In a laboratory-based study on gas hydrate, materials, suitable P-T condition, space, and time, are among the important factors affecting hydrate formation. Similarly these factors are those affecting the formation of natural gas hydrate as well, although appearing in different ways.

　　In natural environment gas hydrate formation mainly involves two kinds of materials – water and gas. Natural gas hydrate generally exists in sediments, so the source of water is pore solution (water). According to ODP investigations, pore water content can be still about 20 % in sediments even at the depth of 1000 m below seafloor, so water is always available for hydrate formation. Two gas origins have been proposed – the biogenic and the thermogenic based on compositional and isotopic analyses of gases from natural gas hydrate. Biogenic gas is thought of being from the degeneration of organic matter in sediments by bacteria, which generally supply gas for in situ hydrate formation. Thermogenic gas is originated from the decompose of organic matter by heat at the depth much deeper than that of hydrate formation, so they have to migrate up to a certain depth where hydrate forms. Although the compositional and isotopic results of the hydrated gases suggest that most of the natural gas hydrates are composed of the biogenic gases, the contents of organic matter in sediments do not support this. For example, TOC in Nankai Trough offshore Japan are generally less than 1.0%, which can have a hydrate saturation rate of ~20% when all the organic matters are converted to methane gas at the same time. It is obvious that the in situ biogenic gas is not enough to saturate the pore space to about 80% as is estimated from pore water chemistry and geophysical logging results. Furthermore the organic matter in sediments cannot be expected to degenerate at the same time. Moreover ODP investigations also concluded that organic matter in sediments dissociated gradually with burial depth but not happened in a short term. Thus migrated gas from the deeper portion of sediments should be the main source for hydrate formation.

　　As limited by stability condition, natural gas hydrate can be formed only in an environment where pressure and temperature condition can stabilize gas hydrate. Thus natural gas hydrates have been found mainly in the permafrost and continental shelf under certain depth. The possible depth for marine gas hydrate formation depends on water temperature and the composition of available gases. According to the observation of water temperature, methane hydrate can be expected to form at the depth deeper than about 500 m below sea level, however this depth can be shallower when thermogenic gas as ethane, propane is available.

　　Gas hydrate generally forms in sediment pores that range from several nm to several mm. When faults or fractures are developed, much larger space in sediment section can be expected. Generally sediment pore sizes range from several nm to about 1000 nm, much larger than the size of unit cell of gas hydrate (12 ? for methane hydrate), so enough space is always available for hydrate formation. However, pore size can affect hydrate formation by narrowing its stability threshold when it is smaller than 10 nm. Field investigation and experimental results also indicate that the saturation rate of gas hydrate in sediments is affected by sediment type: high in sand while low in clay rich sediments.

　　Time for hydrate formation, mainly denoted as induction time in laboratory based experiments, while in natural environment induction time (minutes, hours to several days) can be ignored compared with the time scale of geology (several tens thousand years or much longer). However the concept of time is met in another way in natural environment. Saturation of gas in pore water is the basic condition for hydrate nucleation. It is easy to have water saturated with gas in laboratory, however it takes time in natural environment. The length of the time needed for pore water to be saturated with gas mainly depends on the gas flux from the deeper portion of sedimentary section, it can be very short when the flux is big.