Geologic Emissions of Methane and C2 - C5 Alkanes at the La Brea Tar Pits, Los Angeles, CA
Abstract
Natural hydrocarbon (oil and gas) seeps are widespread in Los Angeles due to gas migration, along faults, from numerous subsurface petroleum fields. These seeps may represent important natural contributors of methane (CH4) and heavier alkanes (C2-C4) for the atmosphere. Methane flux measurements were made from various locations at the La Brea Tar Pits in Los Angeles, CA. Measurements were made using a closed-chamber method and spectroscopic sensors for CH4 and CO2, at 26 oil-asphalt seeps and 188 other sites, without gas manifestations, homogeneously distributed throughout the park. The molecular C1 - C5 composition of gas released from seeps and soil was also analyzed using either FTIR spectroscopy or gas chromatography (GC-FID). Methane emissions from seeps varied from approximately 7 to 54,000 g m-2 day-1, while emissions from soil degassing were between 0 and 9,000 g m-2 day-1. Total emissions were estimated to be in the order of 103 kg day-1 for methane, and at least 10 and 5 kg day-1 for ethane and propane, respectively. The seeping gas exhibited high C1/(C2 + C3) ratios, likely due to molecular fractionation during gas migration from a subsurface petroleum reservoir. Evidence for biodegredation in certain samples was indicated by large i-butane to n-butane ratios. These molecular alterations can be important tracers of natural seepage and should be considered in the atmospheric modelling of the relative contribution of fossil fuel (anthropogenic fugitive emission and natural geologic sources) vs biogenic sources, on local and global scales.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.A31B2166D
- Keywords:
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- 0322 Constituent sources and sinks;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0345 Pollution: urban and regional;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0368 Troposphere: constituent transport and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 3307 Boundary layer processes;
- ATMOSPHERIC PROCESSES