The theory of petroleum migration by molecular solution and a hot, deep origin of petroleum has been criticized because: (1) solubilities of crude oil in water of 10,000 ppm (1 weight percent) are needed to form petroleum deposits, and these solubilities have not been demonstrated; and (2) the geologic long-term thermal stability of petroleum at temperatures above 200ºC has not been demonstrated.
Oil solubility in water has been studied as a function of temperature (100 to 400°C), pressure (100 to 30,000 psi), salinity (0 to 250,000 ppm NaCl), gas composition (CO2, CH4, and N2), and solute boiling-point composition. Petroleum solubilities of as much as 10 weight percent were measured under conditions simulating great depths in petroleum basins. These experiments showed that oil solubility in water increased with both increasing temperature and the presence of gases, and decreased with increasing pressure and increasing salinity above 20,000 ppm.
Organic geochemical analysis of shales from 20,000 to 30,000 ft wells (6,000 to 9,000 m) shows that C15 hydrocarbons survive at these depths (present temperatures above 200ºC), and are locally abundant (1,000 to 3,000 ppm). Analyses of metamorphic rocks show hydrocarbons persisting into the low-temperature metamorphic facies. Experimental diagenesis of heavy-petroleum fractions for periods as long as 90 days shows that thermal-cracking reactions quickly reach a state of apparent equilibrium. Over these time periods no further increase occurs in cracking products at temperatures of 350 to 375ºC in water-wet, closed, high-pressure systems. Such conditions probably occur at depth in petroleum basins. Similar past experiments on oils, kerogens, and asphalts have given similar results. These field and experimental data imply that the concept of a low-temperature thermal destruction of hydrocarbons should be reexamined.
Based on solubility and thermal stability data for hydrocarbons, mass balance calculations show that if primary petroleum migration is the result of molecular solution, it must occur in the range of 300 to 350 C. These high temperatures require very deep burial or short-term heating events that result in temporarily high subsurface temperatures.
Water and hydrocarbons exist at great depths in sedimentary basins. The very high solubility of oil in water at the temperatures corresponding to these depths and the expulsion of deep-basin pore fluids along faults result in the formation of petroleum deposits. Of all the currently available hypotheses, the model of a hot, deep origin and migration of petroleum by molecular solution is the one most consistent with all of the facts observed and related to the occurrence of oil.
Figures & Tables
In the business of petroleum exploration, the topic of migration has enjoyed a great bounty of ideas and opinions while, at the same time, suffering a famine of facts. This publication tontains papers on geologic constraints on migration mechanisms, oil migration limitations, the importance of water-mineral-organic matter interactions, and the nature of shales.