Page 263 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 235

(1) Bicarbonate-sodium type
(a) Class At: primary alkalinity predominates (alkali carbonates and bicarbonates).
(b) Class A2: secondary alkalinity predominates (alkaline earth carbonates and
bicarbonates).
(c) Class St: primary salinity predominates (alkali chlorides and sulfates).

(2) Sulfate-sodium, chloride-magnesium, and chloride-calcium types
(a) Class A2: secondary alkalinity predominates (alkaline earth carbonates and
bicarbonates).
(b) Class St: primary salinity predominates (alkali sulfates and chlorides).
(c) Class $2: secondary salinity predominates (alkaline earth sulfates and chlorides).

CHEMICAL COMPOSITION OF SUBSURFACE BRINES
Petroleum engineers, geochemists, hydrologists, well-log analysts, sedimentologists,
and water chemists all have an interest in classifying water based on its chemical compo-
sition, physical properties, origin, or association with diagenetic processes. Collins (1975)
discussed his compilation of chemical and physical analysis of oilfield brines occurring in
various formations and producing oil and gas reservoirs in the U.S. Geochemists such as
Ortoleva (1994), Bethke (1996) and Giles (1997) approach the problem by employing dif-
ferent strategies. Ortoleva looked at the geochemical self-organization in overpressuring
and compartmentalization in sediments. Giles focused on the resolution of geochemistry
theory with basin modeling aspects. Bethke's methodology is concerned with the analyses
of open and closed fluid systems using computational geochemical reaction modeling.
His reaction model considers the transfer of mass and heat in and out of a system having
an aqueous fluid and one or more minerals, and can accommodate a buffer (an external
gas reservoir) in order to calculate the system's equilibrium state. The reaction path is
determined by the course the equilibrium state takes as it responds to changes in composi-
tion and temperature. Changes in the equilibrium system are audited, thereby monitoring
the reactants (minerals and fluids) influence on the system composition.
How does this fit in with the present research trend on the fluid chemistry relation-
ships in compacting pelitic sediments? Hunt et al. (1998) believe that the cessation
of compaction does not appear to be related to overpressuring, but is a phenomenon
that occurs with hydrostatic-pressured shales. This means that the two-stage, linear
compaction is a normal compaction trend (see section on Field Case Studies). At depths
where compaction no longer occurs, gas generation seems to be the major cause of
overpressures. Now we have all the reaction modeling ingredients (seawater, smectite,
smectite/illite mixed interlayer clays, and illite, and a gas reservoir) needed to explore
Hunts et al.'s premise, and to see if their model matches field results. This could confirm
whether or not the pore waters in shales should be fresher than those in associated
sandstones, and confirm the origin of the fresh water in the overpressure zones. The
alternative to Hunt et al.'s hypothesis is the study by Burrus (1998) on stress-porosity,
using the TEMISPACK finite volume model, showing that compaction disequilibrium is
the dominant cause of overpressures.