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Title: 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 » a rough estimate, this calculation suggests that little confidence should be placed in the resource potential of abiogenic natural gas. In rift or convergent margin basins, hot magmatic fluids can strip methane and other volatiles from metamorphic basement and overlying sedimentary rocks, however, and commercial accumulations of this type of gas may be present. The economic value of such reserves could be impaired by dilution with carbon dioxide and possibly nitrogen. 171 refs., 13 figs., 2 tabs. « less
Authors: ; ; ;
Publication Date:
OSTI Identifier:OSTI ID: 7052010
Resource Type:Journal Article
Resource Relation:Journal Name: United States Geological Survey, Professional Paper; (United States); Journal Volume: 1570
Country of Publication:United States
Language:English
Subject: 03 NATURAL GAS; HELIUM; SYNTHESIS; METHANE; ABIOGENIC GAS; CHEMICAL COMPOSITION; EARTH MANTLE; METAMORPHISM; NATURAL GAS FIELDS; OIL FIELDS; TRAPPING; ALKANES; ELEMENTS; ENERGY SOURCES; FLUIDS; FOSSIL FUELS; FUEL GAS; FUELS; GAS FUELS; GASES; GEOLOGIC DEPOSITS; HYDROCARBONS; MINERAL RESOURCES; NATURAL GAS; NATURAL GAS DEPOSITS; NONMETALS; ORGANIC COMPOUNDS; PETROLEUM DEPOSITS; RARE GASES; RESOURCES 030200* -- Natural Gas-- Reserves, Geology, & Exploration