Page 207 - Geochemistry of Oil Field Waters

194 ORIGIN OF OILFIELD WATERS

earth’s crust is about 17 km, ranging from 5 km under the oceans to about
35 km under the continents (Clark and Ringwood, 1964). Fig. 7.1 illustrates
the various regions of the interior of the earth, with the distance from the
surface of the crust to the center of the inner core being 6,731 km. In this
discussion we are concerned only with the crust to a distance of 0.08% of
the depth to the center of the earth.
Hydrocarbons are believed to have originated from organic material in
sedimentary material which was produced by weathering and erosion of the
earth’s surface. This eroded material is carried away by water, ice, or wind
and redeposited, ultimately forming sedimentary rocks. The major sedimen-
tary, minerals are clays, quartz, calcite, gypsum, anhydrite, dolomite, and
haiite. Most of the large bodies of sedimentary rocks were formed in marine
environments; smaller sedimentary deposits formed in lakebeds and river
floodplains.

Definitions of some water terms

Meteoric water. White (1957) defined it as water that was recently involved
in atmosphere circulation and further that “the age of meteoric groundwater
is slight when compared with the age of the enclosing rocks and is not more
than a small part of a geologic period.”

Sea water. The composition of sea water may vary somewhat, but in general
will have a composition relative to the following (in mg/l): chloride -
19,375, bromide - 67, sulfate - 2,712, potassium - 387, sodium - 10,760,
magnesium - 1,294, calcium - 413, and strontium - 8.
Table 7.1 (Anonymous, 1964) gives a more comprehensive picture of the
constituents found in sea water. The analyses given in Table 7.1. are in parts
per million.

Interstitial water. Interstitial water is the water contained in the small pores
of spaces between the minute grains or units of rock. Interstitial waters are:
(1) syngenetic (formed at the same time as the enclosing rocks); or (2)
epigenetic (originated by subsequent infiltration into rocks).

Connate water. The term connate implies born, produced, or originated
together, connascent. Therefore, connate water probably should be con-
sidered to be an interstitial water of syngenetic origin. White (1957) called
connate water of this definition a fossil water, i.e., water that has been out
of contact with the atmosphere for at least a large part of a geologic period.
As White (1957) pointed out the implication that connate waters are only
those “born with” the enclosing rocks is an undesirable restriction.

Diagenetic water. Diagenetic waters are those waters that have changed
chemically and physically, both before, during, and after sediment consolida-