Page 316 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 285

SUMMARY AND CONCLUSIONS

A study of variations in the chemical composition of subsurface brines has revealed
that salinity generally increases with depth in mature sedimentary sequences. Salinities
many times greater than that of seawater are frequently encountered in permeable sand-
stones, but are higher than the salinity of water in associated shales. This applies mainly
to geologically older sedimentary basins with hydrostatic fluid pressures. In immature
sedimentary basins, where abnormally high formation pore pressures are known to oc-
cur, the difference in the magnitude of salinity between the sandstone and shale waters
is greatly reduced. In any case, the salinity of pore water in well-compacted shales is
often lower than that found in undercompacted shales at comparable depths. Figs. 10-33
and 34 are conceptual models that reveal why pore water in the undercompacted shales
is more saline than in the well-compacted shales. The overprinting of fresh water from
the dehydration of smectite and from the associated salt deposits can lower or increase
the concentrations, respectively. These factors must be considered in using the resistivity
and SP well logs to evaluate abnormal pressure zones.
The fact that interstitial waters in shales is fresher than that in associated sandstones
is further confirmed by the results presented in Table 10-10 and Fig. 10-37. Table 10-10
illustrates pore-fluid freshening by the percentage increase in resistivity with increasing
sample pressure for solutions squeezed out of a fresh marine mud from the Santa Cruz
Basin, offshore southern California (Rieke et al., 1964). Fig. 10-37 presents the chemical
results, which show that both the expelled anions and the cations decrease at about the
same rate with increasing pressure at ambient temperature. The results show that the
ions being removed represent the interstitial electrolyte solution and do not include the
adsorbed cations, and suggest that an analysis for a single ion such as CI- might reveal
nearly as much as the analyses of all the ions. These findings support those of Kryukov
et al. (1962, p. 1365) that the mineralization of interstitial solutions in shales is lower
than that of waters present in the associated sandstones. Water of dehydration (fresh
water) from clay conversion accentuates this difference. This water can be trapped at the
top of overpressure zones by seals or moves into the associated sand bodies. Among the
major dissolved constituents of brines, in normally pressured formations, it is the CI-,

TABLE 10-10
The percentage increase in the resistivity of solutions squeezed out of marine mud with increasing
overburden pressure (after Rieke and Chilingarian, 1974, table 2, p. 240)
Overburden pressure Increase in resistivity (%), as compared to resistivity of solution squeezed out
(psi) at 500 psi
1,000 2.3-6.5
2,000 3.5-15.2
3,000 10.5-19.6
7,000 16.3-32.0
14,000 18.6-37.0
30,000 23.2-45.6
40,000 25.6-48.0