About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

CSPG Special Publications


Facts and Principles of World Petroleum Occurrence — Memoir 6, 1980
Pages 173-192
Geological and Geochemical Principles of Petroleum Occurrence

Primary Migration of Oil and Gas

Kinji Magara


Solubility of natural gas in water under subsurface conditions is relatively high. A large amount of gas can be moved in solution in compaction water derived from shales into interbedded permeable rocks if active generation of natural gas takes place in the shales. Significant pressure differentials between synclinal and anticlinal areas would, in most cases, be sufficient to create a separate gas phase in the latter area.

Primary migration of oil must, however, occur differently, because molecular solubility of oil in water, even at higher temperatures, is relatively low. Most oil may, therefore, migrate in its own phase, along with the movement of compaction water. Capillary restrictions within shales against the movement of oil in the oil phase, may not be very large, because a larger proportion of water in shales, at intermediate stages of compaction, would be semi-solid, thus causing relatively high saturation of oil in the liquid water phase. If the liquid water is squeezed out of the shales by compaction, the oil may also move in its own phase.

In order to keep the effective oil migration in the oil phase, the shales must keep a relatively small amount of the liquid water during active oil generation. This can be accomplished if a stratigraphic section has a good compaction-water-drainage system.

In undercompacted sections, the active oil migration in the oil phase may not take place because of poor drainage conditions and the presence of a large quantity of liquid water in the shale. Most oil must stay in these shales because the concentration of the oil in the liquid water phase is probably too low to cause significant oil migration.

Pay-Per-View Purchase Options

The article is available through a document delivery service. Explain these Purchase Options.

Watermarked PDF Document: $14
Open PDF Document: $24