In this paper, it is assumed that the structure is taken shape by using the horizontal stresses to bend and twist the rocks after they have deposited. According to the assumption, the mechanical model is advanced and the curved surface is used to fit the deformation structural form. The relationship between curvature and fracture form is set up. The maximum curvature direction is considered to be the reservoir fracture direction and the bigger the value of curvature is and the bigger the value of fracture porosity is. The result gotten from the practice in certain oil filed in Xinjiang by using the above principle and method is certified by FMI logging data.