RELATIONSHIP BETWEEN MICROBIAL COMMUNITY COMPOSITION AND GEOCHEMICAL PROPERTIES OF WATERS ASSOCIATED WITH NATURAL CO2 ACCUMULATIONS IN THE SOUTHERN PERMIAN BASIN

The U.S. Geological Survey is investigating naturally occurring carbon dioxide (CO₂) accumulations (>5% CO₂), focusing on the origin and migration pathways of CO₂, evaluating CO₂ resources for potential utilization in enhanced oil recovery (CO₂-EOR), and understanding the relationship between microbial community composition and geochemical properties of CO₂ reservoirs in hydrocarbon provinces such as the Permian Basin. Little is known about microbial communities residing in high-CO₂ reservoirs and how differences in water geochemical composition in high-CO₂ systems may impact those communities. This study further investigates geochemical site characteristics that may drive microbial community composition.

Waters from seven field sites in the southern Permian Basin in west Texas were sampled for this study—three hot springs sampled over two field campaigns (36.9°C – 44.3°C, n=6) and four commercial gas production wells (14.8°C – 24.5°C, n=4). Samples were analyzed for elemental and isotopic geochemistry, and filtered to collect biomass for 16S rRNA gene sequencing. The gas wells had a greater range of pH, alkalinity, and conductivity (5.5 – 7.7 pH, 129 – 5,250 mg/L of CaCO₃, 0.218 – 40.7 mS/cm) compared to the hot springs (6.5 – 7.0 pH, 613 – 710 mg/L of CaCO₃, 11.0 – 13.0 mS/cm). Waters from the hot springs and one of the four gas wells were dominated by Bacteria (>99% of total composition), while Archaea were observed at >10% in the remaining gas wells. No statistically significant (p value = 0.001) associations were observed between the microbial community composition and geochemical parameters of the samples, as p values ranged from 0.006 (Si) to 0.109 (pH and Li). However, hot spring water geochemistry is significantly different from that of gas well water. This finding is supported by previous work studying these same sample sites showing site type (hot springs vs. gas production wells) to be more significant than CO₂ concentration on microbial community composition. Studies of microbial communities within natural CO₂ systems may be beneficial for gaining insight into the relationship between microbial community structure and reservoir geochemistry in areas that may be impacted by long-term anthropogenic geologic CO₂ storage or CO₂-EOR operations.