Aqueous Solubility of Petroleum as Applied to Its Origin and Primary Migration

Abstract

The aqueous solubilities of individual hydrocarbons, petroleums, and petroleum fractions increase with increasing temperatures as determined by solubility data gathered in this study. The rate of solubility increase is uniform from room temperature to about 100°C (212°F) where it increases drastically. At temperatures in excess of 180°C (356°F) petroleum solubilities in water are high enough to account for the formation of petroleum deposits by a molecular-solution primary-migration mechanism. For example, the aqueous solubility of the gas-oil fraction (boiling range 316-371°C, 601-700°F) of the Ghawar Arabian crude oil shows over a 400-fold increase from 25°C (77°F) to 180°C (356°F) from 0.042 ppm to 17.0 ppm. Salinities of over 150,000 ppm NaCl concentration cause drastic exsolution of hydrocarbon from the aqueous phase. Salinities of 350,000 ppm almost totally salt hydrocarbons out of aqueous solution (93-95 percent reduction in solubility). Thus, the pronounced decrease in solubility of petroleums at the lower temperatures at shallower basin depths and from the movement of fresh shale waters into salty sand waters both readily serve to release dissolved hydrocarbons during the upward movement of deep basinal waters. Other investigators have shown that 15-20 percent water, by volume, remains in the clastic sediments at depths below 14,000-18,000 ft (4.27-5.49 km) in the Gulf Coast and other basins. This quantity of water as shown by mass-balance calculations is sufficient to account for the primary migration of petroleum from source rocks by molecular solution. Faults are believed to provide the main pathway for the vertical movement of water and dissolved hydrocarbons from great basinal depths. Eventually the fluids are focused into shallower sands when the fault becomes impermeable to further vertical fluid movement. The carrying capacity for petroleum of a small amount of hot water is many times that of a much larger amount of cold water. Thus, primary migration by molecular solution may be operable to depths of as much as 40,000 ft (12.20 km) until all deep basinal pore water has been removed by compaction.