The present research was undertaken in order to develop a better prediction of abnormally-high formation pressures of different origins and to avoid "condemning" overpressured oil-bearing formations based only on well tests. The authors found that for each formation there is a certain critical effective stress above which the plastic deformation will occur, closing down the avenues (canals, pores, pore throats, fractures, etc.) for the migration of fluids. In the case of synsedimentary origin of abnormally-high pressure (rapid deposition, undercompaction), this critical effective stress(1) can be reached rapidly, as indicated by laboratory compaction tests or a change in the slope of the production versus pressure curves during well tests. On the other hand, in the case of postsedimentary origin of abnormal formation pressure (hydrocarbon generation, upward vertical fluid migration along faults, and expansion of fluids due to an increase in temperature), the preexisting effective stress was lowered due to overpressuring. Compaction tests can help establish the value of this preexisting stress (shale can "remember" its past history). If the existing effective stress is increased due to production and then reaches the critical effective stress, the plastic deformation of reservoir rock will occur upon further increase of the effective stress. The writers confirmed these findings using compaction tests in the laboratory and well tests in the field. Thus they established that knowledge of the origin of abnormally-high formation pressure is of utmost importance.