MAPPING AND INVESTIGATING GEOLOGIC SOURCES OF CARBON DIOXIDE IN CALIFORNIA

Carbon dioxide (CO₂) may be considered both a greenhouse gas and a medium for enhanced oil recovery. Therefore improved identification and characterization of geologic sources of CO₂ is important for future economic and climate risk evaluations. Other important energy-related uses for CO₂ include geothermal heat extraction and dry ice refrigeration. Natural sources of CO₂ are varied, and may be categorized based on geologic setting, origin, and flux. This study focuses on geologic CO₂ in California basins, and summarizes currently known source locations along with details about possible origins of the gas. Complementary work focuses on fluxes of CO₂ in volcanic provinces and future work may focus on subsurface migration pathways, both of which are outside the scope of this study.

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/R_A 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.