ordovician Marine shale gas reservoirs and coal-bed gas reservoirs.The accumulation of oil and gas reservoirs are controlled by structural characteristics and tectonic evolution,such as the range distribution and thermal evolution degree of hydrocarbon source rock,the formation and evolution of the structural trap,improving the reservoir physical property and providing migration channel.Simultaneous,it also can cause the readjustment or destroy the reservoirs.