Page 98 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF FORMATION FLUID PRESSURE DISTRIBUTIONS 79

Bound and interlayer water is in dynamic equilibrium with solid matter of minerals;
therefore, this equilibrium and, thus, compaction of clay are temperature dependent.
With increasing temperature, the energy of Brownian oscillations of water molecules
increases and exceeds the energy of bonds. Thus, with an increase in temperature, layer
after layer of adsorbed water transforms to free water and becomes capable of flowing
through pore space in the direction of lower fluid potential. Transformation of the bound
water into free water occurs with an increase in volume due to the density difference.
For each particular clay bed a major increase in temperature may occur not while it is
being buried and the sediment load is increasing, but in the course of a regression period
when temperature rises due to higher heat flow and, often, vertical migration of fluid.
This may be especially true in the case of fast sedimentation when thick sediments are
accumulated and buried. The rate of heating these sediments is lower than the rate of
sedimentation and burial; thus, rocks are much cooler here than at the same depth in
stable areas: the geothermal gradient may be just half of that in the stable areas. When
sedimentation stops, formations continue to warm and temperature isotherms rise up
the geologic section. During this period, compaction can continue due to temperature
increase even if load of sediments decreases due to erosion.
Changes in shale porosity are also due to the transformation of clay with loss of
water, such as transformation of montmorillonite to illite. This process depends on
temperature increase and availability of an additional amount of potassium. It takes a
long time and its rate is different in geologically different environments. As a result,
montmorillonite ceases to exist at different depth and temperature in different regions. In
tectonically stable regions with older formations the lower boundary of montmorillonite
presence may be about 2000+ m, but in regions of recent and very fast sedimentation
like Azerbaijan it may be deeper than 5000 m.
The combination of load and temperature enhances and changes the process of
deformation of clay. Compaction caused by loading applied at different rates produces
different final porosity in samples of equal initial porosity. Temperature increase and
'melting' of bound water causes appearance of 'dry' contacts between individual
mineral grains that strongly impedes further compaction.
The above discussion indicates that the compaction process includes several relatively
independent components and cannot be reduced to Terzaghi's model and equivalent
depth concept. A single simple model common for all formations and regions cannot
describe it.
In the areas of recent continuous sediment accumulation, burial and compaction, a
correlation between porosity and pressure can be expected and is actually observed.
This correlation may be due to both pressure distribution changed by compaction and
the hydraulic resistance of rocks to the upward water flow, which dissipates the excess
pressure. Utilization of the following characteristics can be suggested:
(1) For the permeability and total hydraulic resistance: total thickness and clay/sand
ratio.
(2) For the rock capability to produce pressure increase: porosity, vitrinite reflectance
(as an indirect indicator of consolidation and, thus, mechanical strength), and the
expandable minerals content in the total clay minerals content.
(3) For the external influence causing compaction: overburden, rate of subsidence,