MAPPING AND INVESTIGATING GEOLOGIC SOURCES OF CARBON DIOXIDE IN CALIFORNIA
The first stage of this study located multiple geologic sources of CO2 in California through database searches, including several legacy U.S. Geological Survey spring and well (hydrocarbon and water) geochemistry studies, commercial inventories of oil and gas field chemistry data, and a literature review. Diverse sources of CO2 were mapped and were broadly categorized according to their geologic setting, such as sedimentary (petroliferous) basin, known geothermal resource area, and structural provinces. This mapping stage identified about 10 key sites for further study, including fieldwork reconnaissance and geochemical sampling.
Geochemical sampling results provided information on potential origins of geologic CO2 in California. CO2 origin may be characterized by gas concentration, isotope geochemistry, and associated noble gas ratios, which all may help distinguish biogenic from thermogenic sources. Results from California include CO2 associated with possible methanogenesis in heavy oil deposits in Ventura Basin legacy fields (δ13C of CO2 from -1 to 14 permil), CO2 associated with possible mantle helium (R/RA ratio of 6.7) in the Salton Sea geothermal system, and CO2 migration through tectonic structures along the San Andreas fault zone (greater than 90 mole percent CO2). Ultimately multiple types of geologic sources of CO2 have been mapped and investigated in California, which is reasonable given the state’s complex tectonic history.