Abstract: Pressure factor of formation is defined as ratio of the measured formation pressure to the static hydraulic pressure calculated at the same depth. Formations with pressure factor over 1.2 are abnormally high pressure ones, which are certainly closed formations. The maximum pressure factor is about 4.0, which occurs only in compressive structures with lateral stresses being larger than overburden pressure. And also the formation rocks must be tight and well cemented. The surrounding rocks of formations must have strong closing ability.

Key words: formation pressure, rock, pressure factor, overburden pressure, abnormal pressure