Fine-grained marine sediments commonly contain organic matter derived from the marine, terrestrial and atmospheric biosphere. Some of this material is degraded by bacterial activity. The by-products either escape to the surface or are buried along with the sediments. Over periods of geological time the sediments are buried to progressively greater depths where thermocatalytic processes break down the remaining organic compounds to form petroleum compounds or coal. The proportions of solid, liquid and gassy hydrocarbons depend upon the nature of the original organic matter, and the temperature and pressure conditions. Some hydrocarbons remain in the sediment and may be converted to inert carbon (graphite). However, petroleum fluids generally escape and migrate towards the surface because of their buoyancy. Impermeable strata may impede migration, so petroleum reservoirs are formed. Gas may be sequestered by the formation of gas hydrates. If unimpeded, or if able to escape through leaky reservoir seals, petroleum will eventually rise to the surface. Within reservoirs, or more significantly, close to the seabed, the rising petroleum may be utilised by bacteria, and re-enter the biosphere. Where flux rates are high, a portion will seep into the water where some will be lost to the hydrosphere, but when flux rates are high, or where water is shallow, some escapes to the atmosphere.

This simple description shows how carbon is cycled through the marine environment (hydrosphere and lithosphere), mainly in the form of hydrocarbons.

Despite the significance to geoscience and indeed to society, of hydrocarbons, this important component of the global carbon cycle is not infrequently overlooked. One reason for this is that the 'return cycle', the migration of petroleum fluids to the surface, is generally considered insignificant. However, studies of shallow gas, gas hydrates, pockmarks, seeps, cold seep communities, methane-derived carbonates, and mud volcanoes have demonstrated the dynamic nature and widespread occurrence of the marine hydrocarbon cycle, and its significance to the lithosphere, the hydrosphere, the biosphere and the atmosphere.