Possible Primary Migration of Oil from Source Rock in Oil Phase: GEOLOGIC NOTES

Abstract

It is now generally believed that most petroleum is generated at temperatures between 60 and 150°C, corresponding to depths of burial of 1,500 to 4,500 m. At these depths shale source rocks have lost most of their water and practically all their permeability. If a good source rock still contains 500 ppm hydrocarbon, it probably has expelled a similar amount. If such a rock was subjected to a porosity loss of 10 percent during the time that it gave up 500 ppm by weight, the ratio of hydrocarbons to hydrocarbons plus liquid is 12,000 ppm, or 1.2 percent by volume of the liquid. There is no possibility of dissolving this much oil in water, even with the aid of solubilizers. Much of the shale surface may be wetted by oil, so that the saturation at which oil will flow as a continuous phase may be less than 10 percent. Furthermore, much of the water in the pores is structured and may behave like a solid. For fluid flow it might be considered as part of the solid matrix, and oil then would form a large fraction of the pore liquid. The relative permeability of the shale to oil then would become greater than to water. As compaction of the source rocks proceeds, shales might expel oil preferentially to water.