The importance of gas in transmitting, marking and altering sediments and of its traces as clues to depositional, paleo-ecological and diagenetic history is not generally appreciated.
Gas generated beneath the seabed is buoyant, and tends to migrate towards the surface. Geological conditions may impede its progress, so shallow gas accumulations are formed. The exact nature of these accumulations depends on the type of sediment they are held in. Also, in certain pressure and temperature conditions, migrating gas may be sequestered by the formation of gas hydrates. Evidence is provided by various forms of acoustic signal recorded on seismic profiles, and is supported by the results of drilling, as well as the occurrence of natural gas seeps at the seabed.
Gas beneath the seabed is not a geological curiosity, but is common-place and widespread.