There are a number of other mechanisms by which gas may move closer to the surface. Diapirism (gravitational instability and sediment overturn) is common in fine grained sedimentary sequences, particularly when the permeability is low, and overpressure builds up through gravitational loading, or via fluid sources, including the generation of hydrocarbons. If fluid sources are strong enough, the sediment may become fluidised, and entrained in a localised upwards flow to form a diatreme (Brown 1990). The flow is driven by a
Watts (1987) stated that fractures can form in a seal when hydrocarbons build up beneath it because the buoyancy pressure in the non-wetting phase can exceed the fracture pressure if the hydrocarbon column height, L, becomes large (Fig. 5); i.e.,
combination of buoyancy of expanding gas and the overpressure. If it reaches the surface, a mud volcano will form. The relationship between methane and mud volcanoes has long been known and hydrate dissociation are implicated as a source of the gas in some cases (Hedberg 1974, Martin et al. 1996, Lance et al. 1998, Kopf et al. 1998).
During the tidal cycle gas bubbles expand and contract in response to the hydrostatic pressure change (Hovland and Judd 1988) and this couples with the poroelastic properties of the gassy sediment (Sills et al. 1991, Fredlund and Rahardjo 1993) to produce strong pressure gradients. If there is some form of valving mechanism, perhaps due to opening or closing of fractures, or movement of bubbles within the pores (Hovland and Curzi 1989) then a directed flux can result even when the driving forces are symmetrical (Wang et al. 1998). This "tidal pumping" action can drive a flow of water through sediments that present an impermeable capillary barrier to gas migration, and Hovland et al. (1999) suggest that such periodic force may contribute to the "pumping up" of fluid pressure in gassy sediments that are interlayered with stiffer sediments.
More sporadic, but potentially larger forcings can be generated by seismic activity. A stress drop in the sediment framework can, counter to intuition, produce significant transient overpressures, while localised dilational strain in the vicinity of faults can open up fracture pathways to a low pressure reservoir or to the oceanic/atmospheric sink (Brown 1994). Seismic pumping or valving has been implicated in the formation of pockmarks and other seabed seep features (Field and Jennings 1987, Hovland and Judd 1988, Hasiotis et al. 1996, Soter et al. 1999).
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