Tectonic Paleogeography Of Early Paleozoic And Its Significances For Petroleum Exploration In The West And South Of Ordos Block

The tectonic Palaeogeography of the Ordos block in the west of the North China Craton and its genetic relationship with the Qilian-North Qinling terrane and the South China Craton,as well as the attributes and petroleum geological conditions of the early Paleozoic marine sedimentary basins superimposed on the Ordos block are hot issues in the field of basic geological research and ancient marine oil and gas exploration in recent years.On the basis of field geological survey and drilling core observation,through the provenance tracing method of zircon U-Pb chronology and Lu-Hf isotopic composition,this paper reconstructs the regional tectonic palaeogeographic pattern in the west and south of Ordos block in early Paleozoic,discusses the tectonic evolution relationship between Ordos block and its adjacent blocks in the west and south.At the same time,combined with the results of sedimentary facies analysis,the regional tectonic environment,primitive basin attributes and tectono-palaeogeographic features of the main tectonic-sedimentary evolution stages of the early Paleozoic in the west and south of Ordos Block are systematically analyzed.In addition,the source-reservoir characteristics and favorable source-reservoir zones controlled by tectono-paleogeography are analyzed and predicted,and the corresponding reservoir-forming models are discussed.The main achievements and new understandings of this thesis can sum up as follows:The provenance tracer results based on zircon U-Pb geochronological analysis show that the clastic materials in the Cambrian and middle-lower Ordovician strata in the west and south of Ordos block are mainly sourced from the metamorphic basement rocks of the western North China Craton(Ordos-Alax)block.Only the early Cambrian Suyukou Formation and Eidacaran Zhengmuguan Formation and Tuerkeng Formation of the Helanshan area,western margin of the Ordos Block,received clastics from the Neoproterozoic volcanic-magmatic rocks in the Alxa Block,indicating that the Alxa Block and the NCC had joined with each other by the late Neoproterozoic,even earlier.It also implies that there is a significant lack of terrigenous clastic or volcanic-magmatic provenance from other adjacent blocks or island arc complex terranes in the Cambrian-early-middle Ordovician epicontinental-epicontinental sea deposits in the southwestern margin of the Ordos block.According to the comprehensive analysis of zircon U-Pb chronology together with regional sedimentary structure characteristics,the Pingliang Formation/Wulalike Formation-Lashizhong Formation in the western and southern margin of Ordos Block and the Xiangshan Group-Miboshan Formation in the eastern part of the Hexi Corridor(or the southeastern margin of Alashan Block)belongs to the Upper Ordovician deep-water flysch deposits.The comparative analysis of regional zircon U-Pb age spectrum and Hf isotope composition shows that they have the characteristics of mixture source from the western North China(Ordos-Alashan)Block,Qilian-North Qinling island arc complex terrane,and East Gondwana continent,indicating that the regional tectonic palaeogeographic environment of the western and southern Ordos block during the late Ordovician is controlled by the ocean-continent distribution pattern in which these adjacent blocks juxtapose with the ocean basin(Proto-Tethys ocean and its branch Qilian-North Qinling Ocean),and the main body experienced the subduction of the late Ordovician oceanic crust and the convergence of the Ordos block and the Qilian-North Qinling terrane(late Caledonian).According to the comprehensive study of tectonic Palaeogeography,the early Paleozoic sedimentary tectonic environment in the west and south of Ordos Block was mainly controlled by the Qilian-North Qinling ocean and the Proto-Tethys ocean.The main body has experienced two proto-type basin evolution stages,namely,cratonic epicontinental basin in the passive continental margin environment facing the Qilian-North Qinling ocean during Cambrian-Middle Ordovician and residual marginal sea basin in the active continental margin adjacent to the Qilian-North Qinling Island arc complex terrane.Among them,the internal structural differentiation of the Cambrian Cratonic epicontinental basin is relatively weak,showing a stable structure of intra-platform shallow depression,platform margin slope break,and fore-platform gentle slope;the Middle Ordovician presents a differentiated structure of intra-platform depression,platform margin L-type uplift and fore-platform slope;the Late Ordovician is characterized by a differential uplift structure in which the ancient land and the residual marginal sea of the platform were juxtaposed.The tectonic paleogeography of "inner depression-edge uplift-outer slope" of the middle and late Ordovician craton epicontinental sea basin mainly controls the petroleum geological conditions of the lower Paleozoic in the study area.In particular,L-type paleo-uplift and its inner and outer high-energy zones have obvious control over granular beach facies dolomite reservoirs and platform margin reef-bank type reservoirs,and also play a key role in the development of source rocks.The outer slope and inner depression of L-type paleo-uplift are the main zones to develop marine source rocks in the basin.The tectonic unconformity formed by Caledonian movements created favorable conditions for the development of high-quality paleokarst reservoirs.The lower Paleozoic in the west and south of Ordos Block mainly includes five favorable source-reservoir zones.