Clathrate hydrates are solid crystalline structures formed by water hydrogen-bonded cages. Each cage is stabilized via dispersion forces exerted by a gas molecule trapped in its interior. Gas hydrates are stable at moderate pressures and low temperatures, above the freezing point of water. Natural gas hydrates can be found in coastal sediments or the permafrost, and their exploitation constitutes an attractive future energy source. It is agreed within the scientific community that natural gas hydrate deposits contain more energy than all the other fossil fuel sources combined. Usually, natural gas hydrate deposits are present in the form of small disseminated particles in marine sediments. Currently, gas hydrate equilibrium models in the bulk are used to estimate the size of hydrate deposits in the ocean floor and investigate gas-production strategies. However, differences between estimates of the depth of the hydrate stability zone within marine sediments and acoustic measurements point towards the importance of the effects of confinement on hydrate thermodynamic stability. A thermodynamic model for the equilibrium of methane gas hydrate inside cylindrical pores was developed in an attempt to better predict the hydrate stability zone within marine sediments (i.e., depths at which natural gas hydrate is stable). The effects of the porous medium on hydrate equilibrium are included in the model in terms of surface energies. Predictions on the displacement of the hydrate stability zone in terms of pore size and other sediment characteristics will be presented and discussed. Accurate prediction of natural gas hydrate equilibrium in sediments is important for the design of gas harvesting strategies.