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Emission of Methane and Heavier Alkanes From the La Brea Tar Pits Seepage Area, Los Angeles
Author(s) -
Etiope G.,
Doezema L. A.,
Pacheco C.
Publication year - 2017
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2017jd027675
Subject(s) - methane , geology , hydrocarbon , petroleum seep , flux (metallurgy) , natural gas , petroleum , oil sands , fossil fuel , plume , fault (geology) , butane , hydrology (agriculture) , environmental chemistry , geochemistry , environmental science , asphalt , geotechnical engineering , chemistry , paleontology , archaeology , meteorology , organic chemistry , biochemistry , physics , history , catalysis
Natural hydrocarbon (oil and gas) seeps are widespread in Los Angeles, California, due to gas migration, along faults, from numerous subsurface petroleum fields. These seeps may represent important natural contributors of methane (CH 4 ) and heavier alkanes (C 2 ‐C 4 ) to the atmosphere, in addition to anthropogenic fossil fuel and biogenic sources. We measured the CH 4 flux by closed‐chamber method from the La Brea Tar Pits park (0.1 km 2 ), one of the largest seepage sites in Los Angeles. The gas seepage occurs throughout the park, not only from visible oil‐asphalt seeps but also diffusely from the soil, affecting grass physiology. About 500 kg CH 4 d −1 is emitted from the park, especially along a belt of enhanced degassing that corresponds to the 6th Street Fault. Additional emissions are from bubble plumes in the lake within the park (order of 10 2 –10 3 kg d −1 ) and at the intersection of Wilshire Boulevard and Curson Avenue (>130 kg d −1 ), along the same fault. The investigated area has the highest natural gas flux measured thus far for any onshore seepage zone in the USA. Gas migration, oil biodegradation, and secondary methanogenesis altered the molecular composition of the original gas accumulated in the Salt Lake Oil Field (>300 m deep), leading to high C 1 /C 2+ and i ‐butane/ n ‐butane ratios. These molecular alterations can be important tracers of natural seepage and should be considered in the atmospheric modeling of the relative contribution of fossil fuel (anthropogenic fugitive emission and natural geologic sources) versus biogenic sources of methane, on local and global scales.