Abstract: Potential Problems Using Vitrinite Reflectance to Calibrate Thermal History Models in the Illinois Basin

Vito F. Nuccio, Joseph R. Hatch

Vitrinite reflectance (R_o) is commonly used to calibrate thermal history and petroleum generation models in sedimentary basins. Laboratory-derived kinetic-R_o profiles are adjusted to match measured R_o values by varying geologic parameters such as paleogeothermal gradients, paleo-surface temperatures, heat flow, and burial history. However, if calibrated to problematic measured R_o values, the resulting thermal-history model may not represent the "true" geologic history of the study area. It is obvious, therefore, that recognition and remediation of problems with R_o data is critical when modeling the thermal and petroleum generation history of a basin.

During thermal-history reconstruction of the Illinois basin of Illinois, Indiana, and Kentucky, we observed that the level of thermal maturity in R_o profiles (constructed from Pennsylvanian Herrin no. 6 coal bed and Upper Devonian-Lower Mississippian New Albany Shale samples) do not always increase with increasing depth of burial and, in some instances, actually decrease with increasing depth. For example, in Wayne County, Illinois, in the central part of the basin, R_o of the Herrin coal is 0.65 percent and R_o of the New Albany Shale, 2,600 feet below, is also 0.65 percent. Based on Rock-Eval pyrolysis, other organic geochemical data, and kinetic-R_o models from published burial and thermal histories, vitrinite reflectance suppression appears to be around 0.35 percent for the New Albany in this part of the basin. For example, T_max values for the New Albany in Wayne County average 450°C; these equate to a R_o equivalent of approximately 0.90-1.0 percent. The kinetic-R_o model for this area also predicts an R_o of about 1.0 percent for the New Albany, similar to the R_o equivalent from T_max.

Organic geochemical data indicate that the level of suppression for the New Albany is variable throughout the basin, depending on the quality of the organic matter and level of maturation. For example, measured R_o values are consistently lower than R_o equivalents from T_max up to a T_max of approximately 455°C, at which point measured R_o seems to converge and is compatible with T_max values. Also, R_o values show an inverse relationship to hydrogen indices (HI); R_o decreases with increasing HI. As a consequence of the misleading R_o data, the present study determines the amount of R_o suppression for the New Albany Shale and uses the corrected values to recalibrate the thermal istory of the Illinois basin.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994