Page 253 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 225

The theories explaining the cause of these high-pressure zones were discussed in the
previous chapters and among others by Chilingarian and Rieke (1976). A symposium
convened at the University of Kansas presented a broad summary on the geochemistry
of subsurface brines and their evolution in geologically mature sedimentary basins
(Angino and Billings, 1969). In 1990, an interdisciplinary conference was held in
England that reported on and reviewed microscopic-scale interactions of aqueous pore
fluids with clay minerals in surface and subsurface environments and contains valuable
ancillary information on the compaction chemistry of pore water (Manning et al.,
1993). The proceedings focused on the mineralogical reactions that take place, such
as authigenic formation of clay minerals, diagenesis of mudrocks in the North Sea,
diagenetic pore-fluid evolution, and the importance of chemistry of pore fluids to the
petroleum engineering and geological concerns with the mechanisms of overpressuring.
Hobson (1954) presented the macroscopic concept of fluid pressure systems to be
either open or closed in explaining the origin of abnormal formation pressures. He
proposed that an idealized closed system is one in which fluid pressures do not dissipate
readily over geologic time, whereas in an open system excess pore pressures decrease
with time. This concept can be extended to include the geochemical fluid reactions
within such pressure system models. To be academically precise, however, a subsurface
fluid system should be categorized from a thermodynamic viewpoint.

Thermodynamic and reaction models

A synopsis of thermodynamic models is in order so that laboratory experimental and
field results can be properly interpreted. The thermodynamic approach includes the fluid
system's state variables such as pressure, volume and temperature, its time-dependency
processes, and physical boundaries of the system. All these factors must be considered
with respect to geologic time and space. A brief discussion is presented here on the
validity of applying this concept.
Subsurface fluid flow systems can be described using the following thermodynamic
models. An open system is defined as one that allows the free flow of both mass and
energy across its boundaries, whereas a closed system allows only the outflow of energy
but not the mass. An isolated system would exhibit neither flow of mass nor energy from
its boundaries. The adiabatic fluid system by definition is closed to mass flow but open
to the flow of energy except for heat (there is no exchange of heat with the adjacent
bounding systems). Under this classification, a leaky fluid flow system is an open
system that only allows a very slow release of mass and energy across its boundaries
over geologic time. The state of these systems depends on the type of system and the
time dependency of the diagenetic processes. The writers realize that with geologic time
all closed and isolated systems will eventually leak.
Giles (1997) posed the following question. Is the application of thermodynamic
classification with respect to diagenesis pointless? His argument is that all burial
systems leak and consequently do not restrain the pore water and their total dissolved
solids. Diffusion will drive mass transfer of ions across the boundary of a 'closed
system'. His point raises several issues. At what scale is one going to examine the
reaction chemistry of pore waters? Scaling is very important with respect to the level of