Page 75 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF ABNORMAL FORMATION PRESSURES 57

Growth faults
According to Dickey et al. (1968), high-pressure zones in the Louisiana and Texas
Gulf Coast region of the United States are related to the particular patterns of block
faulting accompanied by contemporaneous sedimentation and compaction. This 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 diagenesis.
Resistance to the flow of water through the clay is a function of decreasing porosity and
permeability of the clays 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. When clays are compacted, a stage is reached where
the porosity and permeability are so low that the flow of water is completely restricted.
According to Dickey et al. (1968) the growth faults of the Gulf Coast exhibit
the characteristics of slump-type landslides, which in many cases may be due to
old slides that were later buried by sedimentation. The stratigraphic units are thicker
on the downthrown side of the growth faults than they are on the upthrown side,
because during sedimentation there was continuous movement along the fault planes. As
compaction of sediments progresses, the vertical permeability of argillaceous sediments
decreases rapidly. As burial continues, the pore pressure is increased by the mass of the
additional overburden of sediments and temperature increase. In general, abnormally
high pressures are found at depths of 10,000 to 11,000 ft.
Abnormally high formation pressures are encountered in the Niger Delta area in
Nigeria, Africa, where the subsurface structure of the delta is characterized by growth
faults with associated rollover structures, which are caused by gravity (Hospers, 1971).
(Also see Chapter 1 on Growth Faults.)

Transference
Redistribution of excess pore pressure in the subsurface is referred to as transference
(Swarbrick and Osborne, 1998). It is not a primary mechanism in itself for creating
overpressures, but transference may exert a strong influence on many of the pore
pressure profiles seen in the subsurface, and may mask recognition of the underlying
causal mechanism.

Effect of temperature increase on formation pressure (aquathermal pressuring)

Jones (1969, p. 804) pointed out that abrupt changes in temperatures over short depth
ranges are hydrologically critical to the geopressured regime, because the movement
of water is the most important factor in sustaining terrestrial heat flow in the sedimen-
tary basins. Conventional maps of geothermal conditions, however, tend to obscure,
rather than to identify, abrupt changes in temperature. An increase in the geothermal
temperature, as the compacting sediments are subsiding in the basin, causes the pore
fluids (gas, oil and water) to expand more than the enclosing rocks. Such an expansion
would create abnormal fluid pressures in the rocks. There are three modes of heat
transport through fluid-saturated sediments: (1) convective flow of interstitial fluids,