Tectonic sedimentary response and genetic evolution of unconformity surface at bottom of Majiagou Formation in Ordos Basin
The discovery of many large oil and gas reservoirs is closely related to unconformity surfaces.An unconformity surface has developed at the bottom of the Majiagou Formation in the Ordos Basin,which many researchers refer to as the"Huaiyuan Movement"induced unconformity surface.This unconformity is theoretically important for the exploration and extraction of oil and gas from the bottom of the Majiagou Formation.In this study,we discuss the criteria of identification,planar distribution,and genetic evolution of this unconformity based on observations,and the sampling of its field profiles and drilling cores in conjunction with analyses of thin microscopic sections,logging,and geochemistry of the area.The results showed that the lithology of the upper and lower parts of the unconformity have significantly changed.Above the unconformity is the Majiagou Formation containing breccia dolomite,dolomite quartz sandstone,and mudstone dolomite,while below it is the Liangjiashan Formation containing flint strips and nodules,and the top of the Cambrian system containing dissolved fine crystal dolomite as pore powder and mostly parallel unconformities.The number of missing layers in the basin reflects the strength of tectonic movement.Overall,the Huaiyuan Movement in the basin can be characterized as strong in the south and west,and weak in the north and east.The evolution of the unconformity can be divided into three stages.The embryonic stage included the Zhangxia and Sanshanzi sedimentary periods as well as the central uplift of the basin that separated the eastern and western sea areas,and provided the foundation for this stage of the unconformity.In the growth period,most areas of the basin were lifted up during the sedimentary period of the Yeli Liangjiashan Formation,while the underlying strata were exposed and eroded to form unconformity surfaces.The extinction period featured a rise in the sea level during the sedimentation of the Majiagou Formation,because of which marine sediments covered the unconformity surface such that it came into contact with different strata.There are two reasons for the formation of the unconformity surfaces.First,the southern part of the basin was subducted by the Qinling Ocean Plate during the Early Paleozoic,resulting in uplift and erosion.Second,a large-scale decline in global sea levels occurred during this period,and led to the gradual withdrawal of seawater from the basin.
Ordos BasinHuaiyuan Movementunconformity surfacebottom of Majiagou Formationtectonic-sedimentary response
万方数据
维普
