Analysis of the characters of the paleotectonic stress field and its inversion method

Ancient tectonic stress controls structure development degree and distribution of seam, has important effect on reservoir permeability. By measuring actual joints and staging actual measurement joints with stereographic projection method, the paper studied tectonic evolution of Yanchuannan block, features of tectonic stress field and inversion analysis method of ancient tectonic stress. Mohr Coulomb fracturing criterion gives a way to estimate value of ancient tectonic stress, which could be calculated through the changes of rupture angles of conjugated shear joints. Two periods of tectonic stress distribution were simulated, and reflected two periods of ancient tectonic stress field distribution. The maximum principal stress value of Yanshanian tectonic stress field of 70~80 MPa in the southeast region was gradually reduced to 20~30 MPa in the northwest region, and the maximum principal stress value of Himalayan tectonic stress field of 60~70 MPa in the northeast region was gradually reduced to 20 MPa in the southwest region