68 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
a water definitely reported to occur at 4,022 feet, or near the base 
of the main oil measures. It will be noted that this analysis shows 
only about half as much sulphate as analyses 37, 38, and 39. 
Analyses 35, 36, and 40 contain an even lower proportion of 
chloride than analysis 42 and represent the extreme of the reversed 
type. These waters occur on the east flank of the anticline, farther 
north and in shallower territory than those discussed above. Ap¬ 
parently in this locality the connate water has been entirely drained 
away and replaced by meteoric water. In Nos. 36 and 40 primary 
salinity is only 1.4 per cent, and these waters are therefore practically 
pure solutions of sodium carbonate. No. 36 contains no sulphate 
• whatever, but a small amount of the hydrogen sulphide formed by 
the reduction of the sulphate that it formerly contained is still 
present. In Nos. 35 and 40 the sulphate seems high when com¬ 
pared with the chloride, but is negligible in comparison with the 
carbonate. Analysis 41 represents a water similar to No. 40, but 
characterized by higher salinity, both chloride and sulphate. Sul¬ 
phate is relatively high, and the presence of hydrogen sulphide pre¬ 
sumably indicates that alteration is not complete. 
Analysis 43 represents a water similar to No. 41, but occurring 
under very different conditions in the Westside Coalinga field 
This water was obtained from a well drilled to a depth of only 410 feet 
(probably in the brown shale, Oligocene ?) a mile west of the developed 
oil field. The strata dip steeply in this locality and the water sand 
outcrops not far west of the well. The occurrence of an almost 
completely altered water so close to the surface is very unusual, but 
it is known that small quantities of oil or gas are disseminated 
through the formation, and there is said to be an oil seep within a 
mile of the well. Furthermore, only a very small supply of this 
water was found and the action of reducing agents would therefore 
have been localized. Ordinarily, however, the water obtained from 
the brown shale in similar relative locations is only partly modified. 
Analyses 44 and 45 represent waters of the reversed type from the 
northern part of the Midway field. No. 44 was obtained from a 
depth of 3,860 feet or probably several thousand feet below the 
main oil measures, but it is reported that a “stray” oil sand was 
found just below the water. It is said that the production of this 
well increases perceptibly after the rains, which is not inconsistent 
with the fact that the water is chiefly of meteoric origin. No. 45 is 
a similar type of water, though it contains an even smaller propor¬ 
tion of chloride and is less concentrated. It occurs above the oil 
measures and probably in the zone of tar sands. These waters are very 
different from those found 2 miles to the southeast and described in 
the following section. 
