Constraining the Fluid History of a CO₂‐H₂S Reservoir: Insights From Stable Isotopes, REE, and Fluid Inclusion Microthermometry

Abstract

Reservoirs that host CO2‐H2S‐bearing gases provide a key insight into crustal redox reactions such as thermochemical sulfate reduction (TSR). Despite this, there remains a poor understanding of the extent, duration, and the factors limiting this process on a reservoir scale. Here we show how a combination of petrography, fluid inclusion, rare earth element (REE), and carbon (δ13C), oxygen (δ18O), and sulfur (δ34S) stable isotope data can disentangle the fluid history of the world's largest CO2 accumulation, the LaBarge Field in Wyoming, USA. The carbonate‐hosted LaBarge Field was charged with oil around 80 Ma ago, which together with nodular anhydrite represent the reactants for TSR. The nodules exhibit two distinct trends of evolution in δ13C with both δ34S and δ18O that may be coupled to two different processes. The first trend was interpreted to reflect the coupled dissolution of anhydrite and reduction to elemental sulfur and the oxidation of organic compounds and associated precipitation of calcite during TSR. In contrast, the second trend was interpreted to be the result of the hydrothermal CO2 influx after the cessation of TSR. In addition, mass balance calculations were performed to estimate an approximate TSR reaction duration of 80 ka and to identify the availability of organic compounds as the limiting factor of the TSR process. Such an approach provides a tool for the prediction of TSR occurrence elsewhere and advancing our understanding of crustal fluid interactions.

Behavior of rare earth elements in an aquifer perturbed by CO2 injection: Environmental implications

“…REE signatures in fluid inclusions is also a key parameter for enhancing an understanding of the behavior of deep geological fluids. In particular, REE fractionation patterns are good proxies for evaluating the physico-chemical conditions, fluid-mixing, and precipitation processes operating during the genesis of valuable geological resources, such as oil, gas, and ore deposits (Zwahlen et al 2019;Shu and Liu 2019;Perry and Gysi 2018).…”

“…3 5). The Eu/Eu* anomaly was not considered, since interference with BaO during analysis was suspected (Shabani et al 1992;Dulski 1994;Zwahlen et al 2019). Despite the absence of a clear correlation between Ba and Eu (see supplementary materials), the ratio Eu/Ba was >4000, and this may have led to a false positive Eu signal.…”

Phase transitions in natural C-O-H-N-S fluid inclusions - implications for gas mixtures and the behavior of solid H2S at low temperatures

“…Laser Raman spectroscopy allows to determine molecular compositions of complex C–O–H–N–S inclusion gas mixtures 8 , 10 , 20 – 23 . Combined microthermometric and Raman spectroscopic analyses coupled with phase diagrams/EoS provide means to reconstruct the spatial and temporal P–T evolution in mineralizing systems 10 , 24 and gas reservoirs 8 , 21 , 22 , 25 . The latter are often characterized by multiple episodes of coinciding mineralization and gas/oil re-charge.…”

Reaction path modelling illustrating the fluid history of a natural CO2-H2S reservoir

“…The gas trapped in LaBarge Field consists on average of 66% CO 2 , 21% CH 4 , 7% N 2 , 5% H 2 S and 0.6% He (Huang et al, 2007). The H 2 S is thought to be produced by thermochemical sulphate reduction (TSR) whereas the CO 2 is believed to be from a magmatic source (De Bruin, 1991Huang et al, 2007;Stilwell, 1989;Zwahlen et al, 2019). However the timing of the fluids remains controversial.…”

“…Hydrocarbons are thought to have migrated into the reservoir approximately 84 -76 Ma ago (Johnson, 2005) (Figure 2). Different techniques and data sets have been employed to study the influence of CO 2 and H 2 S upon each other (Huang et al, 2007;Kaszuba et al, 2011;Obidi, 2014;Zwahlen et al, 2019): diffusion modelling of the CO 2 -CH 4 distribution concluded that hydrocarbon and CO 2 were introduced at similar times, around 50 Ma ago (Huang et al, 2007) and reaction path modelling has been used to suggest that TSR was triggered by the influx of CO 2 (Kaszuba et al, 2011).…”