SEQUENTIAL EXTRACTION FOR DETERMINING δ81BR AND δ37CL COMPOSITIONS OF ORGANIC MATTER IN COAL BED METHANE AND SHALE DEPOSITS, CHEROKEE BASIN, USA

In a previous study of Coal Bed Methane (CBM) produced fluids from the Cherokee Basin, abnormally high δ⁸¹Br values and a larger isotopic range of δ⁸¹Br than δ³⁷Cl were observed. It was postulated that the high δ⁸¹Br values could be related to the water-rock interactions between organic matters in coals and the produced waters. To assess this hypothesis, a sequential extraction technique is tested on coal and shale core cuttings. The core cuttings used in this study were obtained from the Cherokee Fm. near the Cherokee Basin, south-eastern Kansas, USA. The δ⁸¹Br and δ³⁷Cl signatures in all the rock leach samples vary between -0.66 and +0.72 ‰ (SMOB), -1.00 and +1.39 ‰ (SMOC) respectively. In the different sequential coal leachates, ³⁷Cl is most enriched in the soluble salt fraction, while ⁸¹Br is most enriched in the organic matter fraction. For most coal samples, the exchangeable ion fraction has the lowest δ⁸¹Br values. For shale samples, the heavier bromine and chlorine stable isotope is enriched in the organic matter relative to the other components. The δ⁸¹Br values in the organic matter leachate are lower than the highest values observed in the produced fluids. Because of the location of the samples in relation to the complex fluid flow system, it is unclear whether this is a result of the locations of the fluid and rock samples relative to each other in the complex fluid flow system, or if another process resulted in the high δ⁸¹Br values in the fluid samples.