Gas Hydrates in Sediments of Holes 497 and 498A, Deep Sea Drilling Project Leg 67

Abstract

Gas hydrates are icelike materials that form when specific conditions of temperature, pressure, and gas composition are simultaneously satisfied. Among the first descriptions of gas hydrates under natural conditions was that of Hammerschmidt (1940), who found them in pipelines used to transport natural gas. Milton (1976) indicates that conditions are suitable for the presence of gas hydrates in areas affected by permafrost and cites studies suggesting that large quantities of gas exist in hydrate form. Ordinary ice crystallizes in the hexagonal system, and the voids that result from such crystal geometry are capable of accommodating only hydrogen and helium. Gas hydrates, however, crystallize in the cubic system and occur in Structure I (unit cell = 12 A) or Structure II (unit cell = 17 A) (Hitchon, 1974). These structures have cavities or voids large enough to accommodate small gaseous molecules. Methane, ethane, propane, and carbon dioxide are among the gases commonly found in sediments that can exist in gas hydrate form. Gas molecules with dimensions equal to or larger than w-butane are too large to fit into the voids, whereas very small gas molecules such as helium, neon, and hydrogen are probably too small to form hydrates (Parrish and Prausnitz, 1972). The forces that exist between the gas molecules and the crystalline solid are of the Van der Waals type, thus gas hydrates are not true chemical compounds. The relation is essentially a physical one resulting from cagelike openings that exist in the lattice of icelike solids and the free gas molecules that occupy such openings (Ionescu, 1978). Kvenvolden and McMenamin (1980) have compiled information on several areas of the world's ocean where gas hydrates are thought to exist in seafloor sediment. Most such areas were defined on the basis of a BSR (bottom-simulating reflector) in seismic records. Leg 67 involves an area where BSRs are not readily discernible, but cores recovered at two sites provided abundant evidence for the presence of gas hydrates. Gas hydrates recovered during Leg 67 were invariably associated with stratigraphic sequences containing high-porosity sediments. Material recovered from a core catcher at Hole 497 (Core 39) was an icelike substance