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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Solubility of Petroleum in Water and Its Significance to
Petroleum Migration
By
1973 Matson Award Paper
The aqueous solubilities of individual hydrocarbons, petroleum, and petroleum fractions increase with temperature, the rate of increase being gradual to 100°C and drastic thereafter; eventually co-solubility will be reached at high temperatures. Mass-calculations show that the formation of petroleum deposits can be accounted for by a molecular-solution primary-migration mechanism at temperatures greater than 180°C. Temperature decrease and salinity increase cause drastic ex-solution of hydrocarbons from the aqueous phase. Thus the pronounced decrease in solubility of petroleum at higher salinities and lower temperatures present at shallow basin depths releases dissolved hydrocarbons during upward movement of deep-basinal waters. Faults provide the main pathway for this vertical movement. Eventually the fluids are collected into shallower sandstones when the fault becomes impermeable to further vertical fluid movement.
The model is supported by geochemical-geologic evidence. At depths of 14,000-18,000 ft. (4.27-5.49 km), 15-20 percent water remains in clastic sediments which is more than sufficient to carry the required volumes of petroleum. Experimental and field evidence has shown the almost total conversion of "kerogen" to extractable organic matter in fine-grained rocks at temperatures greater than 300°C. On the other hand, studies in Tertiary Mesozoic basins have shown that at depths shallower than where these temperatures are present the "kerogen" has not thermally degraded at all and the extractable organic matter in shales is immature and unlike crude oil. Microspore and pollen particles in crude oils are derived from sediments much deeper in the section than where the oils are found. Thermodynamic equilibrium temperatures calculated for crude oils are much higher than reservoir temperatures and are in the range predicted by this model.
The model predicts specific geologic and geographic controls on petroleum occurrence. Examination of petroleum deposits and basins confirms the predictions and indicates that this model can be used as a powerful tool in petroleum exploration. Raw model's essence is a search for the first trap off a major fault into the area of greatest sediment thickness. The model can be used for exploration in Tertiary depocenters (Gulf Coast, Niger delta), wrench basins (Los Angeles), upthrust basins (Rocky Mountains), thrust basin (Western Canada), and shelf plays (Western Canada, Mid Continent).
Another implication of the model is the possible existence of a huge new energy resource-crude oil dissolved in hot deep waters of petroleum basins. The possibility exists of tapping these geothermal waters for heat as well as dissolved crude oil. Minimum estimates by mass-balance calculations put the reserves of this resource in range of the trillions of barrels.
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