High resolution borehole micro resistivity image logs (FMI) were used to investigate the origin of fracture structures, with Tarim basin in northwest China as an example. The result demonstrates that structural fractures and non-structural fractures (such as weathered fractures and fractures related to collapse of paleo-caves), extensional fractures and shear fractures, and the filling and solution characteristics of fractures can be differentiated with the help of borehole electric images combined with core calibration and interpretation of traditional well log curves. In addition, the attitudes of conjunctive fractures deduced from the borehole images provide important information for the direction of paleo-structural stress field.