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.