Difference between Normal pore pressure and Abnormal pore pressure

             NORMAL PORE PRESSURE

Normal pore pressure is equal to the hydrostatic pressure of a column of formation fluid
extending from the surface to the subsurface formation being considered In other words, if
the formation was opened up and allowed to fill a column whose length is equal to the depth
of the formation then the pressure at the bottom of the column will be equal to the formation
pressure and the pressure at surface is equal to zero.

Normal pore pressure is not a constant. The magnitude of normal pore pressure varies with
the concentration of dissolved salts, type of fluid, gases present and temperature gradient.
For example, as the concentration of dissolved salts increases the magnitude of normal pore
pressure increases.

                                  ABNORMAL PORE PRESSURE

Abnormal pore pressure is defined as any pore pressure that is greater than the hydrostatic
pressure of the formation water occupying the pore space. Abnormal pressure is sometimes
called overpressure or geopressure. Abnormal pressure can be thought of as being made up
of a normal hydrostatic component plus an extra amount of pressure. This excess pressure is
the reason why surface control equipment (e.g. BOPs) are required when drilling oil and gas
wells.

Abnormal pore pressure can occur at any depth ranging from only a few hundred feet to
depths exceeding 25,000 ft. The cause of abnormal pore pressure is attributed to a
combination of various geological, geochemical, geothermal and mechanical changes.
However for any abnormal pressure to develop there has to be an interruption to or
disturbance of the normal compaction and de-watering process as will be outlined later in
this chapter.

(source: aapgsuez.net)