Mechanism Of Deformation Bands In The Cretaceous Yuanan Basin,Western Hubei,Central China

Deformation bands is a common macro-deformation mechanism developed in high-porosity sandstones,and it is also the most recently recognized macroscopic structural deformation mechanism in the world.As a special structural deformation mechanism,the intensive development of the deformation zone will lead to a significant reduction in the porosity and permeability of the rock,hinder the movement of underground fluids,and affect the exploitation of oil and gas resources and the processing of nuclear wastes.In view of the theoretical and practical significance of the deformation bands,a large amount of researching works have been carried out abroad,but domestic research work in this area is basically in a blank state.In the Cretaceous Yuanan Basin of western Hubei,there are dense deformation belts in the Honghuatao Formation,which are distributed in a planar manner in the basin.In the Yuan'an Basin,the exposed deformation bands is mostly normal fault type,and it is mainly developed under the extended tectonic setting of the Late Cretaceous in the Basin.Through the field measurement of the density distribution of the deformation zone,the identification of the sedimentary facies of the Honghuatao Formation,and the indoor data processing and analysis,the following results are obtained:(1)The deformation bands in the Yuan'an Basin are concentrated in the giant sandstones of the Upper Cretaceous Honghuatao Formation in the middle and southern parts of the basin,especially on the west side of Luyuan Temple,north of Hualin Temple and the northern part of Yejia Bridge.The maximum line density can reach 30 bars/ m,and its distribution range extends southward to the inside of the Jianghan Basin.However,there are few deformation bands in the strata of the Paomagang Formation in the northern part of the basin.(2)By the simple identification and division of sedimentary facies of the Upper Cretaceous strata in the Yuan'an Basin,the stratum of the Upper Cretaceous Honghuatao formation can divided into two distinctly different sedimentary facies,the eolian dune facies and the fluvial facies..By comparing the maps of different types of tectonic types and the stratigraphic sedimentation mapping results,it was found that the deformation bands concentrated in the massive sandstone of the eolian dune facies.Micro X-CT shows that the porosity of eolian sandstone is about 16% on average and the highest is 23%.This is very beneficial to the development of the deformation bands,which is the direct cause of the deformation zone mainly distributed in the safflower formation.(3)On the basis of summarizing the factors affecting the distribution of the deformation bands in the Yuanan Basin,this paper further analyzes the cumulative distribution law of the single deformation bands' width,and the width of the single deformation belt is mainly distributed within the range of 0.5~2.5mm.By contrast,the deformation band width distribution data most conforms to the power rate distribution,and the power rate values are distributed between 0.87 and 1.25.As an early stage of fault development in high-porosity sandstone,the power distribution study of the width of the deformation bands makes up for the lack of early fault-related data.(4)The width of the zone ofdeformed bands is usually tens or even hundreds of times that of a single deformation belt.Contrary to the single deformation zone,the width of the deformation belt will change in the direction of the undulations.In this paper,the deformation band profile is discretized and the processing result is Fourier transformed.On this basis,the two waves that control the periodic variation of the width of the deformed belt are obtained,and the wavelengths is about 125 cm.