Page 21 - Origin and Prediction of Abnormal Formation Pressures

4 E.C. DONALDSON, G.V. CHILINGAR, J.O. ROBERTSON JR. AND V. SEREBRYAKOV

psi/ft). In the geopressured reservoirs of the Gulf Coast region of the United States, at
a depths greater than 3000 m, the pressure gradient increases to about 20.3 kPa/m (0.9
psi/ft). Hence, fluids in the geopressured zones can exhibit pressures greater than 68
MPa (about 10,000 psi). The formation with normal pressure gradient and the geopres-
sured zone above can coexist only if they are separated by barriers that are impermeable
to the vertical movement of fluids over millions of years of geologic time. The pressure
seals (caprocks) above the geopressured zones are impermeable to the flow of fluids.

Undercompaction

Undercompaction of the sediments can occur during rapid sedimentation and burial
of sediments containing a large quantity of clay minerals (Rubey and Hubbert, 1959;
Wilson et al., 1977). The complete expulsion of water does not occur, leaving the
sediments as a loosely bound system of swollen clay particles with interlayer water.
Where rapid deposition involves large quantities of clays, the sand bodies can be
surrounded by clays, and if the loading rate of sediments is high, the permeability will
decrease rapidly. Consequently, the pore fluids are prevented from escaping vertically
through the overlying argillaceous sediments. Support of the overburden load is then
transferred to the interstitial fluids and the formation becomes abnormally pressured
because the fluids are subjected to the load of the newly deposited sediments. Thus
fluids support a greater portion of the total overburden load (see Eq. 1-3).
If the rate of migration of water from the formation undergoing sedimentation is
equal to the rate of sedimentation, the excess fluid pressure created by the increasing
loading will be dissipated and hydrostatic pressure will be maintained at all depths as
the compaction of sediments takes place (Johnson and Bredeson, 1971).

Tectonic compression

Lateral compression can occur in orogenic belts resulting in development of abnor-
mally high pore pressures. Cretaceous mudstones of northern Wyoming (USA) have
been deformed by lateral compression, which has decreased the formation porosity with
consequent fluid expulsion through permeable beds or increase of formation pressure
within the sealed zones (Rubey and Hubbert, 1959). Fluid pressure almost equal to
the overburden pressure was encountered during the initial drilling of the Ventura
Field (California). The presence of these faulted and folded zones suggests that lateral
tectonic stresses are responsible for some of the surpressures that were encountered
(Watts, 1948). Anderson (1927) reported that abnormally high formation pressures were
encountered on the Potwar Plateau of West Pakistan just south of a folding zone in
the foothills of the Himalaya Mountains; high fluid pressures were also associated with
folding in the Khaur Field of West Pakistan (Keep and Ward, 1934).
As a result of the compressive forces, water from shales can be squeezed into the
associated reservoir rocks (sandstones or carbonates), giving rise to overpressures (see
Chapter 8).
A cubic element in the subsurface has nine stress components acting on it: three
principal, normal stresses, cri, acting on the planes normal to the major axes and six