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The aqueous solubilities of individual hydrocarbons, petroleums, and petroleum fractions increase with increasing temperatures. The rate of solubility increase is uniform from room temperature to about 100°C, after which the rate of increase rises markedly. At temperatures above 150°C, solubilities are high enough to account for the transfer of significant quantities of dissolved hydrocarbons in geologic systems.
Salinities of 350,000 ppm NaCl concentration reduce the aqueous solubilities of individual gasoline-range hydrocarbons to 5-7% of their solubilities in fresh water. A temperature drop from 150 to 25°C reduces the aqueous solubility of a whole petroleum by a factor of 4.5-20.5. Thus, the pronounced decrease in solubility of petroleums at higher salinities and lower temperatures encountered at shallow depths readily serves to release dissolved hydrocarbons during the upward movement of subsurface waters.
Other investigators have shown that 15-20% water, by volume, remains in Gulf Coast argillaceous sediments at depths below 14,000-18,000 ft. This quantity of water is sufficient to account for the primary migration of petroleum from source rocks by molecular solution.
Faults are believed to provide the pathways for vertical movement of water and dissolved hydrocarbons from great depth. Eventually the fluids are focused into sands when the fault becomes impermeable to further fluid movement. Field examples are present in the Gulf Coast and Los Angeles basin. This mechanism is restricted to argillaceous basins containing high concentrations of expandable mixed-layer clays that are buried to a minimal depth of 20,000 ft. It does not apply to carbonates or argillaceous limestones in which primary migration can occur at much shallower depths.
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