Hydrocarbons in molecular solution

The solubility of individual hydrocarbons in pure water at 25 °C, as summarized by Tissot and Welte (1984) is given in Table 3.3. Table 3.3 indicates that there is a marked decrease in solubility with an increase in molecular weight (carbon number) for each class of hydrocarbons (alkanes, cyclo-alkanes, aromatics). It also shows that aromatics are more soluble than alkanes for a given carbon number. Polar heterocompounds (organic acids or alcohols) are more water soluble than the corresponding hydrocarbon with the same carbon number (Tissot and Welte, 1984). The hydrocarbon solubilities in water are influenced by temperature, salinity and pressure. 

Gas and liquid hydrocarbon solubilities decrease with increasing water salinity (Price, 1976). 

Temperature and salinity changes have smaller effects on solubility of natural gas in water than that produced by pressure changes. A solubility study by Price (as quoted by Price, 1976) indicated that the presence of gas in solution greatly increases the solubility of crude oil in pure water at temperatures above 250 C to 300 C. 

Some additional geochemical and geohydrological observations are given below in order to answer the question of whether it is likely that hydrocarbon transport in solution can be considered as a primary migration mechanism. 

As was shown in Table 3.3, in general, the liquid hydrocarbons have a very low solubility in water. However, in the zone of peak oil formation, the groundwater in the source rocks will become saturated with hydrocarbons. 

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