CHEMICAL RELATIONS BETWEEN WATER AND HYDROCARBONS. 105 
main oil zone generally becomes heavier with distance from the main 
body and finally passes to tar. Purely on the basis of field evidence, 
therefore, it would appear that among the important factors influ¬ 
encing the gravity of the oil are the distance that the oil has migrated, 
its present distance from the outcrop or its depth below the surface, 
and the extent to which it is or has been subject to contact with 
waters, especially meteoric (sulphate) waters. 
There seems little doubt therefore that the local variations in the 
character of the oil are in part, at least, due to the action of sulphur 
and probably also of oxygen. Sulphate waters descending from the 
surface are to a large extent altered in the zone of tar sands, and the 
tar itself is thereby rendered still more asphaltic. To some extent, 
therefore, the tar sands may be conceived as protecting the main 
body of oil in the sands below. The same process goes on near the 
outcrop, although how far descending meteoric waters have affected 
the main body of the oil since it attained its present position is a 
matter of conjecture. It seems more reasonable to suppose that the 
oil which migrated first and farthest was considerably altered by the 
water that had previously occupied the sands, and that most of its 
alteration took place before it had come to rest in its present position. 
SOLUBILITY OF PETROLEUM CONSTITUENTS IN WATER. 
Certain minor constituents of petroleum or natural gas are soluble 
in water and have been observed in oil-field waters from several 
regions. Aside from the scientific interest that attaches to these 
substances they may in some places be of practical use, for their 
presence in a water has been taken by some to indicate that the 
water has been closely associated with oil or gas. The petroleum 
constituents soluble in water are of two kinds—simple light hydro¬ 
carbons, such as methane, and complex hydrocarbon derivatives, such 
as the naphthenic acids. 
The solubilities of some of the simple gaseous hydrocarbons have 
been investigated, with the results shown in the following table. 
The solubility decreases with increasing temperature, and at 40° C. 
(104° F.) may be only two-thirds to three-fourths of the solubility 
at 20° C. (68° F.). The solubility is generally decreased by the 
presence of mineral salts in the water but is increased to some extent 
by pressure. 
