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# A Rebirth of the Illinois Basin

*"In well-explored areas such as the Illinois Basin, the large lead-zinc-fluorite and fossil-fuel deposits have already been found; simple models of their deposition and occurrence no longer apply very well to locating new deposits.  The challenge we face today in the discovery of new deposits and in the extension of known deposits is to apply new technology and advanced concepts through multidisciplinary team efforts."
Dr. Jennie Ridgley, U.S. Geological Survey

**The Illinois Basin has a long and rich history of energy and mineral production that has been an essential economic component of the region.**
Discovery of new deposits and perhaps new deposit types will be needed to maintain annual revenues of over $3.5 billion from fossil fuel and mineral production in Indiana, Illinois, and Kentucky as known deposits are depleted.  Minerals and energy raw materials will continue to play a vital role in the economic future of the mid-continent region.  The U.S. Geological Survey (USGS) has initiated numerous studies of the basin supporting exploration efforts to locate new sources that will be needed to satisfy future demand for raw materials**The USGS has forged a partnership with the Illinois Basin Consortium (IBC) to advance the geological understanding of the basin.**
The USGS and the State Geological Surveys of Illinois, Indiana, and Kentucky (which form the IBC) have begun to integrate their knowledge of the geologic framework and fluid dynamics of the Illinois Basin as they assess the potential for resources in order to promote economic development.  USGS and IBC have conducted collaborative studies in a region which has produced over 9 billion tons of coal and over 4 billion barrels of oil.  A digital base map of the basin is being compiled in order to display and transfer geologic data efficiently. Public forums, hosted jointly by the USGS and the IBC, are one method used to disseminate information.**USGS studies in the region include detailed mineral resource appraisals and mineral deposit modeling.**
Industrial minerals, such as the Mississippi Valley-type lead and zinc deposits, have been the focal point of studies for decades, yet consensus has not been achieved as to their origin.  Mineral deposit formation appears closely linked in time to the migration of oil.  USGS scientists have formulated geologic models designed to test a variety of hypotheses that explain the relationship between distribution of minerals and hydrocarbon accumulation in the basin.**USGS scientists recently began to examine new hypotheses for mineral deposit occurrences.**
The conventional hypothesis for formation of lead-zinc deposits in the Mississippi Valley district states that ground-water fluids move under the influence of gravity from their original sites to deposit sites.  This simple gravity-flow model is challenged by new geological observations such as unfavorable porosity and permeability, lack of evidence of movement of hot fluids over very long distances in conduit rocks in key zones, and by the role of extensive deep faulting in the basin.  An alternative model suggests that thermal convection related to heat conducted along deep faults may explain more geological constraints than would a gravity-driven flow model. USGS researchers presently are testing these competing models and their applicability to finding new lead-zinc deposits.
**USGS scientists show that revised depositional models for rock units is the key to a better understanding of the geometry of reservoir and conduit rocks.**
A detailed analysis of rock units that have the potential to be reservoirs or conduits for fluids indicates that continuity, and hence porosity and permeability, is more variable than previously thought.  This is important not only for hydrocarbon exploration but also for mineral deposit modeling, waste disposal, and gas storage strategies.  For example, recent data suggest that one crucial horizon contains a sandstone that occurs largely in valleys created by erosion millions of years ago.  These valleys, now at depths ranging from 800 feet to 2,500 feet, are filled with sand that is a potential reservoir.  Techniques used to understand the lateral and vertical variability in porosity and permeability within the framework of these potential reservoir or conduit rocks are being applied to other critical horizons.  This information can then be used by public- and private-sector interests for decision-making regarding exploration, waste and storage management, and recovery strategies.**USGS researchers have recently acquired seismic data that bear on analysis of basin-wide geologic hazards and hydrocarbon exploration.**
In particular, USGS scientists are concerned about recent earthquake activity in the eastern area of the basin along the Wabash Valley fault system. In 1987, this fault system experienced a 5.0-magnitude earthquake.  The well-known and seismically-active New Madrid fault zone extends southwest from the basin, and it is possible that active faults in the vicinity of the Illinois Basin may be controlled by similar processes acting at great depth below the Earth's crust.  New seismic data will provide greater detail of the basin's structure, and may provide clues to future seismic activity within the basin or at its margins.  Seismic data, in conjunction with a new understanding of the thermal history of the basin, may also help explain why petroleum exploration south of the 38th parallel has been unsuccessful.  Evaluation of seismic data may help to identify potential new targets for hydrocarbon exploration where existing deep drilling is sparse or absent.For More Information:Dr. Jennie RidgleyU.S. Geological SurveyMS 939Box 25046Denver Federal CenterDenver, CO  80225(303) 236-9048ridgley@sedproc.cr.usgs.gov
This USGS Fact Sheet can be found at <URL:http://energy.usgs.gov/factsheets/Illinois/illinois.basin.html>Maintained by L. Friedrich  Last updated 6-Jan-1997*