Monday, August 25, 2008

Abiogenic petroleum origin

In considering the truth of the Peak Oil theory it is worth our while to consider alternate theories. The Peak Oil phenomenon follows from normal theories of the origin of fossil oil. Namely that there is a fixed quantity of oil which exists, and that it was formed from buried biological matter which was compressed and heated over millions of years. However the abiogenic petroleum origin theory claims fossil oil came from other sources. It states that natural petroleum was formed from deep carbon deposits, perhaps dating to the formation of the Earth.

Supporters of the abiogenic hypothesis suggest that there may be a great deal more petroleum on Earth than commonly thought, and that petroleum may originate from carbon-bearing fluids which migrate upward from the mantle. The abiogenic petroleum hypothesis predicts that oil is formed in the mantle at temperatures and pressures consistent with the laws of thermodynamics.

If this abiogenic theory has any truth then perhaps the peak oil theory is poppycock.

However most geologists now consider the biogenic formation of petroleum to be supported scientifically meaning that very few geologists find the abiogenic theory to be of any value.

There is methane visible all through the universe, so there must be non-biological sources for some methane. However it's not clear whether the fossil oil came from any source other than the biogenic source of fossilized biological material.

Within the mantle, carbon may exist as hydrocarbon molecules, chiefly methane, and as elemental carbon, carbon dioxide and carbonates. The abiotic hypothesis is that the full suite of hydrocarbons found in petroleum can be generated in the mantle by abiogenic processes, and these hydrocarbons can migrate out of the mantle into the crust until they escape to the surface or are trapped by impermeable strata, forming petroleum reservoirs.

Abiogenic theories reject the supposition that certain molecules found within petroleum, known as biomarkers, are indicative of the biological origin of petroleum. They contend that some of these molecules could have come from the microbes that the petroleum encounters in its upward migration through the crust, that some of them are found in meteorites, which have presumably never contacted living material, and that some can be generated abiogenically by plausible reactions in petroleum.

Proposed mechanisms of abiogenic petroleum include

  • Primordial deposits: Meteorites are believed to represent the major composition of material from which the Earth was formed. Some meteorites, such as carbonaceous chondrites, contain carbonaceous material. If a large amount of this material is still within the Earth, it could have been leaking upward for billions of years.
  • Creation within the mantle: Russian researchers concluded that hydrocarbon mixes would be created within the mantle.
  • Hydrogen generation: Hydrogen gas and water have been found more than 6 kilometers deep in the upper crust, including in the Siljan Ring boreholes and the Kola Superdeep Borehole. Data from the western United States suggests that aquifers from near the surface may extend to depths of 10 to 20 km. Hydrogen may react with dissolved carbon compounds in water to form methane and higher carbon compounds.
  • Serpentinite mechanism: Petroleum could be formed at high temperatures and pressures from inorganic carbon in the form of carbon dioxide, hydrogen and/or methane. This mechanism is supported by several lines of evidence which are accepted by modern scientific literature. This involves synthesis of oil within the crust via catalysis by chemically reductive rocks. A proposed mechanism for the formation of inorganic hydrocarbons[25] is via natural analogs of the Fischer-Tropsch process known as the serpentinite mechanism or the serpentinite process.
  • Spinel polymerization mechanism: Magnetite, chromite and ilmenite are Fe-spinel group minerals found in many rocks but rarely as a major component in non-ultramafic rocks. In these rocks, high concentrations of magmatic magnetite, chromite and ilmenite provide a reduced matrix which may allow abiotic cracking of methane to higher hydrocarbons during hydrothermal events.

    Chemically reduced rocks are required to drive this reaction and high temperatures are required to allow methane to be polymerized to ethane. Note that reaction 1a, above, also creates magnetite.

  • Carbonate decomposition: Calcium carbonate may decompose at around 500 °C.

Some theories of abiogenic origin of fossil oil is similarity with the Fischer-Tropsch process. This process was created in Nazi Germany before WWII. This and similar reactions can create hydrocarbons through direct reactions or reactions with catalysts. Fischer-Tropsch synthesis proceeds from carbon monoxide and hydrogen, while CO2 hydrogenation proceeds from carbon dioxide and hydrogen. Artificial catalytic materials often use rare materials, but some catalysts use somewhat more common materials such as silicon dioxide, aluminum oxide, iron or nickel. Methane production is most common although more complex products such as ethane, propene, propane, and butane have also appeared. The high temperatures needed for direct reactions are reduced to lower temperatures when a catalyst is present.

Searching Google for the abiogenic origin of fossil oil turns up a number of pages which are of very low credibility. Most of the resulting pages are echoing each other and are not from credible scientific journals or other credible sources. It would appear that the abiogenic theory has little in the way of credible backing.

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