PITMAN, JANET K., and MARTIN B. GOLDHABER, U. S. Geological Survey, Denver, CO; THOMAS H. SHAW, City University of New York, NY; and ROBERT C. BURRUSS, U. S. Geological Survey, Denver, CO

ABSTRACT: Basin-Scale Brine Movement in the Midcontinent during the Late Paleozoic: Evidence from the St. Peter Sandstone in the Illinois Basin

Diagenetic and geochemical alteration patterns in the Ordovician St. Peter Sandstone and bounding carbonate rocks of the Illinois basin record late Paleozoic northward brine migration in a large paleoaquifer system. A key tracer for brine migration is fluorine introduced during igneous activity at Hicks Dome at 272 Ma in the southern part of the basin. Igneous processes also contributed to source-rock maturation in southeastern most Illinois. Primary oil inclusions in fluorite from the Illinois/Kentucky fluorspar district demonstrate that mineralization was contemporaneous with hydrocarbon migration. Geochemical data for more than 100 wells show adecrease in trace fluorite mineralization in carbonate rocks immediately overlying the St. Peter to undetectable levels near the Mississippi Arch, indicating a broad northward dispersal of fluoride-enriched brines from Hicks Dome. Similarly, authigenic dolomite from 26 wells in the St. Peter shows south to north trends in stable isotope compositions and iron content. Isotope values vary from as light as -9 per mil ((isotope){13}C, PDB) and -10 per mil ((isotope){18}O, PDB) in the deep, southern part of the basin to as heavy as 0 per mil ((isotope){13}C, PDB) and -1 per mil ((isotope){18}O PDB) near the outcrop in the northern part of the basin. Iron in dolomite decreases from south to north. These trends indicate fluids whose temperature, Fe-content, and organic-derived CO2 concentration was greater in the south than in the north where fluids had cooled along their migration path and isotopically equilibrated with confining s rata. Since the St. Peter Sandstone is a demonstrated migration pathway, any hydrocarbons that entered the paleoaquifer should have had dispersal patterns similar to those of the brines.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.