COMPOSITION OP THE OIL-FIELD WATERS. 
89 
small amount of earths and carbonate can be retained m solution, 
even in the presence of an excess of carbon dioxide, and if this amount 
is exceeded alkaline earth carbonates will be formed and then lost 
through precipitation. In a concentrated solution like sea water the 
maximum value of alkaline earths balanced by carbonate that can 
be retained represents only a very small percentage of the total con¬ 
centration value, and if the sulphate is entirely replaced by carbon¬ 
ate a large proportion of this carbonate, with an equivalent value 
of earths, will be precipitated. 1 The effect on the properties of reac¬ 
tion of this substitution and consequent loss therefore consists in a 
decrease in secondary salinity and an increase in secondary alkalinity 
limited by the solubility of the alkaline-earth carbonates. Sea water is 
characterized by 21.1 per cent of secondary salinity, of which 9.2 per 
cent is contributed by sulphate; hence, the removal of the sulphate 
will effect a substantial reduction in the secondary salinity. The 
increase in secondary alkalinity, however, will be slight, so that if 
the properties are expressed in percentages the value of primary 
salinity will be raised. If it be assumed, first, that the sulphate in 
sea water is entirely removed and an equal value of carbonate intro¬ 
duced, and secondly, that 7.6 per cent, or 2,660 parts per million of 
alkaline-earth carbonates are lost through precipitation, the proper¬ 
ties will change as follows: 
Changes in properties of sea water through substitution of carbonate for sulphate. 
Primary 
salinity. 
Secondary 
salinity. 
Secondary 
alkalinity. 
Concentra¬ 
tion. 
Normal sea water. 
78.6 
21.1 
0.3 
35,000 
Sea water in which all SOUs calculated as CO 3 . 
78.6 
11.8 
9.6 
35,000 
Same after precipitation of assumed excess of alkaline 
fiart.h carbons,tos _ . 
85.1 
12.8 
2.1 
32,340 
Since these reactions take place after the water has been entrapped 
in the sediments they are of course complicated by many indeter¬ 
minable factors, and any attempt to reduce them to exact figures must 
be regarded as speculative. There is little doubt, however, that 
the oil-field brines have been derived from connate water by the 
general processes outlined above, as shown by their properties of 
reaction. (See Tables 8 and 9.) 
The changes involved in the alteration of a meteoric water are 
similar but more extensive. The chief constituents of the surface 
and shallow ground waters of the fields of the west side are sulphate, 
- alkalies, and alkaline earths. (See Tables 1 to 4.) If carbonate 
is substituted for the sulphate in these waters, it may be pre- 
1 Murray, John, and Irvine, Robert, On the chemical changes which take place in the composition of the 
sea water associated with blue muds on the floor of the ocean: Roy. Soc. Edinburgh Trans., vol. 37, p. 481, 
1892-93. 
