28 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
way, tho productive district lies on the Hanks of three folds, but the 
structure is complicated by overturned folds and thrust faults. 
The circulation of the shallower waters is apparently controlled 
chiefly by the topography, these waters being more abundant and 
more highly mineralized in the McKittrick Valley than in the foot¬ 
hills to the west. The deeper circulation, however, is undoubtedly 
affected by the complex geologic structure, and little can be said of 
its course or nature until more complete information is available. 
The oil-bearing zone is generally underlain by a sand carrying warm 
water under considerable pressure. In part of the field the oil sands 
have been flooded, and it is generally believed that the damage had 
been done by this water. Whatever its original source, the water 
that has flooded the oil zone is very uniform in chemical composition 
and is probably the yield of a single water sand. Little variation is 
shown by about 30 analyses of this water at the writer’s disposal, 
and analysis 89 (Table 14, p. 85) is entirely representative. 
MIDWAY-SUNSET FIELD. 
The Midway and Sunset fields are arbitrary divisions of a great oil- 
producing area, and as the geology of both fields is similar they may 
best be described together. The developed area extends west from 
the east slope of the Buena Vista anticline across the Midway syn¬ 
cline and up on its western limb, where the oil-bearing strata outcrop. 
The western limb of the trough is not a regular slope as in the West- 
side CoaJinga field but is broken by several minor anticlines, all pitch¬ 
ing to the southeast. 
The ground-water level in the Midway-Sunset field is lower than 
in the Coalinga field and is generally 250 to 500 feet below the sur¬ 
face. Along the hilly western part of the field the shallower water 
is almost lacking and in some localities is not abundant even to the 
depth of the oil zone. Generally, however, some water is found near 
the tar sands and also between them and the oil. At a variable depth 
below the oil zone, usually less than a hundred feet, abundant sup¬ 
plies of warm water under considerable pressure are found. Owing 
to the scarcity of shallow water this “bottom water” is widely used 
for industrial purposes in this part of the field. 
In the Midway Valley and the Buena Vista Hills, which are the 
surface reflections of a syncline and an anticline, respectively, the 
water conditions are somewhat different. In the valley moderate 
supplies of water may generally be obtained at a depth of 200 to 
500 feet, but in many localities even the shallower water contains 
a remarkably high percentage of sodium chloride. In the Buena 
Vista Hills the water level is generally lower, but the water is simi¬ 
larly salty. The percentage of salt increases with depth, but at a 
variable rate; in some localities water from 700 feet is almost fresh 
