Characteristics Of Geological Structure And Formation Environment Of The Hydrocarbon-rich Depressions In The Sedimentary Period Of Yanchang Formation,Ordos Basin

This paper though the guidance of sedimentology,tectonics and the theory of basin analysis,according a alarge number of field profiles,core observasion,well logging,seismic data and gravity-magnetic data to study Characteristics of geological structure and formation environment of the hydrocarbon-rich depressions in the sedimentary period of Yanchang Formation,Ordos Basin,which contain tectonic activity,filling characteristics,thermal environment and paleoenvironment.It will benefit to know the dynamic environment of the hydrocarbon-rich depression formation and the coupling relationship bwteen the hydrocarbon-rich depression and Qinling orogenic belt.It also have significance for oil-gas exploration of the Ordos basin.Accoding field profiles observasion,core observasion and seismic interpretation we have find a lage number of synsedimentary fault,the fault length is shorter and exists obviously in two bands.It is may relevant with basement rift.The structure style contains extensional structure,compression structure and compound structure.Seismites develop in each oil layer group between chang9 and chang2,turbidite develop in chang9,chang7,chang6,chang4+5 and chang1 oil layer group.The distribution of the synsedimentary fault,seismites and turbidite can cetify that the structure activity was more active in the sedimentary period of the Yanchang Formation.The seismic reflection structure of the Yanchang Formation includes five types of reflection: sub-parallel reflection,parallel reflection,tangent progradation reflection,shingled progradation reflection and disorder reflection.The lake basin began to enter into the initial depression stage during the sedimentary period of Chang10-Chang8,the center of the lake is moving to south in the sedimentary period of Chang8;Chang7 was the strong depression stage of the lake basin;the sedimentary period of Chang6-Chang4+5 was the progressive filling stage of the lake basin;the sedimentary period of Chang3-Chang2 was the stage of lake basin was shrinking;and the sedimentary period of Chang1 was the denudation and extinction stage of the lake basin.On the basis of field profile investigation,core observasion and thin section observasion,there are two types Carbonatite tuberculosis,limestone and dolomite grows in the Chang7 marcellus Shale.Hydrothermal solution was the main formation factor,it certify that the hydrotherm once militate the Yanchang formation?Based on the analysis of core observation and thin section observasion,it is believed that the intrusions found in the Triassic formation of xiang1 well in the south of ordos basin are monz syenite and two igneous.The igneous rock has the characteristics of high GR,high resistance,low acoustic time difference logging response characteristics,and the seismic facies can be devide channel phase and intrusive vein.It is inferred that magma activity exists in the southern part of the Triassic period and the southern part of the Jurassic ordos basin.It is further proved that the tectonic activity of the sedimentary period of the Yanchang Formation is more active.Lacustrine source rocks with the thickness,rich organic matters and the widest distribution generate in Yanchang formation of Triassic in Ordos basin.The source rocks contain three types of lithofacies,black shale,dark massive mudstone and silty mudstone.It can be distinguish through density—neutron overlapping.The Ordos basin was a warm and humid continental freshwater sedimentary environment in chang7,and the lake basin water body was reducing environment,it provide a favorable palaeoclimatic conditions for the organic matter enrichment.