Page 92 - Petroleum Geology

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DENSITY
0.95 g cm-3 1
I 1

1

5000

2

km ft
1-05 cm3g-1 1
SPECIFIC VOLUME
Fig. 4-2. Density and specific volume of fresh water in the subsurface at normal hydrostatic
pressures with geothermal gradients of 26 and 36OC/km (14.3OF and 19.9°F/1000 ft).

ship in the table of Fig. 4-1 is that between water salinity and water density.
Two conclusions are evident: associations must be searched and researched
for causal relationships, and the significant departures from normal associa-
tions must be studied. Abnormality must not be dismissed as a freak, for in
abnormality (as we shall see) may lie the clues to the true causal relation-
ships.
For example, in some oil fields of eastern Venezuela and Indonesia the
density of the oil in successive reservoirs increases with depth, or a trend of
decreasing density is reversed. It is also observed in some of these fields that
the salinity of the associated interstitial water decreases as the density of the
oil increases. Water salinity and oil density are apparently closely related -
but again, closeness of relationship is not necessarily to be interpreted as a
causal relationship, and causal relationships are necessary for understanding.
These reversals indicate that temperature is not the dominant influence. The
real association is probably environmental, affecting both the source ma-
terial of the oil and the salinity of the water; these anomalously heavy oils
may be generated from organic matter associated with a terrestrial environ-
ment, with fresher water. Even in the U.S. Gulf Coast, where the progressive
decrease of mean density with depth has long been recognized (Barton, 1934)