Page 23 - Origin and Prediction of Abnormal Formation Pressures

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

where pp is the pore fluid pressure. If the overburden pressure (~rz) is fixed, and
the effective horizontal stress, Pex, increases more rapidly than the pore pressure is
dissipated from the formation by leakage, the pore pressure will increase until it reaches
a maximum value equal to the overburden pressure (pp = O-z). Then, Pez will be equal
to zero, and crx will increase toward the failure stress of the rock. At this condition,
the superincumbent material can be moved tangentially with negligible resistance.
Approach to this condition depends on the relative rates of the opposing processes: (1)
the rate of the lateral deformation stress (crx) and (2) the rate of pressure dissipation
by fluid leakage. According to Hubbert and Rubey (1959), the application of orogenic
stresses is more effective in promoting the conditions of surpressures than sedimentary
loading in tectonically quiescent geosynclines. Thus, if the rate of increase of applied
orogenic stresses is more rapid than the pore fluid pressure dissipation (through leakage
of the fluid), only the presence of stronger rocks can prevent pp from becoming equal to
~rz.

Faulting

Some high-pressure zones in the Louisiana and Texas Gulf Coast region of the
United States apparently originate from the pattern of block faulting accompanied by
contemporaneous sedimentation and compaction. The process creates lateral seals that,
together with a layer of thick shale overlying the surpressure zones, prevent the loss
of pore fluids from the sediments during compaction and other diagenetic processes.
Resistance to the flow of water through the clay is a function of decreasing porosity and
permeability of the clay as compaction progresses. The hydraulic permeability of clay
is negligible in the geopressured environments. The clay beds have overlain abnormally
pressured formations for millions of years without the release of the pressure by fluid
flow across the clay/shale beds. Apparently when the beds of clay are compacted, a
stage is reached when the porosity and permeability are so low that the vertical flow of
fluids is completely restricted.
According to Dickey et al. (1968) the 'growth faults' of the Gulf Coast exhibit
the characteristics of slump-type landslides and in many cases may indeed be due
to old slides that were later buried by sedimentation. The units are thicker on the
downthrown side of the growth faults than they are on the upthrown side, probably
because during sedimentation there was continuous movement along the fault planes.
During compaction of the sediments while sedimentation was taking place, fluids in the
pores of the sediments normally travel vertically upward. As compaction progressed,
the vertical permeability of argillaceous sediments decreased rapidly and as burial
continued the pore pressure increased due to the mass of the overburden sediments and
temperature increase. The abnormally high formation pressures are commonly found at
depths beginning at about 10,000 ft (3000 m).
Continued sedimentation can cause a shear zone to develop by overloading the
undercompacted shale. Expulsion of the water is accompanied by subsidence of blocks
of sediments. Thus, the contemporaneous faults of the Gulf Coast Basin (USA) are
characterized by the cycle of deposition, expulsion of water, subsidence of blocks of
sediments, and temperature increase.