Analysis Of Meso-Cenozic Structural Characteristics And Prediction Of Favorable Oil And Gas Zone In Liupanshan Basin

The geotectonic position of Liupanshan basin is very special and complex, which is in the North Qilian tectonic belt and in the Hexi Corridor depression zone, besides, Liupanshan basin is also located in the Helanshan-Liupanshan-Longmenshan-Hengduanshan tectonic belt which is one of the most important north-south tectonic belt, so, the tectonic activity of Liupanshan is very strong. The predecessor’s researches about the fault system and structural characteristics for Liupanshan basin is not deep enough, and is the lack of overall research. In this paper, in order to look for the favorable areas for oil and gas accumulation, we use previous data, prospect geological condition in the filed, make laboratory analysis and comprehensive map to study Mesozoic and Cenozoic geological structure of Liupanshan basin. This study is of great significance for the analysis of structural features about basin, which is similar to Liupanshan basin.(1) By analyzing the bouguer gravity anomaly data of Liupanshan basin, we find gravity data is in accordance with regional geologic data very well, which is mainly reflected on the similar nature for the distribution regularity of positive and negative contour with the NW trend of main fault in basin, the distribution regularity of positive and negative contour with the arrangement of positive and negative tectonic units, and the distribution regularity of positive and negative contour with the characteristics about the strength of the strong south west and weak north east in basin.(2) On the local gravity anomalies, the basin boundary fault zone is divided into several segments, besides, by combining the gravity, magnetic, magnetotelluric sounding and other geophysical data with regional geological data, and by combining field investigation with indoor analysis, we found the strike of fault system is mainly showed for NW-SE in Liupanshan basin. What’s more, the fault system can be divided into three styles which is basin-controlling boundary faults, regional framework faults and local tectonic fracture in Liupanshan basin. The east and west boundary fault zone also have the characteristics of a typical section.What’s more, the west boundary fault zone from north west to south east can be divided into Laohushan-Mijiashan fault zone, Damouhuaishan-Wangjiashan-Dalachi-South and West huashan-Moon moutain fault zone(Damouhuaishan-Moon moutain fault zone), Liupanshan fault zone and Longxian-Baoji fault zone. The east boundary fault zone also can be divided into Niushoushan fault zone, Luoshan fault zone, Luoshan-Tanshan fault zone, Tanshan-Guyuan fault zone and Guyuan-Huating fault zone.(3) The block features of the western basin boundary fault zone is embodied in the change of the direction, exposed stratum in fault zone and on both sides, nature about fault and so on. We focuses on the fault characteristics about Damoushan-Moon moutain fault zone and Liupanshan fault zone so that we find ①The west and south huanshan fault which is the east boundary fault in Damoushan-Moon moutain fault zone is thrust nature in early, and sinistral strike-slip on the late. The combination of style on the plane is trellis which is parallel for fault and fold axis intersect with fault by a little angle and is imbricated overthrust structure on cross-section.②The nature of boundary fault in Liupanshan fault zone is a trust fault with strike-slip movement. The combination of style on the plane is fold axes parallel to fault direction and is imbricated overthrust structure on cross-section. What’s more, the opposite thrust fault with fold is developed in the fault zone. By using magnetotelluric sounding data, we find the western boundary fault zone, from the north west to south east, become flatten gradually from rise steep and gentle under and rise steep above on the whole, besides, the deep of fault zone shallow from south west to north east and cutting deep shallow from north to south in deep structural style.(4) The direction of eastern boundary fault zone is SN in Liupanshan basin. There are following changes from North to South in five fault segment.①Qingtongxia-Guyuan fault change from north to south which is thrust with slip fault-thrust with dextral strike or normal fault-thrust fault-thrust fault-thrust with sinistral slip fault.②The eastern fault structural styles change also from north to south which is thin skin and back thrust-thick imbricate thrust nappe and back thrust-single thick thrust nappe-thick and back thrust-high angles and section bending the thrust nappe. Qingtongxia-Guyuan fault in the eastern boundary fault is deep in the Niushoushan and Luoshan fault. Luoshan-Tanshan-Guyuan fault is light.What’s more Luoshan fault has reversed. All the faults are thrust nappe structure.(5) On the base of analysis and summary of previous research, I prepared four geological-geopphysical profiles. Trough the comparative study of the geological profile we have discovered Liupanshan basin has the characters of thrusting nappe style form south west to north east and basement has the uplift trend from south west to north east. Liupanshan basin has different character about structural strength from east to the west and north to south. Strong erosion areas is in northern, central is to weakly deform areas and southern is torsion areas.①Xingrenbao depression is the main depositional center in northern basin and the Carboniferous-Jurassic are deposited in the depreesion. Lihuaping uplift subjected to uplift and erosion in the east of basin and the Carboniferous-Triassic is exposed.What’s more they are folding variants. The Carboniferous and Permian and Triassic strata below the unconformity in Cenozoic Yaoshan uplift an Tongxin depression. Xiangshan uplift has developed thrust with opposite and thrust apptructure indicates that the north is a issqueezing denudation area. ②The basin depositional Center is mainly in Haiyuan and Guyuan depression. The Carboniferous and Cretaceous is mainly sedimentary strata. The basement has the trend of uplift from and sedimentary cover thin from SW-NE. The thickness of fault on the up is greater than lower in the Triassic-Cretaceous. This suggest Qingshuihe fault is sediment fault. The fault has happened tectonic inversion and the overall peformance is the weak sedimentary in the areas. ③ The basin of southern deposition Center is in Guyuan depression and sedimentary strata is Carboniferous and Cretaceous. The southern of basin is in fault interchange and tection activity is very strong. They performance for strongly of inverse in rushed pushed reply constructed and high range broken back oblique in Guyuan depression. Cretaceous performance for a series of thrust nappe and strong deformation within the block.This show pressure and torsion-free zone is in the south of the basin. Distribution from eastern and western is low gravity mainly in the western central depression of and eastern slopes is high gravity. This is possible associated with basement uplift.(6)By comparing the geological-geophysical integrated profiles with its magnetotelluric sounding profile nearby, we can find that: the electrical structure of Liupanshan Basin appeals hierarchies with high-low-high-low-high value features from shallow to deep in the vertical direction. Through the comparative study of magnetotelluric sounding profiles in NE and NS direction, we found that high speed low resistivity zone has the structural styles of deep in SW and shallow in NE, which match the thrust nappe structures in the South Western of Liupanshan basin styles. In addition, by combining local seismic and geological survey data analysis, we find that the regional structural stress in the south west is greater than north east of the basin. Moreover, from the analyses of the fold elements of the whole basin, it confirms that the tectonic stress in the north east of the basin is less than the south west, it also confirms that the compressive stress in Mesozoic-Cenozoic mainly comes from the SW-NE direction. The comprehensive study of fold axial rose diagram and the previous data indicates that over thrust tectonic development is front-exhibition. Comprehensive analysis shows that the area from the South West to the North East of the basin, the tectonic stress field is in a gradual decrease in trend, which shows that the essence is under the influence of Cenozoic tectonic deformation of the Qinghai-Tibet plateau and is controlled by India plate to the North and North-East of compressive stress in Cenozoic.(7)In the ways of previous data,observing rock thin section under a microscope and testing samples in the lab, we know that carboniferous hydrocarbon source rock in Liupanshan basin is mainly producing gas, which belongs to moderately good carboniferous hydrocarbon source rock; And so is the rock of Jura—Trias systems, which has highly organic carbon and maturity; Cretaceous hydrocarbon source rock is mainly producing oil and is high abundance of organic matter but low maturity, which kerogen belongs to I-II. The basin developed some good reservoir rocks of Carboniferous, Jura—Trias and Cretaceous systems. It is analyzed from the view of the development of cap rock that self generating, self storing, self sealing of Cretaceous systems and lower generating, upper storing of Cretaceous—Paleogene systems are the most favorable. We come to conclusion that Xingrenbao depression, east of Haiyuan depression, central and north of Guyuan depression, southwest of armand and the foot-wall thrust—nappe belt(Shagou fault terrace)in moon mountain are all the advantageous oil and gas regions; And the foot-wall of nappe tectonic belt of Qingshuihe in Tongxin depression, Shixiakou fault terrace and Hejiakouzi depression are all potential oil and gas regions.