Abstract: Methane Generation and Petroleum Migration

Hollis D. Hedberg

Methane generation probably has had an important influence on migration and accumulation of petroleum in general.

Most of the huge amount of organic matter in the earth's crust is finely disseminated in muds and shales. Inevitably, a large part of this is eventually converted by either biochemical or thermochemical processes to methane, so that methane is, or has been, widespread in this very common rock type.

In the early stages of sediment consolidation, methane generation is largely the result of bacterial action, and the gas thus produced escapes readily along with large volumes of expressed compaction water, either to the surface or to associated reservoir traps. With increasing depth, bacterial activity diminishes but is overlapped and replaced by thermochemical methane generation which increases in vigor with depth because of increasing temperature. Pore space and interstitial water become greatly reduced, as does also intergranular permeability, but as long as adequate organic feedstock remains in the shale, methane generation will continue and methane concentration will tend to increase.

If, as commonly happens, the volume of methane exceeds the capacity of the interstitial water to take it into solution, free gas bubbles will begin to develop in the pore spaces and an internal fluid pressure will build up within the sediment in addition to the external pressure of overburden--both acting to expel fluids or cause them to migrate. At the same time, however, an inhibiting effect on fluid movement is caused by the presence of both a liquid and a gaseous phase in the sediment (Jamin effect). Permeability is decreased as pressure increases and, unless relieved by fracturing, extensive bodies of impermeable overpressured shale (commonly associated with mud diapirs and mud volcanoes) may result, with consequent effects on migration and accumulation.

Other important effects of methane generation on petroleum migration under appropriate conditions include transport of higher hydrocarbons in solution in methane, and the formation of solid methane hydrates which may constitute important barriers to petroleum movement and perhaps obstacles to liquid petroleum accumulation.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma