Gas hydrates in marine sediments

Abstract

Gas hydrates are naturally occurring ice-like crystalline compounds in which gases are trapped within a lattice of water molecules. The presence of gas hydrates is controlled by temperature, pressure and the availability of appropriate gases and water. The first discovery of gas hydrate goes back to 1810, with the pioneering synthesis of chlorine hydrate by Sir Humphrey Davy (Davy 1811). In the 1930s crystalline substances were observed to form spontaneously within natural gas pipelines in permafrost regions, and these deposits, which were clogging the pipelines, were identified as being hydrates of mixed hydrocarbon gases (Hammerschmidt 1934). The recognition that natural gas hydrates can block gas transmission lines, led the hydrocarbon industry to invest in efforts aimed at understanding gas hydrates, and thus begins the modern research in this subject. Russian scientists (Vasil'ev et al. 1970) were the first to recognize that methane in natural systems could form gas hydrate deposits wherever the pressure and temperature conditions were favourable. These ideas were followed by discovery of gas hydrate, first in the permafrost regions of Russia (Makogon et al. 1971) and Canada's MacKenzie Delta (Bily and Dick 1974), and subsequently in sediments of the Caspian Sea and Black Sea (Yefremova and Zhizhchenko 1974). Interest in these deposits prompted the development of geophysical prospecting tools, which were used to predict the occurrence of gas hydrate in sediments of the Blake Ridge, of the western Atlantic Ocean (Stoll et al. 1971) and elsewhere (Shipley et al. 1979). In the early 1980s, hydrate was recovered from sediments of the Middle America Trench offshore Mexico by the Deep Sea Drilling project (Shipley and Didyk 1982). Since then, deep sea drilling has recovered hydrate from subsurface sediments along the Pacific and Atlantic continental slopes (Kvenvolden 1993). In addition, hydrate has been recovered from many near-surface environments along continental margins worldwide (Mazurenko and Soloviev 2003). The number of hydrate publications, scientific sessions and workshops dedicated to gas hydrate research has increased substantially during the last 10-15 years, reflecting the development of a broad national and international hydrate research effort in this field. The interest in gas hydrates emerges from the awareness that these deposits may play significant roles in global and regional processes with societal and economic significance. A global hydrate assessment, although still uncertain, suggests that methane hydrates might represent an important future energy resource (Kvenvolden 1998; Collet 2002). In addition, other important hydrate questions that have attracted attention include: 1) Is there a feedback between methane hydrate stability and climate? 2) What is the role of methane hydrate in the carbon cycle? and 3) How much does gas hydrate contribute to seafloor stability on continental slopes? The purpose of this chapter is to summarize some of the fundamentals of our current understanding of gas hydrate in marine sediments, its interactions with the environment, and recent findings from ongoing research programs that illustrate key aspects of gas hydrate dynamics. We start with general information on the structure and composition of gas hydrates and address their presence and distribution in the marine sediments based on their thermodynamic stability and environmental conditions. Because here we emphasize topics that are relevant to the scope of this textbook, we review the sources and migration mechanism of gases needed to stabilize the hydrate structure; the chemical and isotopic anomalies associated with hydrate formation; and the interaction of hydrates with fluid flow along continental margins.