Characteristics and controlling factors of fractures in the second member of Xujiahe formation in Songhuazhen-Baimamiao area of west Sichuan depression

The gas reservoir of the second member of Xujiahe formation in Songhuazhen-Baimamiao area of west Sichuan depression is non-conventional fracture gas reservoir.It is mainly compact clastic rock and its primary reservoir space is fracture.The investigation on fracture identification through logging and core observation suggest that three sets of fractures develop in the target sequence. One is NW direction of strike,which is regional fracture formed by NW directional regional stress field,the others are NE and EW directions of strike,which are structural fractures generated by the local structure distortion.The carbon and oxygen isotope data show that main epoch of fractures formation may be on Indosinian,Yanshanian and Himalayan,and the Himalayan is primary period when structural and regional fractures formed.The growth of structural fracture is controlled by such factors as lithology,porosity,lamellation and local structures,and the regional fracture controlled mainly by the regional stress field.Impacted by the present stress field,the NW directional fractures are well opened and should be firstly taken into account when arranging the wells for the exploitation.