One of the simplest hydrocarbon hydrate is the methane hydrate which is structure I hydrate.This was discovered in natural sediment of the artic zone or into subsea sediments. Let us study the equilibrium of methane and pure water because of its natural occurence in arctic zones.
 

This figure shows the pressure-temperature stability diagram of methane and pure water. Four stability zones can be noted. - water ice plus methane hydrate at low temperature and high pressure - water ice plus methane gas at low temperature and low pressure - liquid water plus methane hydrate at moderate temperature and high pressure - liquid water plus methane gas at high temperature and moderate pressures.

From this diagram, we can interpret the different stability zones of methane hydrate in sediments of the artic area. In fact, a sediment is a porous media composed of soil and water which fills the interstices. In arctic area, the temperature is so low that that the soil is permanently frozen. It is called the permafrost. But in penetrating into the soil, temperature increases due to the natural thermal flux coming from the depth of the earth and permafrost disapears. But in some cases, the permafrost can be relayed by an hydrate zone in which the porosity is filled by methane hydrate because pressure is sufficiently high to stabilize the hydrate phase.

It exists a relation between the depth and the pressure but we have to discern the sediment from the water contained into the porosity.

- the sediment is submited to the lithostatic pressure due the mass accumulation of soil above it. This pressure transmits itself through contacts between grains (sand, gravels, silts, clay...). This pressure becomes rapidly enormous.

- the water enclosed in porosity is submited to the hydrostatic pressure resulting from the water column above it. In fact, in first approximation, we can consider that the pressure transmits itself through the continum liquid phase filling the porosity from the earth soil to the depth.

- if the soil temperature is not very low, the temperature of the sediment increases but does not enter the stability zone. If a well is drilled in this area, it is only observed a permafrost zone and then a "normal" sediment zone.

- if the soil temperature is at a sufficiently low temperature, it exists a zone in which the methane hydrate is stable. There is first a permafrost zone, then a zone which can contain ice and methane hydrate, then a zone which can contain methane hydrate and finally the sediment.

In subsea consitions, the situation is quite different. The sea floor temperature cannot be at a temperature inferior to 0°C and so the permafrost cannot exist. Generally, in very deep sea the floor temperature is of few degres. But the hydrostatic pressure can be very high and stabilize hydrates. As a result, it exists a width zone in which the sediment can contain methane hydrate particles filling the porosity.