Tectonic differentiation and distribution of Early Paleozoic Ordos basin
Tectonic differentiation is a common phenomenon on the Earth's surface,shaping its diverse geographic,geomorphic,and tectonic relief.This differentiation arises from the complex interaction of internal geodynamic forces,such as tectonic stress,thermal gradients,and gravitational forces;external geodynamic forces,such as weathering,erosion,transportation,and deposition,which are driven by climatic changes,eustatic sea-level fluctuations,and water activity;and extraterrestrial factors,like interplanetary astronomical influences.Cratons,the basic units of the lithosphere,are characterized by their huge thickness and long periods of stability.However,variations in tectonic differentiation in the interior and along the periphery of the cratons are often observed.Understanding these variations is crucial for comprehending cratonic mobility,genetic mechanisms,evolution,and mineral deposit occurrence,particularly within cratonic basins.This paper focuses on the tectonic differentiation of the Early Paleozoic Ordos basin,recognizing its types and delineating its distribution using extensive borehole data and high-resolution reflection seismic profiles.The study reveals that tectonic differentiation in the Early Paleozoic Ordos basin manifests in two primary ways:surface changes and body changes.Surface changes include unconformities(onlapping,truncation,and downward cutting)and various fault surfaces(normal,reverse,and strike-slip).Body changes are represented by progradation bodies,bioherms(reefs or biomass dunes),and diapiric structures(upward-moving intrusions of salt or magma).These tectonic differentiations combine to form distinct features such as normal fault terraces,rifted troughs,and superimposed uplifts.Their temporal evolution is characterized by complex superimposed structural styles,including successive,newly formed,and modified structures,occurring in passive margins,foreland basins,and superimposed depressions.The Early Paleozoic Ordos basin exhibits distinctive tectonic differentiation marked by three major unconformities(Base Cambrian,Top Cambrian,and Top Ordovician),four paleo-highs(Yimeng,Wushenqi,Qingyang,and Lyuliang uplifts),and three rifted troughs(Fuping-Luochuan,Shenmu-Qingshuihe,and Dingbian sags).These uplifts and sags developed in different tectonic units of the proto-basin and are interrelated in their distribution.The tectonic differentiation of the Early Paleozoic Ordos basin has played an important role in the formation and distribution of Lower Paleozoic hydrocarbon source rocks,reservoir rocks,and their combinations.It has also influenced hydrocarbon generation,migration,accumulation,and ultimately,oil and gas occurrence.The Jingbian super-giant gas field,with reserves exceeding one trillion cubic meters,exemplifies this connection,being located within the Ordovician unconformity weathering crust.Furthermore,a series of gas intervals of the fourth Member,second Member,and the seventh and ninth sub-member underlying the gypsum and salt member of the sixth sub-member of the fifth Member of the Lower Ordovician Majiagou Formation have recently been discovered.These findings demonstrate the promising large-scale exploration potential of the deep-seated Lower Paleozoic Ordos basin.
tectonic differentiationEarly Paleozoicinner dynamic processouter dynamic processgiant oil and gas fieldOrdos basin
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