Abstract
The chapter discusses gas migration, which can occur in the earth's crust as either a continuous or discontinuous phase through porous, water-filled media to the surface. The direction of gas migration in water-saturated formations is generally vertical as the primary force moving the gas is the difference between the specific weights of gas and water (buoyancy). Sources of gases include low-temperature bacterial fermentation, thermogenic breakdown of deeply buried sedimentary organic matter, and the mantle. For biogenic and thermogenic decomposition of organic materials, almost all of the hydrocarbon gases are composed of methane with very few heavier hydrocarbons. Thermogenic decomposition of organic material may give rise to oil and gas under high temperature and pressure. The origins of biogenic and thermogenic gases can be determined by isotopic gas analysis. The chapter also discusses nonhydrocarbon gases and mixing of gases.
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