Integrated Geophysical Interpretation And Oil-gas Favorable Zone Optimization In Structure Of Qiangtang Basin

Located in the central region of Qinghai-Tibet Plateau,the Qiangtang basin has great oil and gas potential.It is considered an important replacement area of petroleum resources in China,which is of great significance for future energy development.Due to the geographical location and complex structural conditions,only limited research has been conducted on the oil and gas exploration of the Qiangtang basin.The complex structure conditions make it difficult for a single exploration method to obtain high-quality results.Therefore,the integrated geophysical interpretation,which gives full play to the advantages and specialties of various geophysical methods,is of great significance for accelerating the petroleum exploration of the Qiangtang basin.This paper is based on collecting and reinterpreting magnetic,electric and seismic data.A comprehensive geophysical interpretation technology is constructed through the forward and inversion fitting of various geophysical data,which is suitable for the complex surface structures and geological structures of the Qiangtang basin.The fault characteristics and stratigraphic distribution of the Qiangtang basin are further studied through structural analysis and profile simulation,followed by the construction of the tectonic framework.Combined with the basic petroleum geological data of the study area,the petroleum geological conditions are evaluated,and the favorable zones are selected.According to the petrophysical data,the interfaces of density,magnetism and resistivity of the sedimentary layers in the Qiangtang basin are defined.The Cenozoic system shows a lower density,while the pre-Devonian basement has a higher density.Sedimentary rocks are no magnetic or weak magnetic,and from the basic rocks to the metamorphic rocks,acidic rocks and sedimentary rocks,the magnetism gradually decreases.The resistivity of magmatic rock,metamorphic rock and limestone is higher than that of clastic rock such as sandstone.By interpreting gravity,magnetism and electricity data,the basin geological structure reflected by various geophysical data is clarified.The consistency of all kinds of data improves the credibility of interpretation.The basement depth of the basin varies significantly,which is shallow in the south and deep in the north,and shallow in the middle and deep in the east and west.Depression areas can be observed both in the north and south parts of the study area.Influenced by intrusive rocks,the pre-Devonian basement in the western central uplift of the basin(from the western Tumen mine to the Anduo-Nierong area)is influenced by intrusive rocks.The basement is exposed in the west part of the study area.There are deeply buried areas of the basement in both north and south depressions.The Qiangtang basin is divided into three first-order tectonic units: the north Qiangtang depression,the central uplift and the south Qiangtang depression,showing the structural pattern of �two depressions and one uplift�.Each first-order tectonic unit can be further divided into several secondary tectonic units.The southern boundary of the southern Qiangtang depression can be regarded as the basin boundary.Fault structures are well developed in the Qiangtang basin.According to the MT,gravity and geology data,there are 41 secondary or first-order faults interpreted in the study area.EW,NWW and NEE faults are the most developed.They are mainly thrust and strike-slip faults.In this study,the strata above the pre-Devonian basement can be divided into four sets of strata(electrical layers): the Cretaceous system to the Quaternary system,the Jurassic system,the Triassic system,the Devonian system to the Permian system.The strata from the Quaternary system to the Cretaceous system are thickly distributed in the piedmont zone and central uplift at the north and south margin of the basin,with a thickness > 400 m,while the thickness in the rest areas varies between 100 and200 m.The Jurassic system is the most widely exposed strata in the Qiangtang area,with thickness ranging from 2000 m to 6000 m.The thickest strata are distributed in the west Tumen deep depression in the south Qiangtang depression and the north Shuanghu deep depression in the north Qiangtang depression.The Triassic system is mostly stably distributed,with a thickness range of 1000 m to 3000 m and an average of 2000 m,being thicker in the north and south depressions and northeast area.The Permian-Devonian systems show the highest thickness in the north Shuanghu deep depression and the west Tumen deep depression.However,due to magmatic activities,the strata have been destroyed in the Anduo area.In the western part of the central uplift,the strata are exposed or wholly eroded.According to the reservoir,cap,and hydrocarbon preservation conditions of the Qiangtang basin,combined with the spatial configuration of source sag,the vertical configuration of source-reservoir-cap combination,the fault characteristics,the magmatic activities,and other factors such as possible trap condition,three grade I(the north Shuanghu deep depression,the Shuanghu-Zarenkewu uplift and the Tumenxi deep depression)and four grade II favorable oil and gas prospect areas.The north Shuanghu deep depression and the Shuanghu-Zharenkewu uplift are selected as the favorable targets for further exploration.In these areas,the structural zones(1)and(2)(with favorable petroleum geological conditions),and(3)and(4)(with shallow burial depth)are the preferred structural zones.This dissertation contains 35 figures,15 tables and 156 references.