Page 256 - Origin and Prediction of Abnormal Formation Pressures

228 H.H. RIEKE, G.V. CHILINGAR AND J.O. ROBERTSON JR.

TABLE 10-1
Chemical composition of seawater, bottom water, and average concentrations of pore water squeezed out
from ocean sediments expressed in g/kg

Ions Seawater Ocean sediments
Normal Red Sea Terrigenous clay Pelagic clay Hemipelagic Red Sea
(>3 cm/10 yr) (< 1 cm/10 yr) bottom burial depth marl, burial
water 223-243 m depth 82 m
Na + 10.5 11.8 10.80 10.80 10.8 11.1 28.7
K + 0.4 0.43 0.30 0.38 0.38 0.52 0.22
Ca 2+ 0.4 0.46 0.40 0.42 0.41 0.41 0.98
Mg 2+ 1.3 1.42 1.08 1.25 1.26 1.18 0.84
C1- 19.1 21.4 19.4 19.5 19.2 19.5 46.1
SO 2- 2.7 3.06 1.05 2.45 2.7 2.7 3.57
HCO 3 0.17 0.15 0.45 0.20 - - 0.15
Data for normal seawater from Goldberg et al. (1971); all other data modified after Manheim (1976). In
Chilingarian et al., 1994, table 5-1, p. 108.

deposition at < 1 cm/1000 years were the only sediments which exhibited low chemical
reactions with the contained pore waters. At shallow burial depths there is not much
change in the pore-water chemistry except when influenced by underlying salt beds. An
example of salt bed influences is illustrated by the chemical composition of the pore
water extracted from Red Sea marl (Table 10-1).
One way of visualizing the changes in pore-water chemistry is presented in Fig. 10-2.
This lumped-parameter analysis illustrates the change in chemistry of oilfield waters
with depth in eight sedimentary basins representing ten different formations. Similar
types of pore-water patterns were shown by Hanor (1987a), using data from Graf et al.
(1966) for the Paleozoic Illinois Basin (U.S.A.). Pore-fluid diagenesis is the term that
was used to denote these changes. Sayles and Manheim (1975) stated that reactions
undetectable in the solid components have a large and readily measurable effect upon
the pore waters.
The geodynamic aspect of the origin of sedimentary basins has a direct influence on
the diagenetic changes that pore waters undergo after deposition. Sedimentary basins
develop in different tectonic settings as shown in Fig. 10-3. The mechanisms involved
in their formation are probably as poorly understood as the diagenesis of their pore
fluids. The writers use this basin classification system to tag each basin and field
example given in this chapter. Contradictions in the geochemical pore-water data from
various basins can be explained by the basin's type and its degree of geologic maturity.
In this chapter, the term delta basin is used to describe those Tertiary to Recent young
depositional sites where present-day deposition is still taking place. A good example in
the United States is the Mississippi delta basin, which is adjacent to, and is considered a
part of, the Gulf Coast crustal collision zone-closed convergent plate margin age basin.
These basins can be described as being immature and forming along the continental
margins, versus those mature basins which are geologically older and now exist directly