The formation of natural-gas hydrates in sedimentary rock

Natural-gas hydrates are a geologically important class of ice-like substances, stable at low temperatures and high pressures. Their distribution and abundance in sediments has been predicted mainly from temperature and pressure data alone. The accuracy of such estimates might be greatly improved by considering additional variables now known to affect hydrate formation. Published laboratory studies indicate that hydrate formation depends strongly on the presence of an interface where the densities of both gas and water molecules are high. Particular attention should therefore be paid to mechanisms which might bring about, or prevent, the existence of interfaces between immiscible hydrocarbon and water phases in the sediment environment. Many such interfaces may occur contiguously to mineral surfaces, which adsorb water, and to organic surfaces which may adsorb dissolved gas. Since recent experiments also indicate that hydrate formation is influenced by the molecular structure of water, the effect of adsorptive surfaces on nearby water structure may be important. We suggest that water bound to some mineral surfaces may be unavailable for hydrate formation unless temperatures and pressures in the sediment are unusually favourable, but that organic surfaces should, if anything, facilitate hydrate formation. Hydrates formed during hydrocarbon migration may be compacted by pressure exerted by separatephase hydrocarbon flow into a dense grain-cementing layer. Hydrate, formed after displacement of most groundwater, may also coexist with gas or oil in reservoirs. Amounts are likely to be small, but their decomposition during drilling may cause unusually high recovery pressures from northern reservoirs. In general, the abundance and distribution of hydrates in sediments should be strongly influenced by sediment properties, and by mechanisms and timing of hydrocarbon migration.