Gas hydrate is an ice-like crystalline form of water and low molecular weight gas (e.g., methane, ethane, carbon dioxide). On Earth, gas hydrates occur naturally in some marine sediments and within and beneath permafrost. Gas hydrates have also been inferred on other planets or their moons.
At the molecular level, gas hydrate consists of gas molecules surrounded by cages of water molecules. Each water cage encloses a space of a particular size, and only a gas molecule small enough to fit within this site can be hosted in that specific hydrate structure. Structure I gas hydrate has 46 water molecules that enclose 8 sites where gas molecules may be hosted. Six larger gas sites are enclosed by water cages with 12 pentagonal and 2 hexagonal faces, while two smaller gas sites occur within pentagonal dodecahedral cavities. Some researchers have likened the water cage structures to buckyballs.
Methane molecules can fit within both the small and large sites in the Structure I lattice, and Structure I methane hydrate is indeed the most common type found in nature. Thus, “gas hydrate” and “methane hydrate” are often used interchangeably by researchers.
Methane hydrate, sometimes colloquially referred to as "methane ice," takes on many forms in sediments. In fine-grained sediments, the methane hydrate can form in small pores and cement the grains, but may not be visible. Gas hydrate has also been recovered in chunks, in veins within sediments, and occasionally in large masses. Gas hydrate should not be thought of as forming within a thick seam, like coal.
Globally, gas hydrate has been recovered or inferred in many continental margin settings and in onshore permafrost or offshore relict permafrost that was flooded by sea level rise over the past ~15,000 years. Gas hydrate has also been recovered from sediments beneath Lake Baikal, Earth’s largest freshwater lake.
It is estimated that 99% of the world’s gas hydrate occurs in marine sediments. This estimate was made before modern drilling of permafrost-associated gas hydrates, but scientists still believe that most of the global gas hydrate occurs in the uppermost hundreds of meters of sediments at ocean water depths greater than ~500 m and close to continental margins.
Except on upper continental slopes (300-700 m water depth), the seafloor of most of the world’s oceans lies within the hydrate stability zone. Apart from a few locations, though, persistent seafloor gas hydrate mounds are relatively rare and not volumetrically important compared to the size of the global reservoir. Gas hydrate is in theory also stable in the lower part of the ocean water column, and gas bubbles rising from the seafloor sometimes form a shell of gas hydrate that usually does not survive very long.
Gas hydrate forms when methane and water combine at pressure and temperature conditions that are common in the marine sediments of Earth’s continental margins and below about 200 m depth in permafrost areas. Globally, gas hydrate has been recovered or inferred in many continental margin settings and in onshore permafrost or offshore relict permafrost that was flooded by sea level rise over the past ~15,000 years. Gas hydrate has also been recovered from sediments beneath Lake Baikal, Earth’s largest freshwater lake.
The theoretical gas hydrate stability curve is calculated for a particular gas mixture and pore water salinity. Methane hydrate can form where local thermal conditions (geotherms) are colder than (to the left of) the gas hydrate stability curve (phase boundary; in red below) at any given depth (pressure). In the diagrams above, the ocean water temperature is cold enough to permit hydrate to be stable at a water depths greater than ~575 m, and gas hydrate is stable in the underlying sediments to a depth of about 225 m below the seafloor. For the permafrost example on the right (above), gas hydrate is stable from about 200 to 600 m within the permafrost and from 600 m to ~1100 m beneath the permafrost.
Just because gas hydrate is stable at a particular location does not mean that it actually occurs there. Methane must be present in excess of its local solubility in sediment pore waters or in the water column for gas hydrate to form and be preserved. Methane hydrate formation can only proceed when sufficient methane is present and when there is available water. Certain conditions (e.g., the presence of saline pore waters or clays) can inhibit gas hydrate formation. Other conditions (high fluid flux) can encourage gas hydrate formation.
The spatial association of gas hydrates with continental margins is related to the availability of ample organic carbon that is being shed from the continents in these settings. Microbes use this carbon to generate methane. Such microbial methane is the most widespread source for methane in most natural gas hydrates, an interpretation that may be biased by the relatively shallow subseafloor depths from which most of gas hydrate samples have been recovered. In some locations and in sediment layers deep beneath the seafloor in petroleum basins, gas hydrates form instead from thermogenic gas that is generated through the deep-seated processes that are similar to those responsible for the formation of oil.
Prior to 1995, there had been no dedicated drilling of gas hydrate-bearing deposits, and estimates of the amount of methane sequestered globally in gas hydrate deposits ranged over several orders of magnitude. Gas volumes are often cited in units of trillion (10^12) cubic feet (TCF), and there are approximately 35.3 cubic feet in a cubic meter. The most careful pre-1990s estimates varied between 10^5 and 10^8 TCF of methane in gas hydrate globally, and numerous researchers devised global estimates between these endmembers as late as the mid-1990s. Since the start of dedicated drilling in 1995, researchers have learned that the saturation of gas hydrates in marine sediments is often far lower than the theoretical amount of gas hydrate that could be hosted in all available pore space. This has led to downward revisions of global and regional estimates. Most studies published in the past 15 years have concluded that between 10^5 and 5x10^6 TCF of methane is trapped in global gas hydrate deposits. While the minimum estimate is more than 4000 times the amount of natural gas consumed in the USA in 2010, only a fraction of the methane sequestered in global gas hydrate deposits is likely to be concentrated enough and accessible enough to ever be considered a potential target for energy resource studies. Also, until recently, there had never been any published estimate for gas sequestered in methane hydrates beneath the ice sheets of the Antarctic continent. This fact highlights the need for further updates to global gas-in-place estimates as new studies emerge.
Researchers lack a fully reliable method for locating gas hydrate in marine sediments or the sediments of permafrost regions. Ideally, the existence and saturation of gas hydrate can be inferred without direct sampling (drilling). In marine settings, seismic reflection techniques have long been used to determine the minimum areal extent of gas hydrates. A bottom simulating reflector (BSR) is a negative polarity (seismically-faster material like hydrate-charged sediments over seismically-slower material like gas-charged sediments) “interface” found in some marine sedimentary sections. The BSR is widely interpreted as the base of the gas hydrate stability zone and derives its name from the fact that it often mimics the gross morphology of the seafloor. Because of BSRs represent a phase transition, they often cross-cut the layering of sediments. The existence of a BSR means that gas hydrate almost assuredly occurs in the overlying sedimentary section. However, gas hydrate has been sampled at many locations lacking a BSR. Thus, BSR distribution provides only a minimum estimate of the area in which gas hydrate might occur. To date, BSRs have not been found in areas with permafrost-associated gas hydrates.
A disadvantage of seismic methods for locating gas hydrate is that the saturation of methane hydrate in pore space must generally exceed about 40% for the most common measure of seismic velocity to be significantly altered. This means that some seismic techniques may miss a significant amount of methane hydrate in areas where the saturation is less than ~40%. Laboratory studies show that electrical methods are more sensitive to lower saturations of gas hydrate. This has fueled interest in the application of electromagnetic (EM) methods for regional characterization of gas hydrate deposits or the joint application of EM and seismic techniques. The sensitivity of electrical properties to a wide range of hydrate saturations is also manifest by the widespread reliance of borehole resistivity logging to identify hydrate-bearing sediments in both marine and permafrost-associated settings.