Research On Deep Structure Of Ordos Basin Based On Gravity And Magnetic Data

The Ordos Basin is located in the western part of the North China block,at the intersection of four different active tectonic belts: the Qinling-Qilian fold tectonic belt,the Helan-Liupan orogenic belt,the Tianshan-Xingmeng fold belt and the Pacific tectonic domain.There are various sedimentary energy minerals such as oil,gas,coal and uranium in the basin,which is an important part of China ’s energy structure.The formation,enrichment and distribution of oil and gas fields are closely related to the deep structure of sedimentary basins.Studying the deep structure of Ordos Basin is of great significance for the exploration of various resources such as oil and gas in the basin.Geophysical method is an important technical means to study deep geological structure.Gravity and magnetic exploration method is widely used in deep and regional geological structure research because of its wide coverage,low cost and high lateral resolution.Based on the regional gravity and magnetic data of Ordos Basin,combined with various processing methods and previous research results,this paper divides the basin fault system,comprehensively studies the characteristics of basin basement zoning,and inverts the depth of Moho and Curie point in the basin.The results and understandings are as follows:(1)The Bouguer gravity anomaly in the Ordos Basin gradually increases from west to east,which may reflect the deep west and shallow east of the basin basement and the variation characteristics of the thickness of the deep crust.Magnetic anomaly reduced to the pole is distributed in high-low phases as a whole.The anomaly in the northern part of the basin is dominated by EW direction,and the anomaly in the central and southern part is mainly distributed in NE direction,which may reflect the difference of basement lithology in the basin.(2)In this paper,the first-order faults in Ordos Basin are divided by the vertical derivative of the normalized total horizontal derivative(NVDR-THDR),analytical continuation and vertical derivative,and 10 first-order faults are inferred.The scale of this kind of faults is large and the extension is long,which is mainly the boundary of the basement lithology of the basin.The faults in the north of the basin are mainly EW-trending,and many NE-trending faults are developed in the middle.The characteristics of gravity and magnetic anomalies on both sides of the faults are obviously different,which are manifested as large linear anomaly zones,cascade zones or boundaries of different gravity and magnetic anomalies.The second-order faults are divided by gravity and magnetic methods,and 28second-order faults are inferred.These faults control the uplift-depression pattern and sedimentary cover distribution of the basin.The first-order faults in the central and southern part of the basin are staggered by the second-order faults.This type of fault shows a certain scale of linear anomaly zone,cascade zone,beaded anomaly,etc.(3)The Ordos Basin has obvious zoning characteristics.In this paper,the polar magnetic anomaly is the main method,and the basement of the basin is divided more carefully by means of upward continuation and vertical first derivative.The northern part of the basin is mainly EW-oriented,and there are several NE-oriented high and low magnetic anomaly areas in the south-central part.It is speculated that the main reason for the regional high magnetic anomaly is that the basement lithology in this area is mainly composed of gneiss,granulite and other strong magnetic rock masses,while the basement lithology in the regional low magnetic anomaly area is mainly marble,phyllite,migmatite and other weak magnetic rock masses.The first-order fracture may be the boundary of regional high and low magnetic anomaly characteristics.Based on the analysis of the characteristics of gravity and magnetic anomalies in the basin,the gravity and magnetic 2.5D human-computer interaction inversion was carried out.The results confirmed the zoning characteristics of the basement lithology of the basin,and showed that the basement of the basin as a whole showed the undulating characteristics of high east and low west.(4)The Paker-Oldenburg interface inversion method is used to invert the depth of the Moho in the Ordos Basin.The results show that the depth of the Moho is 28-44 km.The western part of the basin is mainly the Moho depression area,and the southeast is the Moho uplift area.The power spectrum method was used to calculate the depth of the Curie in the basin.The results show that the depth of the Curie is 20-40 km.The north side of Hangjinqi is an EW-trending Curie uplift area,and the Qin ’an-Yanchuan area is a NE-trending Curie uplift area.Conventional oil and Mesozoic tight oil and gas are mainly distributed in the cascade zone of the depth change of the basin and the uplift area of the Curie.