Abiogenic hydrocarbons and mantle helium in oil and gas fields
Abiogenic hydrocarbons and mantle helium in oil and gas fields Most carbon degassed from Earth's interior is released as volcanic and hydrothermal carbon dioxide along oceanic spreading ridges and as volcanic and geothermal carbon dioxide in continental regions. The chemistry of the earth's interior is poorly understood, however, and methane, rather than carbon dioxide, may be the dominant form of carbon throughout much of the mantle. According to one hypothesis, methane from the mantle is continuously injected into the deep crust at lithospheric plate boundaries, ancient suture zones, and other areas of crustal weakness such as large meteorite impact sites. Where introduced beneath sedimentary basins this methane could accumulate in the conventional structural and stratigraphic traps in which we find petroleum. Geochemical evidence strongly argues that crude oil is of sedimentary (biogenic) origin, but the origins of natural gas are more complex and the proportion that may be derived from mantle (that is, abiogenic) sources is unknown. Using a geometric mean of 3x10[sup 6] for the molar CH[sub 4]/[sup 3]He ratio in uncontaminated, mantle-derived fluids from spreading ridges, mantle plumes and summit fumaroles of arc volcanoes, the median abiogenic methane content of commercial gases is estimated to be less than 200 ppm by volume (range=0 to 12,000 ppm). While admittedly more »
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