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New study lends credence to abiogenic petroleum theory, which means there may be more oil in our future than we thought

A new study demonstrates how high hydrocarbons could be formed from methane deep within the Earth, aside from the compression and heating of ancient animal remains over the eons. Fused-methane oil would be far less common than your typical petroleum, of course, but the study shows abiogenic hydrocarbons could conceivably occur in some of the planet’s high-pressure and high-temperature zones.

Scientists at Lawrence Livermore National Laboratory used supercomputers to simulate what would happen to carbon and hydrogen atoms buried 40 to 95 miles beneath the Earth’s crust, where they would be subjected to prodigious pressures and temperatures.

They found at temperatures greater than 2,240 degrees F and pressures 50,000 times greater than those at the Earth’s surface, methane molecules can fuse to form hydrocarbons with multiple carbon atoms. Interactions with metal or carbon sped up the fusion process, the researchers said. These conditions are present about 70 miles down, according to an LLNL news release.

Methane, CH4, has one carbon and four hydrogen atoms; high hydrocarbons, like propane and butane, have more carbon atoms.

About 99 percent of all the hydrocarbons in oil and natural gas are derived from the compressed, heated remains of ancient living organisms like zooplankton and algae. These critters were buried under layers of sediments five to 10 miles beneath the surface of the Earth.

In the 19th and 20th centuries, some scientists believed hydrocarbons could form from abiogenic (non-biological) processes, too. The existence of methane on several solar system bodies shows hydrocarbons can exist without organic ingredients. But the theory fell out of favor, in part because no one ever found any abiogenic oil deposits.

The LLNL researchers don’t claim to know where such deposits would be, nor did they examine whether or how such deep deposits could ever migrate higher into the mantle where they could be retrieved. But the researchers say abiogenic hydrocarbons are technically possible in some settings like rifts or subduction zones, according to Giulia Galli, a professor at UC-Davis and senior author on the study, which appears in the Proceedings of the National Academy of Sciences.

“We don’t say that higher hydrocarbons actually occur under the realistic ‘dirty’ Earth mantle conditions, but we say that the pressures and temperatures alone are right for it to happen,” she said.

PopSci News, 15 April 2011

[Lawrence Livermore National Laboratory]

Hydrocarbons in the deep earth

A snapshot taken from a first-principles molecular dynamics simulation of liquid methane in contact with a hydrogen-terminated diamond surface at high temperature and pressure. The spontaneous formation of longer hydrocarbons are readily found during the simulations.


Can Hydrocarbons Survive in the Hot, Pressured, Mantle?

Earth’s hydrocarbons are typically formed when organic matter is trapped in sediments on the bottom of Earth’s oceans, seas, lakes, swamps, and bogs. Over a period of time, exposed to varying temperatures, pressures, and anaerobic conditions, the organic matter is transformed into hydrocarbons such as natural gas, peat, coal, and oil of various types.

Sediments trapped in oceanic crust (as opposed to continental crust) are subducted into the Earth’s mantle after dozens of millions of years — and exposed to very high pressures and temperatures. Many geologists had presumed that any hydrocarbons that had not migrated out of these subducted sediments, would be destroyed in the oxidising environment of the mantle. But a variety of research over the past several years suggests that not only can hydrocarbons survive the heat and pressure of the upper mantle — new short-chain hydrocarbons may actually be created within the mantle.

… conventional geochemists argued that hydrocarbons could not possibly reside in Earthʼs mantle. They reasoned that at the mantleʼs depth—which begins between 7 and 70 kilometers below Earthʼs surface and extends down to 2,850 kilometers deep—hydrocarbons would react with other elements and oxidize into carbon dioxide. (Oil and gas wells are drilled between 5 and 10 kilometers deep.) However, more recent research using advanced high-pressure thermodynamics has shown that the pressure and temperature conditions of the mantle would allow hydrocarbon molecules to form and survive at depths of 100 to 300 kilometers. Because of the mantleʼs vast size, its hydrocarbon reserves could be much larger than those in Earthʼs crust. _PDFLivermoreLabPDF

The notion that hydrocarbons generated in the mantle migrate into the Earth’s crust and contribute to oil-and-gas reservoirs was promoted in Russia and Ukraine many years ago. The synthesis and stability of the compounds studied here as well as heavier hydrocarbons over the full range of conditions within the Earth’s mantle now need to be explored. In addition, the extent to which this ‘reduced’ carbon survives migration into the crust needs to be established (as in, without being oxidized to CO2). These and related questions demonstrate the need for a new experimental and theoretical program to study the fate of carbon in the deep Earth,” the expert adds. _Softpedia


Now for the first time, scientists have found that ethane and heavier hydrocarbons can be synthesized under the pressure-temperature conditions of the upper mantle -the layer of Earth under the crust and on top of the core. The research was conducted by scientists at the Carnegie Institution’s Geophysical Laboratory, with colleagues from Russia and Sweden, and is published in the July 26, advanced on-line issue of Nature Geoscience. _Geology.com


So far there is no strong evidence that large quantities of economically important hydrocarbons are being generated within the mantle, with subsequent migration up into the crust — where humans can access them. But it seems quite likely that new gaseous hydrocarbons do migrate from the mantle into the crust — in some quantities — and contribute to gas deposits of various types, including methane clathrates.

What is more interesting to me than the abiotic generation of hydrocarbons is the fate of billion year old hydrocarbons of biological origin which find their way into the upper mantle through geologic upheaval. No doubt some of this hydrocarbon will survive as medium chain alkanes, although I suspect most will end up as methane or ethane. Some will get caught up in volcanic activity and be converted to CO2 — or get ejected into the atmosphere or ocean as CH4. But what is the proportion of each product? How much will end up in a typical oil & gas “trap” in the crust where they can be economically extracted?

We will learn more about that over time. But between the abiotic gases and the truly ancient hydrocarbons that have survived the eons, it is likely that there is far more hydrocarbon in the deep Earth than geologists typically allow themselves to dream.

 

Al Fin Energy