Abstract: Western Hubei-Eastern Chongqing region is rich in oil and gas resources, and the cap rocks are Jurassic mudstone and Silurian mudstone from top to bottom. However, this region has been suffered long term uplift and erosion, which may result in cap rock generating fractures. To investigate mechanical properties of mudstones during geological evolution, three typical tectonic mudstones deposited in the Jurassic Shaximiao formation, Tongzhuyuan formation and Silurian Longmaixi group, are selected to conduct triaxial unloading tests under five different confining pressures from 10 to 50 MPa. The results show that the unloading effect of these three mudstones is enhanced with the increase of unloading confining pressure, and the Jurassic mudstone increases in unloading deformation, the Silurian mudstone mainly exhibits the lateral expansion along the unloading direction. In comparison with the stress state of rock failure in the normal triaxial compression test, under unloading condition, the unloading strength of the mudstone and the internal frictional angle are increased. The mudstone lateral fracture coefficient is negative exponential distribution and decreases with the increase of the confining pressure. The mudstone with short sedimentary history is more prone to damage during the uplift and denudation process. The rock fracture pattern is governed by the rock structure and the stress environment. The mudstone of Shaximiao formation presents a single shear section failure, and the fracture is gentle, while the fractures of the mudstones of Tongzhuyuan formation and Longmaxi formation transit from a single shear cross section to the conjugate shear with increasing the confining pressure. The fracture of Tongzhuyuan formation mudstone is smooth, but the fracture of Longmaxi formation mudstone is jagged.

Key words: mudstone, geological era, unloading, lateral expansion coefficient, lateral rupture coefficient, fracture mode