The remote Bering Sea seems an unlikely location for a major natural gas province. Yet the region may hold thousands of trillions of cubic feet of gas resources.
At a Jan. 11 meeting of the Geophysical Society of Alaska, U.S. Geological Survey Senior Research Geologist David Scholl described how what appear to be massive methane hydrate bodies pepper the south-central Bering Sea subsurface in a region that straddles the divide between the U.S. and Russian economic zones. The hydrates occur in a flat area of the abyssal plain, comparable in size to Nevada and Utah combined, Scholl said.
The hydrate bodies appear as strange looking features in seismic data collected during the Cold War.
“The reason we found these things was because of anti-submarine warfare,” Scholl said. “… We were chasing submarines around and doing acoustic work.”
Trapped methaneMethane hydrate, often referred to as gas hydrate, consists of a crystalline substance in which a lattice of water molecules traps methane molecules (methane is the primary component of natural gas).
The hydrate crystals remain stable within a certain range of temperature and pressure, known as the methane hydrate stability zone. If moved outside the stability zone, the hydrate crystals decompose into water and methane gas. When decomposed the crystals yield about 170 times their volume in methane, Scholl said.
Hydrates can occur on the seafloor if the water temperature and pressure falls within the stability zone. Alternatively, the hydrates can occur underground, within rock formations that lie within the stability zone. Within rocks the hydrates may be dispersed in rock pores, or form nodules, layers or massive aggregations, Scholl said.
On Alaska’s North Slope there are extensive gas hydrate deposits within underground rock formations around the base of the permafrost zone — a government and industry team is currently engaged in a multi-year project to determine whether it is possible to viably extract natural gas from these hydrates.
Seismic featuresThe scientists engaged in the Cold War era research observed many strange-looking anomalies in the deep Bering Sea seismic sections. In profile, these anomalies looked like giant mushrooms, with the heads of the mushrooms typically about 1,200 feet below the seafloor and about 3 miles across. The stalks of the mushrooms would extend vertically downwards for several thousand feet to basement rocks that lie underneath the sediments that blanket the seafloor.
Typical water depths in the region where the structures were found are around 3,800 meters (12,500 feet).
Scholl explained that because no seismic line crossed the same structure twice, it was impossible to say whether in plain view a structure was circular or of a more elongated form.
The structures showed up on seismic profiles in part because they contained areas of heightened seismic signal amplitude, and in part because within the structures the reflections of rock strata would be deflected out of the horizontal.
In the head of the mushroom seismic reflections from otherwise horizontal rock strata would be deflected upwards, a phenomenon that geophysicists term “pull up,” while in the stalks the reflections would bend down, a phenomenon known as “push down.” Pull up is caused by material with relatively high sound velocity — the reflections of the seismic sound reach the surface seismic receivers more quickly than normal, thus making the reflections appear closer to the surface. Push down is the opposite effect, where a relatively low sound velocity slows down the seismic signals.
The scientists termed the Bering Sea structures VAMP structures, an abbreviation for velocity and amplitude anomaly.
“We didn’t know what these things were,” Scholl said.
But the scientists could see that the structures occurred in clusters of several hundred. And thousands of the structures dot the deepwater region of the south-central Bering Sea.
Evidence for hydratesAfter the physics of methane hydrates became known in the early 1970s it became evident that the VAMP structures related to gas hydrates in the subsurface.
The hydrate has a relatively high sound velocity that would account for the pull-up effect in the head of the VAMP. Methane reduces the sound velocity, so that the push down in the mushroom stalk provides evidence of a gas chimney. Gas in the chimney would bubble upwards from deep within the seafloor rock strata and feed the hydrate accumulating in the head of the mushroom.
“A couple of percent gas (in the rock) and your (sound) velocity will drop 20 or 30 percent,” Scholl said.
But how can the scientists be so sure that the seismic anomalies result from underground methane and methane hydrate?
It’s mainly a question of the location of the methane hydrate stability zone. When you project the known temperature and pressure gradients downwards below the seafloor you find that these gradients cross the boundary of the hydrate stability zone at the bases of the VAMP heads. Go below the heads and gaseous methane would be stable. Go higher and methane hydrate is stable.
In fact a distinctive seismic reflection known as the bottom simulating reflector, or BSR, also depicts the seismic velocity contrast at the bottom of the region of presumed methane hydrate and generally runs parallel to the sea floor (apparently, the “BS” portion of BSR was originally coined from a driller’s response to being asked to target the reflection. The phrase “bottom simulating” was later thought up as more polite terminology).
The push down effect in the stalks of the VAMP mushrooms increases with depth, presumably because of the cumulative velocity effect of sound passing to deeper and deeper levels through gas in the gas chimneys. That shows that the gas is flowing from great depth within the chimneys and, thus, indicates that the gas is thermogenic, Scholl said (thermogenic gas forms from the heating of organic material, as distinct from biogenic gas that results from bacterial decomposition of the material).
Geologic explanationAnd there’s a plausible geologic explanation for a gas hydrate kitchen under the deep Bering Sea.
The seismic sections show that near-horizontal sedimentary strata, anywhere from about 3,000 feet to 30,000 feet thick, blanket a pre-existing, mountainous rock basement with immense troughs and ridges. The VAMP structures tend to occur over the higher points in that basement.
Some drilling done in association with the acoustic survey work found that the sedimentary strata consist of two distinct sequences. The younger and shallower of these sequences consists of rocks called turbidites that are typically laid down from sediment flows in deep water. The Bering Sea turbidites started to form about 2.5 million years ago, Scholl said. Under the turbidites lies an older sequence of mudstones containing huge quantities of diatom remains — diatoms are tiny algae with silica cell walls. The older sequence of strata appears to extend back into the Miocene epoch that began around 26 million years ago.
The existence of diatoms in the mudstones indicates that in the geologic past the Bering Sea was organically productive, just as it is at present. And organic material from buried diatoms and other organisms would feed thermogenic gas production in the region.
Hot basinBut what about the necessary heat for the process?
The geologic setting of the Bering Sea results in an exceptionally high heat flow out through the Earth’s crust in the region. The subsurface thermal gradient is about 60 C per kilometer, Scholl said.
“That means that in a couple of kilometers (6,000 feet) you’re in the gas window,” Scholl said.
Not only that, but the alteration of diatom-containing mud under the effects of temperature and pressure would likely release further heat, with the assemblage of sediments acting like a water laden and organic-rich blanket enveloping an already hot basement and effervescing methane gas.
The seismic data show little evidence of geologic faulting that might provide conduits for the methane to flow towards the surface. The scientists have instead proposed a theory involving the gas chimneys forming in fracture systems in the mudstones. Evidence from similar rocks in California shows that the original diatom-containing sediment shrinks and fractures as is solidifies — those fractures could form embryonic gas chimneys that would later become self-perpetuating, as hot gas flowed towards the surface.
The fact that the VAMP structures tend to occur over high points in the oceanic basement remains something of a mystery. One theory is that compaction of the sediments over these high points might have accelerated the fracturing and gas chimney formation.
Massive resourceSome simple calculations show that the Bering Sea VAMP structures may contain vast amounts of methane. Assuming that the structures are circular and assuming a minimum gas concentration in the chimneys, a typical vamp might contain anywhere from 0.5 trillion cubic feet to 1 tcf of methane. Multiply that by the potential number of VAMP structures and you arrive at some fairly mind-boggling numbers.
But before anyone gets too excited about developing this spectacular resource they might want to consider that the gas lies many miles from land under more than 12,000 feet of water in one of the world’s harshest marine environments — economic extraction of the gas seems implausible in the foreseeable future.
And scientists would like much more information about the physics of the structures, their geometry and their distribution — among other things, that information would enable much more complete gas resource estimates.
“What we want very much to do is to get up and acoustically map both the horizontal and vertical characteristics of a number of VAMPs,” Scholl said. “… No vessel has ever gone out there to take a look at one of these things using modern navigation equipment.”
But, given the cost of vessel-based research, these investigations seem unlikely to occur for a number of years. Meantime the methane hydrate “mushrooms” of the Bering Sea remain a tantalizing feature of the region.
“The Bering Sea is really infected with these guys,” Scholl said.