CHEMICAL RELATIONS BETWEEN WATER AND HYDROCARBONS. 99 
phate, but this observation merely indicates that the two changes are 
the result of the same process and does not throw much light on the 
proportions involved. In the transition from normal to altered 
waters in the oil fields the increase in carbonate is roughly propor¬ 
tional to the decrease in sulphate, but the loss by precipitation of 
alkaline-earth carbonates prevents the deduction of exact figures. 
The assumption made on page 88 that for the value of sulphate re¬ 
moved from the water an equivalent value of carbonate is intro¬ 
duced, is perhaps the best that can be made at the present time and 
is fairly adequate if the water alone is considered. As a matter of 
fact, however, the waters in the zone of alteration contain sufficient 
half-bound carbon dioxide to allow the formation of bicarbonate 
almost exclusively, and in addition many of these waters contain 
considerable free carbon dioxide. The amount of free and half-bound 
carbon dioxide in the zone of alteration seems disproportionately large 
in relation to the amount of hydrogen sulphide, even if the ready 
oxidation of hydrogen sulphide is taken into account. 
The apparent disparity between the total amount of carbon 
dioxide formed and the amount of sulphate removed may be due to 
the fact that all the carbon dioxide is not derived from the oxidation 
of hydrocarbons. Several reactions are known by which carbonate 
may be derived from inorganic sources. Hilgard 1 2 finds that a 
solution containing free carbon dioxide in the presence of sodium 
sulphate dissolves calcium carbonate and forms sodium bicarbonate 
and a precipitate of gypsum. This reaction would partly explain 
the disappearance of sulphate and the formation of carbonate, but 
it would not account for the formation of hydrogen sulphide. If it 
is assumed, however, that the hydrogen sulphide is derived through 
the reduction of sulphate, the presence of free carbon dioxide may 
be explained by the following reaction, first investigated by Bechamp : 3 
CaC0 3 + 2H 2 S = Ca(SH) 2 + H 2 0 + C0 2 . 
Under other conditions hydrogen sulphide may unite with calcium 
carbonate to form calcium sulphate and sulphur. 3 It is evident, 
therefore, that the disappearance of sulphate and the formation of 
carbonate may be the net result of several reactions. As the strata 
in the oil fields of the San Joaquin Valley do not contain much 
calcium carbonate, the reactions just discussed have probably not 
entered largely into the development of the chemical character of 
the waters, but the possibility that they have played some part 
should be duly considered. 
1 Hilgard, E. W., The geologic efficacy of alkali carbonate solution: Am. Jour. Sci., 4th ser., vol. 2, pp. 
100-107,1896. 
2 Bechamp, A., Recherches sur l’etat du soufre dans les eaux minerales sulfurees: Annales ctnmie et 
phys., 4th ser., vol. 16, p. 234,1869. 
3 Spezia, G., Sull’ origine del solfo nei giacimenti solfiferi della Sicilia, 1 orino, 1892. 
