T224 Environmental Impacts Studies for Energy Resources

Wednesday, 7 November 2018
8:00 AM - 12:00 PM
Room 135

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

Paper number: T224-6
Program: Technical Sessions
Time: 9:15 AM – 9:30 AM

Authors

Abstract

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/RA) 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/RA ratios (>0.2) and δ13C CO2 values around -5‰. Heterogeneous gas sources are expected in complex structural settings.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-323128

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