104 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
Valley fields has migrated up from lower formations, and there is 
reason to believe that some of the variation in its composition and 
properties is the result of changes that have taken place during this 
migration. The free chemical energy of reacting substances and the 
time of contact are potent factors in chemical change, and these factors 
should be recognized in accounting for variations in the properties of 
oil that has migrated. An oil that is the first to traverse a given 
course comes in contact with reacting substances at their highest 
potential, and therefore becomes changed more radically than does 
the oil that follows it at the same rate. As soon as these reacting 
substances have become exhausted, then oil may pass them unaffected. 
Again, oil that moves with extreme slowness and remains a very long 
time in contact with reacting substances may undergo changes just as 
marked as though it had moved more rapidly in a new channel. In 
general, therefore, that portion of the oil which migrated first or 
farthest will be the most altered. Moreover, the oil nearest the sur¬ 
face or nearest the outcrop of the oil-bearing zone may be further 
altered by fresh supplies of descending sulphate waters. Therefore, 
the oil around the upper edges of the main body should as a rule be 
the most altered or, in other words, the heaviest and most asphaltic. 
This reasoning is well borne out by the variations in the gravity 
of the oil in the valley fields. In the deeper portion of the Midway- 
Sunset field, for example, the specific gravity of the oil ranges between 
0.933 and 0.875 (20° and 30° Baume), but as the outcrop is ap¬ 
proached the oil becomes heavier and most of the wells nearest the 
outcrop produce oil of specific gravity about 0.985 to 0.972 (12° to 
14° Baume). A part of this difference is probably due to the escape 
of the more volatile constituents of the oil in the zone along the out¬ 
crop, but variation in gravity several miles away from the outcrop 
can hardly be explained in this way. Furthermore, as a general rule, 
to which there are local exceptions, the highest producing oil sand 
carries heavier oil than the sands below. In portions of the Coalinga, 
Midway, and Sunset fields the producing oil sands are overlain at a 
distance of several hundred feet by the tar-sand zone, which contains 
sands partly impregnated with a very heavy, viscous tar. Some of 
the sands in this zone carry water of the modified type (sulphur 
water), and most of the tar sands become water bearing farther down 
the dip. So far as the writer knows this very heavy tar has never 
been analyzed, but the analyses of several samples of oil of 0.993 • 
(11° Baume) gravity show about 1.15 per cent of sulphur, and it 
may be presumed that the tar carries at least as much as this. In a 
general way the tar-sand zone marks the farthest limit of migration 
of the oil. It has been observed that oil which has migrated for 
some distance into sands that lie in angular unconformity with the 
