CLASSIFICATION OF THE OIL-FIELD WATERS. 43 
and in this type of water the earths constitute as much as 10 per 
cent of the total mineral content. 
In sea water the ratio of magnesium to calcium by weight is about 
3, or if expressed in terms of reacting values, about 5. It is therefore 
interesting to note that in that type of oil-field water which the writer 
considers connate, and which closely resembles sea water in all other 
respects, the average ratio of the reacting values of magnesium to 
calcium is only 0.71. In some of the surface waters and shallower 
ground waters, especially those in the Coalinga field, the ratio is 
almost as high as in sea water, hut this is accounted for by the fact 
that these waters are a part of the drainage of areas in which ferro- 
magnesian (Jurassic) rocks outcrop. The deficiency of magnesium 
in the deeper waters can not be so easily explained. At high tem¬ 
peratures magnesium is deposited from solution as basic carbonate or 
hydrate, 1 and it is possible that the temperatures of 100° to 125° F., 
which prevail in the oil fields at depths of 1,000 to 3,000 feet, have 
had some effect in lowering the concentration of magnesium. It is 
also possible that the magnesium is removed as magnesium silicate, 
though no evidence was obtained in the field to support this view. 
In the table on page 92 are given the average ratios of magnesium 
to calcium in the several types of water in the Coalinga, Midway, and 
Sunset fields, only the analyses included in this report being repre¬ 
sented. The variations in these ratios are discussed in more detail 
below, but it may be pointed out here that the average ratio in the 
Coalinga field is considerably higher than in the Midway and Sunset 
fields, and that the ratio in the brine is only about half as great as in 
the other types of water. 
Sulphate .—In the normal ground waters everywhere on the west 
side of San Joaquin Valley sulphate is the predominating acid radicle, 
especially near the surface. In some of the shallow waters from the 
Coalinga field sulphate and its equivalent basic radicles constitute 
over 85 per cent of the total mineral content. Outside of the oil 
fields most of the deeper ground waters are also characterized by a 
high concentration of sulphate, but this figure bears no constant rela¬ 
tion to depth. Within the oil fields, however, sulphate decreases 
with increasing depth and practically disappears at a certain distance 
above the oil zone. Many of the waters associated with the oil do 
not contain even a trace of sulphate, and most of them carry less 
than 1 per cent, although in the Eastside Coalinga field sulphate is 
usually higher and some of the waters may contain as much as 5 per 
cent. The tables of analyses below show the ratio of sulphate to 
sulphate plus chloride by reacting value in all the waters discussed, 
and in many of these waters the ratio of sulphate to carbonate as well. 
i Davis, W. A., Studies of basic carbonates: Soc. Chem. Ind. Jour., vol. 25, p. 796,1906. 
