NOBLE GAS GEOCHEMISTRY FOR CHARACTERIZING GEOLOGIC CO2 SOURCES IN SELECTED CALIFORNIA BASINS

Research on characterizing geologic sources of CO₂ is critical because it is a powerful greenhouse gas and a valuable energy gas when used for enhanced oil recovery. In California, CO₂ accumulations were initially categorized according to their geologic setting: sedimentary (petroliferous) basin, known geothermal resource area, and structural province. Accumulations may be further characterized by gas composition, isotope geochemistry, and associated noble gases. Noble gas fractionation patterns may suggest differing mechanisms for gas partitioning from a parent fluid, more clearly defining fluid systems involved in free gas formation (e.g., oil versus groundwater), and may allow for interpretations of gas genesis when coupled with bulk gas and carbon isotopic composition.

We present data from high- and low-CO₂ natural gas samples from four heterogeneous basins in California. These natural accumulations typically have high helium isotopic ratios (R/R_A) regardless of bulk gas composition, suggesting mantle or magmatic sources. These mantle signatures, however, may be concealed by ⁴He-rich gas containing radiogenic crustal He, typical in natural gas generation and migration in petroliferous basins. CO₂ sourced from oils displays characteristic air-like Kr and Xe enrichment, though without matching air-like ²⁰Ne/³⁶Ar ratios. This mismatch suggests a trapped, sedimentary (i.e., crustal) source for atmospheric Kr and Xe, released during oil formation and primary migration. CO₂-poor spring waters display noble gas partitioning between air-saturated water and atmospheric components resulting from possible CO₂ dilution or degassing. Therefore, these spring gases are less confidently attributed to the mantle and may also have a crustal genesis. CO₂-rich samples from known geothermal resource areas may be attributed to a mantle genesis based on their R/R_A ratios (>0.2) and δ¹³C CO₂ values around -5‰. Heterogeneous gas sources are expected in complex structural settings.