The Characteristics And Controlling Factors Analysis Of Fractures In Palaeozoic Shale,Fengyi Block,Qianbei Area

Fenggang belongs to the north Guizhou Province,which is one of the main shale gas exploration and exploitation fields in China.Fractures in shale are essential for shale gas exploitation.With the foundation of outcrop observation,data of well drilling and logging,this paper analyzes the fracture development characteristics in Lower Paleozoic Niutitang formation.The controlling factors for fracture development are discussed from aspects of tectonic stress,TOC content and fragile mineral content.The dominant controlling factors are recognized.Tectonic stress field simulation of target formation during Xishan period is done by ANSYS software.Fracture development of Fenggang area is predicted based on fracture characteristics and controlling factors.The main conclusions of this thesis are as follows:In general,fractures in Niutitang shale formation is rather developed.Fractures of Niutitang Formation from drilling core of Yongxin 1 are well developed from 2008 to 2022m.Most of fractures are of tectonic origin;over a half of fractures are of high-angle;the width of fractures generally vary from 0.2mm to 1.0mm;the length of fractures are mostly within 10 cm.Logging response shows conductive fractures and resistive fractures are developed,between which,resistive fracture is the dominant.The strike of conductive fractures and minor fault is NNE-SSW,the dip of them is SSE;the strike of resistive is S-N,the dip of them is W-E.The controlling factors of fractures in study area are tectogenesis,TOC content and fragile mineral content.Tectonic factor are the principal factor that controls fracture development followed by TOC content and the fragile mineral content of the shale.The tectonic active contribute to the fracture development;the density of fracture and TOC content as well as fragile mineral content shows positive correlation.The hole enlargement while drilling led to the converse of Young's Modulus and Poisson's Ratio of shale rock,where fractures are developed.The Niutitang Formation of research area experienced NW-SE trending compressive stress during Xishan Period the maximum principal stress(MPS)of which varies from-309.457 Mpa to-50 Mpa.High MPS value shows on and around folds,meanwhile the low MPS value shows on and around faults.The shear stress are mainly sinistral shear varies from 20 Mpa to-50 Mpa.The fracture development prediction of Niutitang Formation of study area during Xishan Period shows the fold areas in the southeast and north of study area are of fracture-developed potential.The development of fracture is of high correlation with tectonic activities,areas around folds are defined as ?-?potential fracture-developed level;areas around faults are defined as ?-? potential fracture-developed level.