Internal Structure Characteristics Of Fault Zones In Sand-mud Interbedded Formations

Fault is regarded as a fluid transport channel or a barrier depending on its internal structure,the fault zone consists of two elements of fault core and damage zone.The fault core is the part that absorbs most of the displacement,including fault rocks,lens,slips,etc,while secondary micro structures developed in the damage zone.In this paper,based on the investigation of the deformation mechanism and internal structure of fault zones in sandstone and mudstone,in the hope of exploring the evolution of the internal structure of fault zone and the law of fluid migration,and in order to lay foundations for fault sealing,fluid migration along faults and hydrocarbon accumulation near fault,we choose two fault zone anatomical objects: one is the low porosity sand-mud interbed formation in Liujiang River basin of Qinhuangdao,the other one is the high porosity sand-mud interbed formation in Qaidam Basin,deeply study fault micro deformation mechanism,micro structure type and structural characteristics of fault zone.It is indicated that main deformation mechanism in low porosity sand-mud interbed is fracture from sectional anatomy of fault zones in Heishanyao village of Liujiang River basin.Fault core is mainly developed breccia,fault gouge,mudstone smear,lens and slip surface,while multi-directional fractures in damage zone.The fracture density decreases with the increase of distance from the fault core.Comparison of internal structure characteristics of different fault zones,we get fault evolution model in low porosity formations : the fault zone has a gradual evolution from one element model with non-fault core to two element model with discontinuous fault core and to two element model with continuous fault core.Anatomy in the fault zone from Qaidam basin high porosity sand-mud interbed shows,the microstructure in high porosity sandstone is called deformation band which has a lower permeability compared with undeformed rock.The types of deformation bands can be further divided into disaggregation band,cataclastic band and phyllosilicate bands.The evolution in high porosity sand-mud interbed fault zone depends on the development of the deformation zone,the stress of rock begins to produce a single deformation band because of the strain hardening,the cluster deformationband is formed,and then strain softening occurred,forming a sliding surface and evolving into a fault.Low porosity rocks tend to form high permeability fault zone,while high porosity rocks tend to form low permeability fault zone,which is called reverse transformation of fault to reservoir and produces great differences between fault seal type and hydrocarbon accumulation pattern.Because of non-closed in fault zones,the main seal type is the lateral lithologic docking in fault reservoir formed in low porosity formations,sealing ability is controlled by the regional cap rock and lithology and form block fault reservoir.While sealing type of fault reservoir in high porosity formation is fault rock seal,oil and gas accumulation near fault depends on fault core and local caprock which can be formed as layered reservoir.