Characteristics Of Ordovician Subsalt Faults In The Eastern Ordos Basin

In recent years,with the continuous breakthrough in the exploration of supra-salt natural gas in the Lower Paleozoic Ordovician in Ordos Basin,it has been shown that the Ordovician has high quality natural gas resources,which brings hope for the exploration and research of the sub-salt natural gas,and the deep and ultra-deep natural gas has been paid more and more attention.In the eastern part of the basin,there is a set of carbonate platform deposits in the Lower Paleozoic Ordovician,among which there are several sets of gypsiferous rocks and carbonate rocks.The migration and enrichment of natural gas are closely related to faults,so it is of great significance to study the Ordovician sub-salt faults in the eastern Ordos Basin.However,due to the existence of gypsiferous rock,the structure in this area is relatively complex,so it is very important to study the influence of gypsiferous rock distribution,lithologic assemblage and related deformation characteristics on faults.Based on the logging data,logging data and regional 2-D seismic data of 26 deep Wells drilling through the Ordovician system in the study area,this paper takes the sub-salt faults as the research object and applies the theories of tectonic geology and tectonic chronology,combined with aeromagnetic data to carry out research.Based on the analysis of the logging curves of 25 Wells drilled into the Ordovician gypsiferous rock in the study area,the logging facies characteristics of gypsum rock and salt rock are determined.It is considered that the salt rock has the characteristics of high acoustic time difference,high resistivity,low density and high natural gamma ray.The gypsiferous rock has higher acoustic time difference than salt rock,but lower resistivity than salt rock.Compared with carbonate rocks,gypsum rocks have high natural gamma ray and high resistivity,but they have lower acoustic time difference and higher density than carbonate rocks.Using drill in the Ordovician in the study area of 26 Wells,different thickness and percentage statistics and combined with peripheral basin outcrop made paste thickness of salt rocks figure,dividing the distribution range and think gypsum salt mainly distributed in the eastern depression depression subject area,cream salt rocks are circling the distribution on the edge of the salt and depressions,rock area and cream quality Yun Yan slope zone are mainly distributed in the restricted platform.Based on the interpretation of 2D seismic data in the study area,the gypsum rock deformation can be divided into two major types and six small types: stable sedimentary type,flow type,eye ball type/spindle type,cap rock slip type,basement involved type and mixed type.Based on 2D seismic data,the tectonic style of the Ordovician sub-salt fault in the east of Ordos Basin has been determined.It has obvious zoning characteristics.The paste rock area is characterized by plate fault,and the paste rock area and the salt rock area are mainly developed by salt-related fault structural style,including shear fault-bend fold,fault-propagation fold and decollement fold.There are few "herringbone" structural patterns and plate-like fault patterns.Through the eastern basin,the Ordovician fault interpretation and plane combination set out of the Ordovician boundary fault in late 116,the four boundary fault in late 110,fracture level is small,mainly in the north east,north north east structural trend,there are a few faults in north west,intensive distribution and fault in the plane constitute two fault zone,including salt zone fault extension distance far away,The extension distance of fracture in gypsum rock area is short.The formation mechanism of the Ordovician sub-salt faults is mainly controlled by three aspects.The basement/pre-existing structures of different scales have inherited influence on the Ordovician faults.Different gypsum rock lithologic assemblages control the structural style and development of faults.The late reformation and the superposition of multi-stage tectonic events have regional influence on the distribution and formation of the Ordovician sub-salt faults.