Variations of organic matter transformation in response to hydrothermal fluids: Example from the Indiana part of the Illinois Basin

Thermal maturation of organic matter in sedimentary rocks is a complex process controlled by multiple parameters. In this study, we examined the thermal history of one location in the Indiana part of the Illinois Basin. A total of 21 samples spanning the time interval from the Ordovician to the Mississippian were selected for maturity assessment and chemical structure characterization of vitrinite/vitrinite-like particles, solid bitumen, and alginite. The reflectance of vitrinite/vitrinite-like particles (VRo) reveals erratic variations with depth from 0.61% to 1.18%. Solid bitumen reflectance (SBRo), ranging from 0.43 to 0.92%, is slightly lower than VRo across this maturity range in this borehole. In addition to documenting reflectance variations with depth, variations within single organic matter particles were analyzed in the Upper Devonian New Albany Shale and Upper Ordovician Maquoketa Shale samples; these samples were targeted because they contained more organic matter than the samples from other stratigraphic units. SBRo shows significant variations within a single solid bitumen particle with a difference up to 0.81%. Significant maturity-related chemical differences within individual particles of solid bitumen and alginite were also revealed using micro-FTIR technique. An increase in temperature with depth of burial is certainly the key factor in controlling the thermal transformation of organic matter in sedimentary rocks. However, it is apparent that this mechanism alone does not explain the stratigraphic variations of VRo throughout the core and reflectance variations within individual solid bitumen and alginite particles. We suggest that the flow of hydrothermal fluids through the New Albany and Maquoketa Shale caused the observed thermal maturity anomaly. High concentrations of chlorine, vanadium, chromium, and zinc also suggest that organic matter transformation was enhanced by the flow of hydrothermal fluids in the study area.