The Manifestations And Hydrocarbon Significance Of The Late Caledonian Movement In Southwest Of Ordos Basin

The southwest of Ordos Basin is located at the intersection of the Qinling-Qilian Orogenic Belt and the North China Plate.After the reconstruction of the multi-stage tectonic movements such as the Caledonian Movement,Yanshan Movement,and Himalayan Movement,the geological and tectonic characteristics are more complicated.In the multi-phase tectonic movements,how the Caledonian Movement has transformed and affected the oil and gas geological conditions of the Lower Paleozoic,especially the Ordovician marine carbonate rocks,has become a hot issue in marine oil and gas exploration in the Ordos Basin in recent years.This article takes the late Caledonian Movement as the main line of study,combined with the field outcrop in the southwest of the basin and the seismic profile,drilling and logging data inside the basin,the manifestation of the late Caledonian Movement in the southwest of the unconformity type,structural deformation characteristics and geological response were compared in time and space,and then the effect of late Caledonian Movement on natural gas accumulation was discussed.The analysis of Caledonian unconformity types in the southwest of Ordos Basin showed that the contact relationship between the upper and lower Paleozoic from the edge of the basin to the interior of the basin changed from high-angle unconformity to low-angle unconformity,and then transitioned to parallel unconformity,indicating that the intensity of tectonic activity in the late Caledonian gradually weakened towards the interior of the basin.According to the distribution characteristics of the upper and lower remaining strata of the unconformity interface in the late Caledonian,the geological maps of the pre-Carboniferous and post-Ordovician in the Ordos Basin were compiled,it can be speculated that the tectonic movement in this period caused differential uplift and erosion in different areas in the southwest of the basin.The central ancient upwelling area and the ancient upwelling area are the areas with the strongest uplift and erosion;the uplift and erosion in the central and east areas of the basin are strong;the differential uplift and erosion is strong in the south area of the basin;the central part of the west margin of the basin has the weakest uplift.The structural analysis of the field outcrop and the seismic profile in the basin in the southwest of Ordos Basin shows that the structural deformation in the late Caledonian is different in different regions.The southern part of the basin appears as a northward thrusting tile-like thrust nappe structure,and it is composed of various types of fold structures;in the west,it mainly shows simple fold deformation of the Upper Ordovician;the seismic section shows the top of the Ordovician from the edge of the basin to the inside,it was gradually cut off and sharpened.The outcrop and drilling of Ordovician in the southwest of Ordos Basin generally contain tuff interlayers,the analysis of tuff characteristics and thickness through the logging curve shows that the cumulative thickness in the basin can be up to 1.2m,and it has a feature of thinning from southwest to north.Zircons U-Pb dating of the typical Ordovician tuffs in the southwest of the basin and outcrop in the field was performed at the end of the Late Ordovician(444-449 Ma).Geochemical information shows that the source rocks are all from the volcanic island arc environment.At the same time,the deposition of events such as gravity flow,traction flow and slump accumulation in the southwest of the basin also reached the largest scale in the Late Ordovician,indicating that the Qinling-Qilian Ocean northward subduction reached its peak in the Late Ordovician,the series of event deposits is the geological response to the northward subduction of the Qinling-Qilian Ocean in the late Caledonian.The late Caledonian tectonic movement in the southwest Ordos Basin has an important influence on natural gas accumulation.In addition to the formation of Ordovician weathered crust reservoirs in the Jingbian gas field in the middle of the basin,the weathered crusts formed by this movement also developed weathered crust reservoirs in the Yichuan-Huanglong area in the southern of the basin,thereby forming weathered crust-type gas reservoirs;In the central part of the western margin,karst fracture-cavity reservoirs;in the central of the western,karst fracture-cavity reservoirs were formed under the influence of the Caledonian tectonic movement,which could form karst fracture-cavity trap gas reservoirs.