Primary migration

Price, L.C. (1976) Aqueous solubility of petroleum as applied to its origin and primary migration. Am. Assoc. Petrol. Geol. Bull. 60, 213-244. 

Drummond S. E. and Palmer D. A. (1986) Thermal decarboxylation of acetate II. Boundary conditions for the role of acetate in the primary migration of natural gas. Geochim. Cosmochim. Acta 50, 813—824. 

Talukdar S., Gallango O., Vallejos C., and Ruggiero A. (1987) Observations on the primary migration of oil in the La Luna source rocks of the Maracaibo Basin, Venezuela. In Migration of Hydrocarbons in Sedimentary Basins (ed. B. Doligez). Editions Technip, Paris, France, pp. 59-78. 

Some additional geochemical and geohydrological observations are given below in order to answer the question of whether it is likely that hydrocarbon transport in solution can be considered as a primary migration mechanism. 

Micelles are molecular aggregates formed in solutions of surface-active agents (surfactants compounds that orient at an interface such as between oil and water) (McAulifFe, 1980). Micelles may contain up to 100 or more surfactant molecules with a nonpolar (hydrophobic) end on the inside and a polar (hydrophilic) end on the outside. In 1959, Baker first advanced the concept of solubilization of hydrocarbons in (soap) micelles as a possible primary migration mechanism. The possible role of soaps, i.e. salts of organic acids, in primary migration was supported by Cordell (1973). The concept was considered attractive because it also explains how the practically water-insoluble hydrocarbons can solubilize in groundwater at relatively low temperatures. However, the likelihood of micellar solution as an effective primary migration mechanism has been seriously questioned by many authors (for instance Price, 1976 Hunt, 1979 Tissot and Welte, 1984). The main problems associated with micellar solution are ... 

Micelles appear to be larger than the small size of the pore throats in the source rock at a depth of burial corresponding to the main zone of oil and gas generation. According to Tissot and Welte (1984), primary migration of hydrocarbons in micellar solution would definitely be limited down to a depth of about 2000 m. 

Hence, the role of micellar solution of hydrocarbons in primary migration can be considered to be limited. 

Organic matter / kerogen in source rocks occurs primarily in a solid finely disseminated condition, but is often very inhomogeneously distributed being concentrated in layers (Momper, 1978 Jones, 1980). The minimum hydrocarbon saturation necessary to initiate primary migration may be reached sooner in layers with relatively high concentrations of organic matter. 

Leythaeuser et al. s (1982) study showed that the composition of the gas accumulated in a reservoir after transportation from a source rock where diffusion was the initial process of primary migration is controlled by three principal factors the initial concentration of the individual hydrocarbons in the source rock, their relative differences in diffusion rates, and the source rock thickness. Diffusion through water-saturated rock is probably a relatively minor primary migration mechanism, in view of the total amounts of... 

Figure 3.13 Conceptual model of a primary migration system at three scales (after Stainforth and Reinders, 1990. Reprinted with permission from Organic Geochemistry, Vol. 16, nos. 1-3, Copyright 1990, Peigamon Press Ltd.).

After primary migration has taken place, a certain proportion of the generated hydrocarbons remains in the pore system of the source rock (Hunt, 1979). The oil fraction that remains in the source rock will be cracked to gas as the source rock is buried to greater depths and temperatures (Section 3.1.5). The effect of primary migration of hydrocarbons can be indicated by the expulsion efficiency. The petroleum expulsion efficiency is the ratio of the expelled petroleum and the sum of the generated and initial petroleum and can vary from zero (no expulsion) to 1.0 (complete expulsion) (Cooles et al., 1986). The expulsion efficiencies are not uniform in time and space (Leythaeuser et al. 1987b). They depend on the tsrpe of source rock, its richness and thermal maturity and the primary migration mechanism. 

Different primary migration mechanisms are probably responsible for the transport of hydrocarbons through the hydrocarbon-generating source rocks in sedimentary basins. The three major mechanisms of primary hydrocarbon migration seem to be ... 

The effectiveness of the different primary migration mechanisms is determined by the subsurface characteristics of both the hydrocarbons to be moved and the medium through which the movement takes place. 

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