Page 264 - Origin and Prediction of Abnormal Formation Pressures

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

Pore waters can be classified based on their origin as (1) syngenetic (formed at the
same time as the enclosing rocks), and (2) epigenetic (owing their origin to subsequent
infiltration of meteoric and other waters into already formed rocks). The main processes,
which alter the chemistry of buried waters, are: (1) physical (gravitational compaction);
(2) chemical (reactions involving minerals, organic matter and interstitial solutions); (3)
physicochemical (filtration through charged-net clay membranes, adsorption and base
exchange); (4) electrochemical; and (5) biochemical.
Data provided by Hanor (1981) illustrate that the geochemical properties of fluids
being expelled from recently deposited sediments of the Mississippi River delta undergo
a compositional change. He attributes these changes to early diagenetic processes
of bacterial respiration, mineral precipitation, and possible fractionation due to the
presence of clays having high exchange capacity.
Changes in the concentration of pore-water fluids during the process of compaction,
as reported by different investigators and presented in the following section, are based
on field and laboratory data. Conceptual models relating the results to gravitational
compaction and the generation of overpressures are also presented.

Salinity variations in compacting sandstones and associated shales
Much of the available data on the composition of oilfield brines pertains to water
from permeable formations and only in a few instances are data on the composition of
pore water from associated shale beds are reported in the literature. De Sitter (1947)
noted that the salinity of formation waters in sandstones varies from that of fresh water
to ten times the salinity of seawater.
The distribution of salinity of pore water present in the young geosynclinal sediments
(recent deposition in the crustal collision zone-closed convergent plate margin) along
the U.S. Gulf Coast is well documented by a number of investigators. Timm and
Maricelli (1953, p. 394) stated that high salinities up to 4.5 times that of normal
seawater characterize the pore waters in Miocene/Pliocene sediments. Where the
relative quantity of shale is large and the degree of compaction is high, pore waters have
salinities as low as one-half that of normal seawater. Fig. 10-4 illustrates their concept
that the formation waters in downdip, interfingering, marine sandstone members, have
lower salinities than that of seawater. These sandstones have proportionately less volume
than the associated massive shales. More massive sands updip have salinities greater
than that of seawater, because of lack of influx of fresher waters from shales.
Myers (1963) studied the chemical properties of formation waters, down to a depth of
12,400 ft (3780 m), in four producing oil wells in Matagorda County, Texas. Salinities
of pore waters ranging from 5000 ppm to 12,500 ppm were found below 10,000 ft (3048
m) in each of the four wells, as compared to salinities of about 70,000 ppm above that
depth. Myers commented that in the deeper section the proportion of massive shale is
large and the sands are near their downdip limits. These results were in close accord
with those of Timm and Maricelli (1953).
Kharaka et al.'s (1977) study of the geochemistry of geopressured geothermal waters
from the Frio Clay in the Texas Gulf Coast indicated that the salinity (total dissolved
solids) of water in the geopressured zone ranged from 20,000 to 70,000 mg/1. Water