Page 98 - Petroleum Geology

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than the solids because of its association with petroleum. Geology that ig-
nores the water in the rocks is largely meaningless: one can describe rocks
without considering their water, but one cannot understand them.
All water in the rocks contains some dissolved solids. These solids, in
terms of their dissociated ions, are:

calcium Ca’’ \ carbonate C03’- )
bicarbonate HC03-
cations
sodium Na’ sulphate SO4’ -
magnesium MgZ +
potassium K+ I chloride CI-
There is a long list of minor constituents that includes iron, aluminium,
boron, fluoride, copper, silver, tin and vanadium (the last also occurs in oil
shales and in association with petroleum).
The chemistry of ground water, like that of petroleum, is a subject in it-
self. For general purposes, however, the geologist needs to know the salinity
of the water in a particular rock unit, usually expressed in parts per million
(p.p.m.) total dissolved solids. For electrical log interpretation, this may be
converted to NuCl equivalent - the solution of sodium chloride that would
have the same electrical conductivity as the more complex solution.
The salinity of surface water varies from almost nil in many rivers and
lakes, to that of sea water, which is about 35,000 p.p.m. total dissolved
solids. In the subsurface, however, salinities vary from very low in fresh-
water aquifers to well over that of sea water. Not only does the salinity
commonly differ significantly from that of sea water, but also the ionic
composition. The relative proportions of the ions vary greatly. Hence it is an
unjustifiable over-simplification to regard formation water as sea water
buried with the rocks. Formation waters commonly contain 80,000--100,000
p.p.m. total dissolved solids; and salinities around 300,000 p.p.m. are
known.
Variations in ionic composition (as we have noted) are considerable, and
rarely reflect the composition of sea water. Sulphate is significantly low in
most oil-field brines, but commonly high near base metal sulphide deposits.
Carbonate and bicarbonate are unimportant in sea water and usually unim-
portant in oil-field brines; but they are relatively important in many fresh
waters. However, it must be remembered that the great bulk of formation
water analyses comes from water associated with petroleum in the subsur-
face, and may not therefore be representative.
It is pointless to speculate on the origin of the salinity of formation water
at this stage. Fresh ground water near the surface may be regarded as meteoric
(rain water that has infiltrated and percolated into the rocks), and much of
the water of artesian aquifers (when fresh or brackish) may also be regarded
as meteoric. Interstitial water that is regarded as original, in the sense that it
was the medium in which the sediment accumulated, is called connate -