Provenance Of The Upper Paleozoic Shan1-He 8 Formation In Southern Ordos Basin And Its Influence On The Sedimentary Filling Pattern

The Upper Paleozoic of the Ordos Basin is rich in natural gas resources.After more than40 years'exploration and development,great progress has been made in Shan 1 member of Shanxi formation and He 8 member of Shihezi formation,while there has been no major breakthrough in the southern basin.One of the main constraints is the lack of recognition on provenance and sedimentary filling pattern of the southern basin.Based on the detailed investigation of the field profile around the basin and the observation of the basin core,this study combines basin drilling data with logging interpretation materials,through the methods of paleocurrent direction measurement,the analysis of heavy mineral assemblage characteristics,the observation of sandstone components under the microscope,the observation of rock debris characteristics and the law of plane distribution.specified the direction and location of the provenance of Shan 1 and He 8 formation and how the provenance control and effects the deposition phase sequence assemblage,the deposition phase and the temporal spatial distribution of the sand body development.It is found that higher similarities appears between Shan 1 and He 8 formation in paleocurrent direction,heavy minerals,mineral debris and debris composition characteristics and its plane distribution law.Besides,it is also found that there are few changes on provenance and deposition distribution in Shan 1 and He 8 formation featuring great inheritance.However,discrepancies are also found in the study including the filling materials provided by the basin's surrounding orogenic belt is gradually increasing;the sediment characteristics in the southwest and southeast of the basin are quite different.The provenance study found that the study area was mainly controlled by two provenance directions,southwest and southeast,while the influence range of provenance in the north was limited,the southwest provenance is mainly from the Qin-Qi orogenic belt in the southwest,and the southeast provenance is mainly from the north Qinling orogenic belt in the southeast.the sequence stratigraphic analysis of outcrop profile on the periphery of the basin and the sequence combination characteristics of in-basin well connection profile,the varation process from High-stand system tract system-lowstand system tract-transgressive systems tract is presented in segment of Shan 1 formation-lower segment of He 8 formation-upper segment of He 8 formation.High-stand system tract system developed at the top of upper segment of He 8 formation in some areas The early stage is dominated by accretion and progradation,the middle stage is dominated by progradation sequence while the late stage is gradually transformed into the regressive sequence.The above phenomenon indicates that during the filling process of the sedimentary strata in the southern part of the basin,the sedimentary filling effect of the Shan 1 stage is gradually enhanced from the early to the late stage.The filling effect of the lower He 8 stage is the strongest,and the provenance supply is sufficient.What's more,the grain size of the upper He 8 is obviously thinner than the lower part,Therefore,filling effect is gradually weakened,and the whole stage shows the evolution process of the lake water from deep to shallow and deepening.According to the outcrop profile around the basin,the facies signature characteristics of the core in the basin and the logging facies analysis,and combined with the regional sedimentary background,it is considered that the Shan 1 stage of the southern basin is a meandering river delta environment and the He 8 stage is a braided river delta environment.Controlled by the direction of source supply and the distribution of sedimentary facies belts,the horizontal distribution of sand bodies in Shan 1 formation and He 8 formation are southwest-northeast,southeast-northwest,and the sand bodies in southeast are more developed than those in southwest.Among them,the development of sand bodies in Shan1³ of Shanxi Formation is relatively limited,the range of Shan 1^1-2 sand bodies is gradually expanding;the sand bodies in He 8 Formation are most widely distributed,and the thickness of sand bodies is obviously larger than that of Shan 1 sand bodies,which also confirms that the materials supply of He 8 is more abundant.This study will provide important scientific basis and data support for the exploration and development of natural gas in southern Ordos Basin.