481
14.1 Introduction
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.
14 Gas Hydrates in Marine Sediments
GERHARD BOHRMANN AND MARTA E. TORRES
14.1 Introduction
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.
14 Gas Hydrates in Marine Sediments
GERHARD BOHRMANN AND MARTA E. TORRES