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The U.S. Geological Survey Gas Hydrates Project

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Gas Hydrates Primer

What is Gas Hydrate?

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 occurnaturally 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 thatenclose 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 twosmaller 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 typefound 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 canform 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.

Where Does Methane Hydrate Occur?

Globally, gas hydrate has been recovered or inferred in many continental margin settings and in onshore permafrost or offshore relict permafrost that wasflooded by sea level rise over the past ~15,000 years. Gas hydrate has also been recovered from sediments beneath Lake Baikal, Earths largest freshwaterlake.

It is estimated that 99% of the worlds gas hydrate occurs in marine sediments. This estimate was made before modern drilling of permafrost-associated gashydrates, but scientists still believe that most of the global gas hydrate occurs in the uppermost hundreds of meters of sediments at ocean water depths greaterthan ~500 m and close to continental margins.

Except on upper continental slopes (300-700 m water depth), the seafloor of most of the worlds oceans lies within the hydrate stability zone. Apart from a fewlocations, though, persistent seafloor gas hydrate mounds are relatively rare and not volumetrically important compared to the size of the global reservoir. Gashydrate 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 hydratethat usually does not survive very long.

How Does Gas Hydrate Form?

Woods Hole Science Center

Water molecules (1 red oxygenand 2 white hydrogens) form apentagonal dodecahedronaround a methane molecule (1gray carbon and 4 green

hydrogens). This represents 2of the 8 parts of the typicalStructure I gas hydrate

molecule.

Scanning electron microscope image of gashydrate, courtesy of L. Stern, USGS GasHydrates Project

Methane bubble emitted at a seafloor

seep on the Blake Ridge diapir in2000. The bubble formed a shell of gashydrate as it rose.

Known and inferred locations of gas hydrate occurrence. Map compiled by theUSGS.

The hydrate resource pyramid showing therelative amounts of gas hydrate in the globalsystem. The hydrates at the top of the pyramidare most likely to be exploited for energyresources. After Boswell and Collett.

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Gas hydrate forms when methane and water combine at pressure and temperature conditions that are common in the marine sediments of Earths continentalmargins and below about 200 m depth in permafrost areas. Globally, gas hydrate has been recovered or inferred in many continental margin settings and inonshore permafrost or offshore relict permafrost that was flooded by sea level rise over the past ~15,000 years. Gas hydrate has also been recovered fromsediments beneath Lake Baikal, Earths 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 thermalconditions (geotherms) are colder than (to the left of) the gas hydrate stability curve (phase boundary; in red below) at any given depth (pressure). In thediagrams 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 stablein 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 about200 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 methaneis present and when there is available water. Certain conditions (e.g., the presence of saline pore waters or clays) can inhibit gas hydrate formation. Otherconditions (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 inthese settings. Microbes use this carbon to generate methane. Such microbial methane is the most widespread source for methane in most natural gashydrates, an interpretation that may be biased by the relatively shallow subseafloor depths from which most of gas hydrate samples have been recovered. Insome locations and in sediment layers deep beneath the seafloor in petroleum basins, gas hydrates form instead from thermogenic gas that is generatedthrough the deep-seated processes that are similar to those responsible for the formation of oil.

How Much Gas Hydrate Exists?

Prior to 1995, there had been no dedicated drilling of gas hydrate-bearing deposits, and estimates of the amount of methane sequestered globally in gashydrate deposits ranged over several orders of magnitude. Gas volumes are often cited in units of trillion (10^12) cubic feet (TCF), and there are approximately35.3 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 numerousresearchers devised global estimates between these endmembers as late as the mid-1990s. Since the start of dedicated drilling in 1995, researchers havelearned 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 allavailable 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 ofnatural 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 andaccessible enough to ever be considered a potential target for energy resource studies. Also, until recently, there had never been any published estimate forgas sequestered in methane hydrates beneath the ice sheets of the Antarctic continent. This fact highlights the need for further updates to global gas-in-placeestimates as new studies emerge.

How to Find Gas Hydrate

Researchers lack a fully reliable method for locating gas hydrate in marine sediments or the sediments of permafrost regions. Ideally, the existence andsaturation of gas hydrate can be inferred without direct sampling (drilling). In marine settings, seismic reflection techniques have long been used to determinethe minimum areal extent of gas hydrates. A bottom simulating reflector (BSR) is a negative polarity (seismically-faster material like hydrate-chargedsediments over seismically-slower material like gas-charged sediments) interface found in some marine sedimentary sections. The BSR is widely interpretedas 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 BSRsrepresent a phase transition, they often cross-cut the layering of sediments. The existence of a BSR means that gas hydrate almost assuredly occurs in theoverlying sedimentary section. However, gas hydrate has been sampled at many locations lacking a BSR. Thus, BSR distribution provides only a minimumestimate 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 themost common measure of seismic velocity to be significantly altered. This means that some seismic techniques may miss a significant amount of methanehydrate in areas where the saturation is less than ~40%. Laboratory studies show that electrical methods are more sensitive to lower saturations of gashydrate. This has fueled interest in the application of electromagnetic (EM) methods for regional characterization of gas hydrate deposits or the joint applicationof EM and seismic techniques. The sensitivity of electrical properties to a wide range of hydrate saturations is also manifest by the widespread reliance ofborehole resistivity logging to identify hydrate-bearing sediments in both marine and permafrost-associated settings.

Gas hydrate stability fields for nominal marine and permafrost settings, after Ruppel,2007.

The interplay between pressure-temperature in thesubseafloor, gas hydrate stability, methane solubility inpore waters, and the occurrence of gas hydrate and freegas. After Xu and Ruppel, 1999.

Overall controls on the thickness ofthe gas hydrate stability zone andthe occurrence of a BSR, from

Ruppel and Kinoshita, 2000.

Gas hydrate cemented sands recoveredat the Milne Point BPXA/DOE/USGS site

(Mt Elbert) in 2007 in a permafrost

setting near Prudhoe Bay, North Slope,Alaska. The black liquid is drilling fluid.

Gas hydrate recovered in shallow layers

just below the seafloor during piston

coring in the Gulf of Mexico.

Gas hydrate recovered from a seep inthe deepwater Arctic Ocean aboard the

USCG icebreaker Healy in 2009.

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Further Reading

z Boswell, R. and Collett, T.S., 2011. Current perspectives on gas hydrate resources. Energy and Environmental Science, 4, 1206-1215.z Collett, T.S., et al., 2011. Permafrost associated natural gas hydrate occurrences on the Alaskan North Slope. Marine and Petroleum Geology, 28, 279-

294.

z Collett, T. S., et al., 2009, Natural Gas Hydrates: A Review, in T. Collett, A. Johnson, C. Knapp, and R. Boswell, eds., Natural gas hydratesEnergyresource potential and associated geologic hazards: AAPG Memoir 89, p. 146 219.

z Ruppel, C., 2007, Tapping methane hydrates for unconventional natural gas, Elements, 3(3), 193-199.z Ruppel, C., 2011, Methane hydrates and the future of natural gas, Supplementary Paper #4, The Future of Natural Gas, MIT Energy Initiative study, 25

pp.z Ruppel, C. and M. Kinoshita, 2000, Heat, fluid, and methane flux on the Costa Rican active margin off the Nicoya Peninsula, Earth and Planetary Science

Letters, 179, 153-165.z Trehu, A.M. et al, 2006, Gas hydrates in marine sediments: lessons from ocean drilling, Oceanography, 19, 124-143, 2006.z Xu, W. and C. Ruppel, 1999, Predicting the occurrence, distribution, and evolution of methane gas hydrate in porous marine sediments from analytical

models, Journal of Geophysical Research, 104, ,5081-5096.

U.S. Department of the Interior | U.S. Geological Survey

URL: woodshole.er.usgs.gov/project-pages/hydrates/primer.htmlPage Contact Information: WHSC WebmasterThis page last modified on Monday, 14-Jan-2013 04:50:57 EST

A weak Gulf of Mexico BSR (yellow) imaged by the USGS usingmultichannel seismic data collected in the Keathley Canyon area, leaseblock 151, in 2003. Note that BSR cuts across the sediment layering

Timeline of past drilling activities conducted by countries, private sector firms,government agencies, and academe that have helped to refine global gashydrate estimates and possible future drilling and production testing .

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