Organic matter commonly makes up a substantial part of freshly deposited muds. During postdepositional history, much of this organic matter is decomposed by biochemical and thermochemical processes, with accompanying generation of methane within the sediments. Likewise during postdepositional history, weight of overburden brings about compaction of mud sediments through expulsion under pressure of interstitial pore water. However, at the same time as pressure on the fluid is tending to increase because of compaction and other factors, the permeability of the sediment tends to be reduced by closer packing of clay particles. Thus it may happen not infrequently that the escape of fluids from a mud or shale body may fail temporarily to keep up with the causes tending to incre se fluid pressure, and in such cases these bodies will become overpressured and undercompacted with respect to surrounding sediments. The generation of methane gas within these mud and shale bodies is an important added factor which tends further to accentuate, or even create, their overpressured, undercompacted state, both by building up additional internal pressure, and also by further impeding fluid expulsion because of the development of a second phase (gas) in the pore fluid.

Because of the unstable, semifluid nature of methane-charged undercompacted muds or shales, they frequently find expression in mud or shale diapirs or in mud volcanoes. The importance to petroleum exploration of a causal relation between methane generation and undercompacted shales, shale diapirs, and mud volcanoes is principally that these features thus become indicators of areas or intervals where hydrocarbons have been, or are, actively originating. Although the nearby area of these overpressured features may be unfavorable for petroleum accumulation because of a lack of extensive reservoirs, the surrounding facies, above, below, and laterally adjacent, may be particularly favorable in their relation to both reservoir and source environments.