Page 24 - Origin and Prediction of Abnormal Formation Pressures

INTRODUCTION TO ABNORMALLY PRESSURED FORMATIONS 7

Abnormally high formation pressures are also encountered in the Niger Delta in
Nigeria, Africa. This delta is characterized by growth faults caused by gravity creating
zones of high pressure overlain by thick shale beds.
There is no doubt that several other mechanisms were active as supplementary
pressure generators during the origin of the high pressures found in zones where
growth faults predominate, and also in the maintenance of the high pressures after
their generation. Although several mechanisms may have made their contributions,
gravitational loading and tectonic compression probably exerted the greatest influence
on pore pressures and hydrocarbon/water migration. Therefore, knowledge of the
vertical and lateral orogenic stresses in the depositional basins is of major importance
for interpreting the abnormal fluid pressure environments and anticipating the location
of oil and gas reservoirs associated with the abnormally high pressures.
The analysis of fluid-rock stress conditions has many other applications: earthquake
prediction, hydraulic fracturing, compaction of rocks during their geological history,
and the deformation of rocks in subsiding formations. The same theoretical basis
applies for the solution of deformational problems by earthquakes and hydraulic effects
that dissipate tectonic stresses through small earthquakes; and deformations caused by
oil, gas, and water production. There is the curious generation of earthquakes up to
magnitude 5 created near Denver, Colorado, USA, by the injection of waste fluids into
the fractured gneiss using a 3700-m deep well. The increase of subsurface pressure
disturbed the fluid-rock stress equilibrium and promoted sudden slippage along fracture
planes (faults), some with very deep epicenters up to 5500 m deep (Evans, 1966).

Diapirism

The Jurassic age Louann Salt underlying deep sediments of Louisiana and Texas in
the United States was thinned by diapiric flow during the period of rapid sedimentation
that began with the uplift of the Rocky Mountains at the beginning of the Cenozoic era.
Salt was squeezed gulf-ward by sand and clay deposits forming domes and ridges, with
some diapirs rising through the entire thickness of the overlying deposits. As the depth
of burial continued, the increases in temperature induced dehydration of the clays within
the buried zone and contributed to the shearing stresses.
The salt became ductile and flowed like a viscous plastic under pressure and at
elevated temperatures, such as those encountered in deep subsurface formations: 93~
(approximately 200~ at 3700 m (12,000 ft). The low density and strength of salt readily
allowed development of domes when the density of overlying sediments exceeded the
salt density. Salt was pushed upward penetrating the overlying sedimentary structures
and acquiring a sheath of pliable clays, or shales, around parts of the salt diapir. The
term sheath refers to the predominantly shale material which is out of place between
the salt stock and the younger sedimentary rocks. The sheaths originate from folding
of the clay bed and deposits of younger sediments against the dome, or from faulting
of the clay bed which is then pressed into its position between the salt dome and the
flanking sediments (Fig. 1-3). Structural features generally associated with salt domes,
such as the configuration of the sheath, indications of uplift, subsidence at the surface,
and development of rim synclines, are a consequence of the physical properties of