The mantle is a dense, hot layer of semisolid rock approximately 2,900 kilometers thick. The mantle, which contains more iron, magnesium and calcium than the crust, is hotter and denser because temperature and pressure inside Earth increase with depth. Because of the firestorm-like temperatures and crushing pressure in Earth’s mantle, molecules behave very differently than they do on the surface. _Source
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 Earth?s mantle. They reasoned that at the mantle?s depth—which begins between 7 and 70 kilometers below Earth?s 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 mantle?s vast size, its hydrocarbon reserves could be much larger than those in Earth?s crust. _PDF LivermoreLab
“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.
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.
By. Al Fin