THE WATER AND GAS GEOCHEMISTRY OF HYDROCARBON-RICH SEEPS FROM THE KAROO BASIN

The production of unconventional shale gas and oil continues to increase throughout the US and abroad. Because of the environmental sensitivities of the water-restricted Karoo Basin located in South Africa, the South African government developed a plan for a team of domestic and international experts to evaluate the baseline water quality of the region prior to potential shale gas development. As part of the baseline water quality assessment, we analyzed water quality and water and gas geochemistry. Our analyses utilized a multi-tracer approach for evaluating baseline gas and salt chemistry within this ecologically sensitive region. Because one of the most common risks associated with shale gas development is stray gas contamination of groundwater, we focused the current contribution on our efforts to characterize the gas geochemistry of naturally hydrocarbon-gas-rich groundwaters from diverse locations throughout the Karoo Basin. The current study includes a comprehensive evaluation of major gas geochemistry (e.g., N₂, Ar, CO₂), hydrocarbon gas geochemistry (C₁-C₅), compound-specific stable isotopes of hydrocarbons and CO₂ (δ²H-CH₄, δ¹³C-CH₄, δ¹³C-C₂H₆, δ¹³C-CO₂), noble gas geochemistry (He, Ne, Ar, Kr, Xe), tritium-helium (³H-³He) age-dating, water chemistry (pH, Eh, δ²H-H₂O, δ¹⁸O-H₂O) and salt chemistry (e.g., Na, Ca, Cl, Br). Like studies from other areas across North America, we identify a significant positive correlation between the abundance of salts, elevated helium, higher levels of ethane, and measurable higher order aliphatic hydrocarbons in thermogenic methane that also displays more enriched values of δ¹³C-CH₄. By comparison, we also identify and distinguish a cohort of helium-poor, biogenic methane with more negative values of δ²H-CH₄, δ¹³C-CH₄, and significantly higher values of C₁/C₂+ (i.e., lower levels of ethane and higher order aliphatic hydrocarbons). In conclusion, our multi-tracer approach allows us to constrain the plausible scenarios and geological histories of the hydrocarbon gas migration from hydrocarbon sources to shallow aquifers. This work further demonstrates the capacity to differentiate biogenic and thermogenic contributions in hydrocarbons within shallow aquifers that experience oxidation and molecular and isotopic fractionation of hydrocarbons.