Clathrate hydrates are solid crystalline structures in which water hydrogen-bonded cages enclose gas molecules in a unit volume equivalent to one hundred and sixty unit volumes of gas at standard conditions. The conditions for the formation and stability of gas hydrates involve high pressures and low temperatures, a few degrees 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 source of energy. In the case of marine sediments, natural gas hydrate can be present as disseminated particles, hydrate lenses, or massive structures. A thermodynamic model developed to describe the equilibrium of gas hydrates in sediments is discussed in this work. The model is analogous to the analysis performed to predict frost damage in soils. Effects of the porous medium on hydrate equilibrium are included in the model in terms of surface work and grain stress. The objective is to better describe the hydrate stability zone within marine sediments (i.e., depths at which natural gas hydrate is stable), and to predict the characteristics of natural-gas hydrate deposits. Accurate prediction of natural gas hydrate equilibrium in sediments is important for the design of future gas-hydrate exploitation strategies.