GEOTHERMAL ARCHITECTURE OF THE ILLINOIS BASIN AT THE TIME OF COALIFICATION

Jorge E. MARIÑO,

Department of Geology, University of Illinois at

Urbana-Champaign, IL,

[email protected]

Previous research in the Illinois basin indicates that coal maturity is incompatible with burial depth, and the highest maturity occurs near a region of known hydrothermal alteration. A preliminary reanalysis of vitrinite reflectance (VR) in deep wells suggest that the paleogeotherm in the basin contains an inflection point by the Mississippian-Pennsylvanian boundary. The purpose of this study is to: (1) test the existence of a stratigraphically controlled inflection by using additional VR analysis and other indicators of temperature like conodonts and petrography; (2) build on previous fluid-flow models (Basin2) by testing if changes in permeability can explain the inflection.

Preliminary paleogeotherms obtained so far indicate that to the south, where VR values are highest, the paleogradients are divided into: (1) a gentler gradient near the surface adjacent to the coal bearing units, and (2) a steeper gradient below the coal bearing units. The conodonts by the inflection point display saccaroidal and pitted surfaces and the petrography reveals the presence of baroque dolomite and dolospar cements with bright colors under cathodoluminescence light. Those three features have been associated with hydrothermal activity.

Preliminary results show that the inflection in the paleogeotherm is probably due to lateral influx of heat by hydrothermal fluids passing through Carboniferous aquifers. These fluids interacted with Precambrian basement rocks prior to entering the Mississippian-Pennsylvanian aquifers through pre-existing faults.

AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid