Page 261 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 233

TABLE 10-2
An overview of proposed mechanisms to account for the origin of chemical differences in subsurface pore
waters (after Chilingarian et al., 1994, table 5-2, p. 111)

Reference Mechanism(s)
Washburne (1914) Subsurface evaporation and juvenile water additions.
Richardson (1917) Leaching of disseminated salt and salt diffusion from salt bed.
White (1957) Burial diagenesis of seawater.
Berry (1959) Chemical osmosis.
Chave (1960) Trapped remnants of seawater moved by sediment compaction.
Von Engelhardt and Gaida (1963) Ion exchange capacity of clays under compaction.
Bredehoeft et al. (1963) Membrane filtration by clays.
Powers (1967) Alteration of smectite to illite during deep burial.
Serruya et al. (1967) Electrical potentials (electrodiagenesis).
Mangelsdorf et al. (1969) Molecular settling.
Hitchon et al. (1971) Trapped pore water diluted by fresh water recharge and
concentration by clay membrane filtration.
Sayles and Manheim (1975) Interaction between sediments and water contained in their pore
spaces.
Carpenter (1978) Infiltration of subaerial brine.
Stoessell and Moore (1983) Salt related brines diluted by mixing with seawater.
Hanor (1987b) Thermohaline overturn of pore water.

(1970) confirmed these effects. Sayles and Manheim (1975) stated that temperature is
the most single significant factor affecting the composition of pore waters.
With respect to laboratory-simulated compaction studies, problems arise from (1) the
magnitude of pressure used to squeeze out the pore water (chemistry of water changes
with pressure), (2) analytical techniques involving minute amounts of squeezed-out pore
waters, (3) specimen preparation, and (4) contamination involved in collecting the pore
water.

Palmer and Sulin water classifications
A concise discussion of Palmer and Sulin's water classification methodologies is in
order so the reader can quickly comprehend the meaning, formulate relationships, and
categorize subsurface water chemistry results based on these classification schemes.
Both schemes could be useful in making comparisons with published subsurface water
analysis data. The foundation for such analytical schemes is the composition of the
seawater system, i.e., contents of (Na +, K +, Ca 2+, Mg 2+, CI-, SO 2-, and CO 2-) in
H20. Schoeller's system for the most part addresses petroleum-reservoir waters and the
reader is referred to Schoeller (1955) and Collins (1975) for details.

Palmer's classification
Palmer (1911) devised his water classification system based on the chemical salinity
(salts of strong acids) and alkalinity (salts of weak acids). Briefly, the concept of the
chemical salinity is that all cations (positive ions) and certain anions (negative ions),
such as chloride, sulfate, and nitrate, can cause salinity. Alkalinity depends on the