--> Geochemistry and Characterization of Geologic Methane and Carbon Dioxide Sources in Southern California

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Geochemistry and Characterization of Geologic Methane and Carbon Dioxide Sources in Southern California


Methane (CH₄) and carbon dioxide (CO₂) are greenhouse gases (GHG), and a focus of climate change mitigation. These efforts often describe anthropogenic GHG emissions and ignore the, albeit smaller, contribution of geologic sources. Nevertheless, these geologic sources could become economic resources if explored and exploited properly. This work focuses on better characterizing geologic sources through fieldwork and geochemistry in southern California. Fieldwork was conducted in the San Joaquin, Ventura, and Los Angeles (LA) Basins, and seep gases were sampled and analyzed for alkane composition and isotope geochemistry. Free gases were sampled from oil well annular space, and from bubbling tar pits and warm springs. The tar pits exist over a legacy oil field with legacy abandoned oil wells and at the edge of an extensive oil field bisected by a local fault. The oil wells and warm springs are located in mature hydrocarbon provinces. The oil well and tar pit gas sample origins are likely thermogenic with δ¹³C-CH₄ measurements ranging from -40 to -48‰, whereas the warm spring samples may be biogenic with δ¹³C-CH₄ around -54‰, though some oil well samples also approach the biogenic range. These observations match published results for other tar seeps in the LA Basin. A Bernard plot [δ¹³C-CH₄ vs. C₁/(C₂+ C₃)] confirms most oil well gas samples have a thermogenic origin, whereas tar pit and warm spring samples plot in the mixing zone between thermogenic and microbial. Volume percent of CH₄ versus other light alkanes is referred to as gas wetness and may range up to 30%. The measured seep gases were relatively dry and despite their association with thermogenic oil, the tar pit samples have less than 2% wetness. This may be due to uptake of C₂ to C₅ alkanes by bacteria present in the soil column below the seep. Separately, the average ethane to methane ratio (by volume) for the surface seeps is about 1%, whereas the ratios for the oil well gas are an order of magnitude higher. This disparity agrees with published results comparing seep gas to LA Basin commercial natural gas. Difference in isomeric (i)-butane to normal (n)-butane ratios between the tar pit samples (larger ratios by one order of magnitude or greater) and oil wells was also noteworthy and may be explained by preferential bacterial degradation of n-alkanes over i-alkanes. The CH₄ observations are supported by the CO₂ sampling results. The tar pit samples had δ¹³C-CO₂ value around 25‰ indicative of methanogenesis or oil field biodegradation. The oil well and warm spring gas δ¹³C-CO₂ measurements are lighter, possibly indicating kerogen decarboxylation or hydrocarbon oxidation, respectively. The hydrocarbon and non-hydrocarbon gas geochemistry reveals heterogeneous seep origins and migration pathways, as expected given California’s complex sedimentary and structural geology settings.