Stable Bromine Isotopic Composition Of Coal Bed Methane (Cbm) Produced Water, The Occurrence Of Enriched Br-81, And Implications For Fluid Flow In The Midcontinent, Usa

This study characterizes the delta Cl-37, delta Br-81, and Sr-87/Sr-86 of coal bed methane produced fluids from Pennsylvanian Cherokee Group coals of the Cherokee Basin in southeast Kansas, USA. The delta Cl-37, delta Br-81, and Sr-87/Sr-86 values range between -0.81 and +0.68 parts per thousand (SMOC), -0.63 and +3.17 parts per thousand (SMOB), and 0.70880 and 0.71109, respectively. A large percentage of samples have delta Br-81 above +2.00 parts per thousand. Two fluid groups were identified on the basis of K/Br, Br/Cl, and Ca/Mg ratios, temperature, He content, delta H-2, delta O-18, delta Br-81, and Sr-87/Sr-86. Both fluid groups have geochemical similarities to fluids in Cambrian, Ordovician, and Mississippian units. Lower salinity and higher temperature fluids from deeper units are leaking up into the Cherokee Group and mixing with a higher salinity fluid with higher delta Br-81 and more radiogenic Sr-87/Sr-86. Variation in delta Cl-37 indicates an unknown process other than mixing is affecting the salinity. This process does not appear to be related to evaporation, evaporite dissolution, or diffusion. Insufficient data are available to evaluate halide-gas or water-rock interaction, but halide-gas interactions are not likely a significant contributor to high delta Br-81. Rather, interactions with organically bound bromine and soluble chloride within the coal could have the strongest effect on delta Cl-37 and delta Br-81 values.