Page 77 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF ABNORMAL FORMATION PRESSURES 59

Perhaps the most obvious feature of the geothermal-gradient maps of the northern
Gulf Coast Basin is its conformity with the structural map. Elongate areas, beneath
which the geothermal gradient is lowest, overlie the axis of the depositional basin (Gulf
Coast geosyncline). Sediments which overlie the deepest portion of the Gulf Coast
geosyncline would appear, then, to possess the highest thermal conductivity. Jones
(1969, p. 807) stated that if they do not, then they must form a thermal sink and are
now storing heat energy received from below; their temperature must inevitably rise.
The endothermic diagenetic processes occurring in these argillaceous sediments, such
as the dehydration of montmorillonite, require the addition of heat and, thus, reduce the
amount of heat flux to the overlying sediments.
Jones (1969, p. 808) concluded that, by checking the upward flow of water from the
saturated sediments beneath the shales, the sealing clay beds have caused a reduction of
the geothermal flux above and overheating of the undercompacted sediments below.
When the interstitial fluids cannot escape the sediment, subsurface temperature
changes can result in changes in pore pressure, especially if gas is present in the
interstitial fluid. As sediments and pore fluids are buried deeper during sedimentation,
the temperature rises and if the fluid cannot escape, the fluid density would decrease and
volume will increase. If the fluid cannot escape, the effective pressure (grain-to-grain
stress) decreases and the pore pressure increases; thus, the interstitial fluids support
more of the overburden pressure (see Poston and Berg, 1997, pp. 13-16).
Calculation of pressure abnormality due to changes in temperature is presented in
Chapter 5.

Decomposition of organic matter

Organic matter (or kerogen), which constitutes a substantial part of freshly deposited
muds, converts to liquid and gaseous hydrocarbons during diagenesis and catagenesis.
The resulting fluids released during these transformations can create, or accentuate,
the overpressured, undercompacted, state of the compacting clay sediments: (a) by
increasing the pore pressure; and (b) by further impeding the expulsion of interstitial
pore water through the development of a second gas-fluid phase. Methane bubbles
dispersed in water reduce the permeability of the rock to either phase (Chilingarian et
al., 1995).

Gas migration
As mentioned in Chapter 1, one mechanism which is responsible for the formation
of abnormal pressures and yet not fully recognized is the migration of hydrocarbons
(mostly gas) from the lower to upper horizons along faults. One such example is
presented here.
Larichev and Timurziev (1987) studied petroleum geology of pre-Jurassic formations
in the Mangyshlyak Peninsula on the eastern shore of the Caspian Sea in relation
to the neotectonic movements (Fig. 2-22). One of the tectonic characteristics that
they investigated was the gradient of the amplitudes of recent vertical movement
defined as the amplitude per unit horizontal distance. First, the map of recent vertical