Structural Evolution And Cambrian-ordovician Tectono-paleotopographic Reconstruction In The Yubei And Adjacent Areas,Tarim Basin

Ordovician carbonate rocks in the Yubei and adjacent areas are important oil and gas reservoirs.Based on analysis of logging and seismic data in the study area,this paper presents a three-dimensional structural model,and analyzes the assemblage relation features and spatial distribution characteristics of the major faults in the Yubei and adjacent areas.The characteristics of tectonic evolution in the Yubei and adjacent areas were revealed by analyzing the balanced geological cross-section.For tectono-paleotopographic reconstruction of the Yubei area from the early episode of the Middle Caledonian to Early Hercynian,the impression method and residual thickness method were used,combined with reconstruction of paleobathymetry,based on the different tectonic characteristics of the intervals in the research area.Furthermore,the controlling effect of tectonic geomorphology on the Karst reservoir was analyzed.The faults in this area are mainly divided into shovel-shaped thrust faults beneath the Lower-Middle Cambrian Awatage Formation gypsum salt rocks,and thrust faults over the Cambrian gypsum rocks.In the eastern part of the study area,the development time of the faults was earlier than that of faults in the central and western regions,and the deformation was more severe.The eastern part of the Yubei area is dominated by an imbricate thrust structure,and secondary faults are well developed.The trend of the faults is generally NE.The Yubei 1 and Yudong 3 structural belts are segmented,which shows that the factors controlling the faults differ in different layers.Between the early episode and episode I of the Middle Caledonian,Yubei and the adjacent areas was a north-dipping slope,high in the south and low in the north.The study area was uplifted and eroded in episode III of the Middle Caledonian.From the Late Caledonian to Early Hercynian,the Yubei area existed in a regional uplift stage,with large-scale absence of Silurian�Middle Devonian deposition.At this stage,the Yubei and adjacent areas were high in the east and low in the west.During the Himalayan orogeny,Markit was inclined to the south because of tectonic inversion in the Bachu-Markit area.In this period,the structure of the Yubei area was high in the northeast and low in the southwest.Based on calculation of the paleo-water depth of the wells,the Penglaiba Formation in the Yubei area was characterized by restricted platform facies,and the water depth ranged from 3.73 to 4.02 m in the eastern part of the study area.The Yingshan Formation formed in an open platform setting,and the water depth ranged from 4.88 to 9.86 m.At the time of deposition,the water depth ranged from 20.10 to 49.97 m in the fault-sag in the east,and the water depth ranged from 4.88 to 9.86 m in the central part.From the Penglaiba to Yingshan formations,water was mainly distributed in the north;the Yubei area was high in the south and low in the north.The tectono-paleotopographic reconstruction shows that a karst highland developed in the southwest,a karst slope developed in the central part,and a karst depression developed in the north and northwest in the early episode of the Middle Caledonian in the Yubei area.In episode I of the Middle Caledonian,the karst highland developed to the south of the central part,the karst slope developed in the central and southwestern part,and the karst depression developed in the southern area between wells PSB1-x and PSB2 in the Yubei area.In episode III of the Middle Caledonian,the karst highland developed to the south of the central part,the karst slope developed in the central and southwestern parts,and the karst depression developed in the western part of the study area.In the Early Hercynian,the karst highland developed in the east with striped distribution,the karst slope developed in the central part,and the karst depression developed in the western part of the Yubei area.From the early episode of the Middle Caledonian to the Early Hercynian,the core of the paleo uplift was transferred to the east,and the migration of the karst highland was consistent with this transfer.From the early episode of the Middle Caledonian to the Early Hercynian,the tectono-paleotopographic was strongly eroded on the karst slope,which was a favorable karst development zone in the central part of the Yubei area.The weathering karst crust and the limb of the fault zone fold of the Penglaiba and Yingshan formations is a favorable area for exploration.