Physical analysis of oil migration through a water-saturated, hydrophilic, porous sediment suggests the following mechanisms of migration: (1) continuous-phase flow, (2) colloidal dispersion in water, and (3) molecular solubility in water.

The first mechanism, continuous-phase flow, is a major factor in controlling the movement of oil through porous reservoir rocks, the migration of oil into traps, or leakage of oil out of traps. Relatively high saturation of hydrocarbons (15-25 per cent of pore volume) must exist to create the continuity of the oil phase necessary for migration by this mechanism. The relatively low residual hydrocarbon saturations observed in many shales considered as probable source beds suggest that migration by this mechanism has not occurred therein.

The second mechanism, colloidal dispersion in water, appears to be a major factor in controlling the primary migration of oil out of many typical source beds. These migrating colloids may vary from the partially reduced organic complexes found colloidally dispersed in sea water and sea-bottom muds to oil solubilized by naturally occurring soap micelles or other solubilizing agents. If this mechanism has occurred in a shale, the organic content of the shale source bed may be low, and the residual hydrocarbon content in per cent of pore volume may be almost nil.

Two primary requirements for the operation of this second migration mechanism are the following.

1. The source-bed mineral surfaces must be hydrophilic. Experimental evidence indicates that the predominantly sodium-based mineral surfaces found in a normal marine sediment are generally hydrophilic and, therefore, could be source beds; whereas the predominantly calcium-magnesium-based mineral surfaces of many non-marine sediments may commonly be oleophilic and, consequently, probably are non-source beds.

2. The organic colloid, soap micelle, or other solubilizing agent must be stable and mobile within the source bed and must become unstable, immobile, or dissociated somewhere

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within the reservoir bed. The flocculation or dissociation of these colloids appears to be greatly affected by the Donnan-equilibrium-controlled anion-exclusion and cation-absorption properties of high-electrical-charge-density materials such as shales.

When the migration is terminated by either dissociation or flocculation, the resulting finely dispersed unstable organic particles or oil droplets will start to aggregate. Consequently, buoyancy will cause them to rise (or fall) through the water phase to the top (or bottom) few inches of the porous reservoir. If the oil or oil-forming material accumulates in the top few inches of the reservoir bed in sufficient concentration to subsequently produce extensive oil-phase continuity, then additional migration by the continuous-phase flow mechanism can occur.

The third mechanism, molecular solubility in water, may be a significant factor in selectively transporting certain hydrocarbon fractions and thereby modifying the oil characteristics.

If only the first mechanism were operative, source beds should be detectable by their high residual hydrocarbon content. If the second mechanism were commonly operative, such source beds could not be identified simply by measuring the residual oil saturation; and if source beds of this type are throughout most marine sedimentary sections, the limited occurrence of major oil production must be related to conditions required for continuous entrapment and preservation of oil since the time of origin.

The entrapment of oil is primarily controlled by the first mechanism--continuous-phase flow. Therefore, a critical evaluation of this mechanism under both hydrodynamic and hydrostatic conditions throughout the geologic history of an area is recommended for finding both broad oil provinces and specific oil fields.

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