With a distinct element code, we analyzed a compartmentalized North Sea oil reservoir's mechanical behavior during fluid pressure drawdown. Special emphasis was given to the affect of compartment pressure reduction on the activation of existing faults, the change of the fault-sealing properties as a result of fracturing of the intact rock mass, and the determination of the existing faults' behavior during fluid pressure drawdown with respect to shear displacement and changes in the fault/fracture conductivities. The simulations showed that several segments of the existing faults in the vicinity of the drawdown region could be activated, Some faults with shear stresses below peak showed large shear displacements and were therefore considered to be activated. Shear fracturing of the intact rock was found to be possible in several zones around the drawdown region.