| In the Illinois Basin (IB) the coals with the highest rank are not located within the deepest part (depocenter) of the basin, but rather occur to the south near to an intrusion, additionally the analysis of vitrinite reflectance in deep wells suggests that the paleogeotherm of the basin contain an inflection at the Mississippian-Pennsylvanian boundary. These observations suggest that topographically driven regional flow of hydrothermal fluids through the Upper Paleozoic Lower Paleozoic aquifers raised the temperature and caused an inflection(s) found in the paleogeotherms. The purpose of this dissertation was to; characterize the paleogeothermal architecture of the basin, through analysis of vitrinite reflectance; determine if there is independent evidence for regional hydrothermal fluid flow in the basin at the time of coalification, through the study of conodont alteration, CL petrography, study of fluid inclusions, and measurement of stable-isotopes; and model the basin using the Basin2 finite-difference program. The results demonstrate that paleotemperatures decrease from south to north, and that the character of the paleogeotherm changes markedly with location. In the Southern Zone the paleogeotherm overall is very irregular. In the Central Zone, the slope of the paleogeotherm displays a single notable inflection. And in the Northern Zone of the basin, the paleogeotherm has a straight slope that parallels the present-day geotherm. The results from conodont alteration, CL petrography, study of fluid inclusions, and measurement of stable-isotopes suggest that there was an episode of hydrothermal fluid flow through the IB, that the flow event was of short duration, and that the flow was channeled through Mississippian strata. The paragenetic sequence above the unconformity is different from that below, suggesting that, the unconformity separates two distinct paleohydrologic systems. The modeling indicates that the best scenario is one with a transient heat source at the southern end of the basin representing the intrusion, with a topographic gradient sloping from south to north that drives flow; and with a high permeability zone in Mississippian strata. Such a scenario can produce the high temperature to the south, the decrease in temperature to the north and a small inflection at the Mississippian-Pennsylvanian boundary. |
