Exploring for Hydrothermal Dolomite-Hosted Fields in the Illinois Basin

Hickman, Robert G.¹, W. Norman Kent², and Jeff R. Martin^2
1 Structural Solutions, Sugar Land, TX
² Kent GeoScience Associates, Richmond, TX
³ GETECH Inc., Stafford, TX

Fields reservoired in hydrothermal dolomite are widespread across the Michigan basin, Appalachian basin, and adjacent arches and basin margins in southern Ontario, western Ohio and eastern Indiana. While Trenton-Black River (TBR) fields have received the most attention, hydrothermally dolomite reservoirs are also important in several other Paleozoic carbonate units in these basins. Additionally, hydrothermal brines have generated secondary porosity within Lower and Middle Paleozoic sandstone units.

Understanding the role of basinal brines, and the brine migration pathways are keys for understanding the development and distribution of these reservoirs. The brines moved laterally from the deeper parts of the basins via deep aquifers and migrated vertically through permeable zones including faults, pre-existing karst, and windows created by unconformities. Hydrothermal reservoirs develop in these areas of focused, vertical flow.

Peripheral lead-zinc deposits, and fluid inclusion and isotopic data from aquifers and mineral deposits provide clear evidence of the extensive migration of hot brines toward the margins of the Illinois basin. Production from Ordovician rocks demonstrates the presence of a working Lower Paleozoic hydrocarbon system in the basin. Historic Trenton fields in the Illinois basin were all found by deeper drilling on anticlines with shallow production. However, permeability in the Trenton pools is erratically distributed and is related to fracturing and diagenesis. Deliberate exploration for hydrothermal dolomite-hosted fields should focus on mapping the distribution of the Mt. Simon and St. Peter aquifers, unconformities, fluid flow information derived from isotopic and fluid inclusion data, and the intersection of reactivated basement faults.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004