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The solubility of a whole crude oil (44° API) in methane has been measured (with water present) at temperatures of 50° to 250°C and pressures of 740 to 14,852 psi, as have the solubilities of two high molecular weight petroleum distillation fractions at temperatures of 50° to 250°C and pressures of 4,482 to 25,266 psi. This was done to evaluate the gaseous solution mechanism for primary petroleum migration. Both increases in pressure and temperature increase the solubility of crude oil and petroleum distillation fractions in methane, the effect of pressure being greater than that of temperature.
The data of this study, compared to previous work in dry systems, show that the presence of water in the system greatly increases the solubility of crude oil in methane. The presence of water also drastically lowers the temperature and pressure conditions required for cosolubility.
Qualitative analyses of the crude oil solute samples showed that with increases in temperature and especially pressure, the solute condensate became essentially identical to the original crude oil. The n-paraffin distributions (as well as overall composition) of the solute condensates are controlled by the temperature and pressure of solution and exsolution, as well as by the composition of the original starting material. It appears possible that primary migration of gaseous solution could "strip" a source rock of crude oil-like components leaving behind a bitumen totally unlike the migrated crude oil. The data of this study demonstrate that previous criticisms of primary petroleum migration by gaseous solution are invalid: that primary migration by gaseous solution cannot occur based on the inadequacy of methane to dissolve sufficient volumes of crude oil or to dissolve the highest molecular weight components of petroleum (tars and asphaltenes).
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