The Origin of Methane (and Oil) in the Crust of the Earth
Thomas Gold
U.S.G.S. Professional Paper 1570, The Future of Energy Gases, 1993
Abstract
The deposits of hydrocarbons in the crust of the Earth have long been regarded by many investigators as deriving from
materials incorporated in the mantle at the time of the Earth's formation. Outgassing processes, active in all geological
epochs, then transported the liquids and gases liberated there into porous rocks of the crust. The alternative viewpoint, that
biological debris was the source material for all crustal hydrocarbons, gained widespread acceptance when molecules of
clearly biological origin were found to be present in most commercial crude oils.
Modern information re-directs attention to the theories of a non-biological, primeval origin. Among this information is the
prominence of hydrocarbons—gases, liquids and solids—on many other bodies of the solar system, as well as in interstellar
space. Advances in high-pressure thermodynamics have shown that the pressure-temperature regime of the Earth would
allow hydrocarbon molecules to be formed and to survive between the surface and a depth of 100 to 300 km. Outgassing
from such depth would bring up other gases present in trace amounts in the rocks, thus accounting for the well known
association of hydrocarbons with helium. Recent discoveries of the widespread presence of bacterial life at depth point to
this as the origin of the biological content of petroleum. The carbon budget of the crust requires an outgassing process to
have been active throughout the geologic record, and information from planets and meteorites, as well as from mantle
samples, would suggest that methane rather than CO2
could be the major souce of surface carbon. Isotopic fractionation of
methane in its migration through rocks is indicated by numerous observations, providing an alternative to biological
processes that have been held responsible for such fractionation. Information from deep boreholes in granitic and volcanic
rock of Sweden has given support to the theory of the migration of gas and oil from depth, to the occurrence of isotopic
fractionation in migration, to an association with helium, and to the presence of microbiology below 4 km depth.
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