| The Songliao Basin is located in the eastern section of the Central Asian Orogenic Belt and is an important oil and gas-bearing basin in China.Its formation and evolution are affected by the tectonic system of the Paleo-Asian Ocean,the Mongol-Okhotsk Ocean and the Paleo-Pacific Ocean.With the continuous growth of energy demand,deep oil and gas exploration has become a new way to increase production.Therefore,in-depth analysis of the lithospheric structure of the Songliao Basin under the influence of tectonic systems and exploration of the influence of lithospheric structure on oil and gas resources are not only the frontier issues of geoscience research,but also the urgent task of the investigation and evaluation of deep oil and gas resources in the Songliao Basin.However,due to the lack of deep geophysical observation data,the relationship between tectonic systems superposition and lithospheric structure and the spatial distribution of the Upper Paleozoic are not clear,and whether there is a Carboniferous-Permian oil and gas resource pool with potential for oil generation in the deep is urgent to be solved.This paper provides new evidence for solving the above problems by constructing the magnetotelluric model of the crust and mantle of the Songliao basin and the non-seismic geophysical model of the basement.First of all,138 large space distance broadband magnetotelluric sounding sites in the Songliao Basin are processed and analyzed.The phase tensor analysis results show that the underground structure has strong three-dimensional(3-D)characteristics.The 3-D inversion of the full impedance data is carried out to obtain the model of the regional resistivity in the Songliao Basin.Secondly,non-seismic geophysical data are collected,processed and analyzed for the area of the well SK-2,and the local non-seismic geophysical 3-D model is obtained.The inversion results of the two models show that the upper crust of the Songliao Basin is widely distributed in the NE and NW directions with high-conductivity anomalies,and there are orthogonal low-resistivity anomalies(～10Ω·m)at the intersection,and the vertical high and low resistivity overlap,in which the low-resistivity is mainly a "basin" structure with the thick middle and thin sides.The lower crust and upper mantle of the basin have high-conductivity characteristics that are rooted in the deep and upwelling diagonally on both sides.A large area of high-conductivity anomaly occurs at a depth of about 45 km at the junction of the central depression area and the northern plunging area.In the non-seismic geophysical model,it is found that the basement resistivity in the area of the SK-2 has a large range of variation,many abnormal bodies are gathered,and the gravity and magnetic anomalies are distributed in a wide range,with the overall distribution in NE and NW directions.And there are three characteristic combinations: 1.High-density,low-magnetism and low-resistivity anomaly combination,2.Low-density,low-magnetism and high-resistivity anomaly combination,3.High-density,high-magnetism and high-resistivity anomaly combination.The results show a heterogenous lithosphere with anomalous low-resistivity zones(～10 Ω·m).The position of upper-crustal features(<10 km)correlates with a network of fault systems and rift basins.Based on estimates of high crustal temperatures,mid-lower crustal features(15–35 km),some of which appear near seismic reflectors,are attributed to partial melts(～5%)containing moderate-high water contents(4.5 wt.%).A deep(>45 km)feature is interpreted as an asthenospheric upwelling near the center of the Songliao Block,where seismic evidence indicates a thin lithosphere.It is proposed that the collision,convergence,and suturing of blocks caused by the closure of the Paleo-Asian Ocean and the bidirectional convergence between the Mongol-Okhotsk and Paleo-Pacific Ocean may have produced weak tectonic zones throughout the lithosphere.They provided a structural basis for the later upward movement of fluids and melt,likely due to hydrous upwellings caused by the subduction of the Paleo-Pacific system.According to the combination of density,magnetic susceptibility,resistivity and physical properties,it can be inferred that the basement of the area of the well SK-2 is mainly composed of Upper Paleozoic strata and intermediate-acid intrusive rocks.Moreover,according to the geophysical characteristics,there are three superimposed relationships between the Upper Paleozoic and the overlying Mesozoic.i)upper Mesozoic and lower Paleozoic,with intrusive rocks intruding into the interlayer;ii)intrusive rocks are underlain in Mesozoic;iii)Paleozoic is directly buried in Mesozoic.The complex superimposed relationship is related to the late transformation of the tectonic system.According to the conditions of oil and gas generation,the first superimposed relationship is considered to be the most favorable for the generation,storage and migration of oil and gas.According to the law,it is proposed that the joint of the Heiyupao sag and the Sanzhao sag in the research area is a good oil and gas accumulation unit.To sum up,this paper not only constructs the electrical structure model of the deep lithosphere of the Songliao Basin and the regional gravity,magnetic and electrical three dimensional inversion result models of the area of the well SK-2,but also gives the relationship between tectonic systems superposition and lithosphere structure and fine structure of Upper Paleozoic strata,which provides a new idea for understanding the structural evolution and oil and gas exploration of the Songliao Basin. |
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