olelog

There is widespread evidence that petroleum originates from biological processes. Whether hydrocarbons (oil and gas) can also be produced from abiogenic precursor molecules under the high-pressure, high-temperature conditions characteristic of the upper mantle remains a disputed question.

A hypothesis that oil can be created by non-biological mechanisms originated in Russian and Ukrainian scientific circles in the 1950s. Put briefly, it proposed that petroleum forms deep in the Earth’s mantle under extremely high pressure and temperature through a reaction between carbonates, iron oxides and water. This process goes on continuously, and the petroleum migrates upwards through the lithosphere. At issue is the formation of complex hydrocarbons. There has never been any doubt that simple hydrocarbons such as methane can be formed by inorganic processes.

If true it was speculated it might be possible to find oil deep under the Siljan Impact Crater (Sweden), and they drilled here in the 1980’s and 1990’s, among other things in the hope to find oil. The hypothesis went that (inorganic) methane (gas) migrates upwards from the mantle and transforms into oil in the upper crust in igneous rocks (like granite). The Siljan ring is a crater of granitic rocks overlain by soil formed by a large meteorite impact 360 million years ago. The impact was postulated to have created fractures at great depth through which gas and oil would have been able to migrate. Although a little bit of oil was found, the drilling was more or less a failure – as far as oil and gas is concerned. Many scientists (though not everybody) thought that that was the final end of the abiotic oil story.

Apparently the dream has been awakened again.

As I said Scientists have debated for years whether some of our oil and gas (hydrocarbons) could also have been created deeper in the Earth and formed without organic matter. Now for the first time, according to a study published in the July 26, advanced on-line issue of Nature Geoscience, scientists have found that ethane and heavier hydrocarbons can be synthesised under the pressure-temperature conditions of the upper mantle —the layer of Earth under the crust and on top of the core (red in the figure below).


Click on image to enlarge!

Using a diamond anvil cell and a laser heat source, scientists first subjected methane to pressures exceeding 20 thousand times the atmospheric pressure at sea level and temperatures ranging from 704°C to over 1 227 °C. These conditions mimic those found 65 to 150 km deep inside the Earth. The methane reacted and formed ethane, propane, butane, molecular hydrogen, and graphite. The scientists then subjected ethane to the same conditions and it produced methane. The transformations suggest heavier hydrocarbons could exist deep down. The reversibility implies that the synthesis of saturated hydrocarbons is thermodynamically controlled and does not require organic matter.

The results from the study seem to support the suggestion that hydrocarbons heavier than methane can be produced by abiogenic processes in the upper mantle.

Reference:
Kolesnikov et al.
Methane-derived hydrocarbons produced under upper-mantle conditions
Nature Geoscience
Published online: 26 July 2009
doi:10.1038/ngeo591

• http://www.nature.com/ngeo/journal/vaop/ncurrent/abs/ngeo591.html
• http://www.ciw.edu/news/hydrocarbons_deep_earth
• http://www.scientificblogging.com/news_articles/peak_oil_not_if_deep_earth_hydrocarbon_theory_true
• http://www.npd.no/English/Aktuelt/Nyheter/22.5.2007+Fossilt+drivstoff+uten+fossiler.htm?print=true
• http://www.physorg.com/news167835116.html
• http://www.eurekalert.org/pub_releases/2009-07/ci-hit072409.php
• http://www.eurekalert.org/multimedia/pub/15568.php?from=141472
• http://www.sciencecodex.com/deep_earth_hydrocarbon_discovery_oil_and_methane_without_organic_matter
• http://www.planetthoughts.org/?pg=pt/Whole&qid=2982&src=rss
• http://www.sciencedaily.com/releases/2009/07/090726150843.htm
• http://www.energy-daily.com/reports/Hydrocarbons_In_The_Deep_Earth_999.html
• http://www.abc.net.au/science/articles/2009/07/28/2638484.htm

In Danish:
• http://ing.dk/artikel/100215-olieproduktion-i-laboratorier-nye-forsoeg-vaekker-forskernes-haab?utm_medium=rss&utm_campaign=nyheder



PS of 28 July 2009:
Glenn Reynolds files the story under “THINGS I’D LIKE TO BE TRUE” at http://pajamasmedia.com/instapundit/82538/

Can slabs of oceanic crust descend as deep as to the core-mantle boundary? Is cold mid-ocean-ridge basalt dense enough to sink so deep into the mantle?

Do mantle plumes exist? And if so, can they well up from as deep as the core-mantle boundary?

These and other core-mantle boundary related questions are occupying many earth scientists. They just want to know.

Most of our earlier knowledge of the Earth's interior came from studying seismic waves (after earthquakes). It became clear that the boundary between the core and the mantle was something special. S-waves (secondary wave or shear wave) stopped. They cannot propagate through liquids. Seismic velocity is linked to the density of the medium through which the waves travel. P-wave (primary wave) velocity in general increases with increasing density - In liquids, however, the speed will be less. The core is much denser than the mantle.

When it was discovered that a thin layer directly above the core-mantle boundary had some mysterious properties it had to get a name consistent with the naming of the earth’s layers in the middle of last century. It is now still referred to as the D" ("D double-prime" or "D prime prime"). The D" name originates from the mathematician Keith Bullen's designations for the Earth's layers. His system was to label each layer alphabetically, A through G, with the crust as 'A' and the inner core as 'G'. In his 1942 publication of his model, the entire lower mantle was the D layer. In 1950, Bullen found his "D" layer to actually be two different layers. The upper part of the D layer, about 1800 km thick, was renamed D’ (D prime) and the lower part (the bottom 200 km) was named D" (pronounced "dee double prime"). A layer that produces strange seismic properties.

Research into the mysteries of the D“ layer has advanced spectacularly since 2004, when Japanese researchers found that high temperatures and pressures like those existing in the D” layer transform perovskite, the major mineral in Earth's mantle. The publication in the journal Science of Post-Perovskite Phase Transition in MgSiO₃ is seen as a turning point. Since then many papers on this topic have followed. The latest I have seen is Radiative conductivity in the Earth's lower mantle by Goncharov et al. in today’s (13 November 2008) issue of Nature.

The discovery of post-perovskovite has profound implications for the chemical, thermal, and dynamical structure of the lowermost mantle (the D” region). Several major seismological characteristics of the D” region can now be explained by the presence of post-perovskite, and the specific properties of the phase transition provide the first direct constraints on absolute temperature and temperature gradients in the lowermost mantle. A discussion of the current understanding of the core–mantle boundary region can be found in Discovery of Post-Perovskite and New Views on the Core–Mantle Boundary Region by Kei Hirose and Thorne Lay in Elements of June 2008 (DOI: 10.2113/GSELEMENTS.4.3.183 - start downloading of the full article as large pdf-file by clicking here).

The Mg²⁺ site in post-perovskite is smaller than in perovskite, resulting in a volume reduction of 1.0–1.5%. Unlike perovskite, the post-perovskite phase has a layered structure of the SiO₆ octahedra, which may lead to a large contrast in some properties with perovskite.


Scenario for the D” region. The D” seismic discontinuity is caused by the perovskite (Pv) to post-perovskite (PPv) phase transition. Post-perovskite may transform back to perovskite in the bottom thermal boundary layer with a steep temperature gradient. The large low-shear-velocity provinces (LLSVP) underneath upwellings (forming plumes) possibly represent large accumulations of dense MORB-enriched materials. The solid residue formed by partial melting in the ultralow-velocity zone (ULVZ - thin reddish zone on image) might also be involved in upwelling plumes. Such thin (20-40 km) ULVZ-layers are mainly (but not only) found under the Pacific Ocean and under Africa.

• http://www.eurekalert.org/pub_releases/2008-11/ci-eht111008.php
• http://www.cems.umn.edu/research/wentzcovitch/highlights/science_now_040324.htm
• http://olivine.ethz.ch/~artem/NewMinerals.html
• http://www.ucsc.edu/news_events/press_releases/text.asp?pid=978
• http://www.nature.com/ngeo/journal/v1/n1/full/ngeo.2007.44.html (full paper in HTML)

For my (increasing number of) Scandinavian readers there is review in Norwegian at Geoportalen - http://www.geoportalen.no/planetenjorden/jordensindre/revolusjon/ . The article here by Reidar G. Trønnes, Natural History Museum, Universitty of Oslo, on the Earth’s Interior is somewhat easier to consume, and is well illustrated.

Maybe I ought to add that some of the topics discussed in the (review) papers are still controversial.

I had just started writing a new post concerning a paper that could change our view on mantle plumes, when I became aware of a weird piece in Der Spiegel. The post on which I began will now have to wait till tomorrow, but in some ways the two articles are related.

On January 25 2008 Spiegel Online International brought an article titled Where Continents Go To Die - A New Look into the Center of the Earth.

Let me start with saying that I don’t believe in this theory (yet), but find it worth discussing. So here we go.

Old, cold plates are pushed down into the Earth's mantle on the continental edges, where they collect large amounts of iron. Weighted down by the iron, the plates sink farther and farther into the Earth's mantle. There, at a depth of 2,900 km, they settle into "plate graveyards”. Heat and pressure in the depths trigger chemical processes, causing the plates to deposit their load of heavy elements. Once liberated of this burden (a few hundreds of millions years later), they become lighter than their surroundings, causing them to rise and as mantle plumes they make their way toward the surface (at hot spots etc.). Well, that is the biggest convection cell that I have ever heard of, and I must stress that this is only a (new) theory, that in no way has been proven by facts, as far as I know.

More about the mantle plume bit in my next post here.


• http://www.spiegel.de/international/world/0%2C1518%2C531023%2C00.html
• http://www.spiegel.de/international/world/0,1518,grossbild-1078154-531023,00.html




PS:

1. In another context (science journalists/journalism) Chris (goodSchist) had an indirect comment to this post over at Clastic Detritus (Response#2).
   
2. There is a dedicated mantle plume website at http://www.mantleplumes.org/ with links to lots of (free) papers on mantle plumes.