SHALLOW GEOPHYSICAL ANALYSIS OF GAS MIGRATION WITHIN THE GEOLOGIC FORMATIONS ABOVE A NATURAL GAS STORAGE FIELD IN NORTH-CENTRAL, ILLINOIS

The natural gas stored in the Ancona gas storage field is naturally reservoired in an anticline structure associated with the LaSalle Anticlinorium. Gas is injected into the Mt. Simon Formation, a basal Cambrian sandstone, around 1600 ft. below ground surface. The Eau Claire limestone provides a natural cap. Ordovician, Pennsylvanian and Quaternary units also overlie the storage field. Reservoir engineers have observed a small percentage of injected gas being lost and crop damage at the surface has been spotted at times. A gravel deposit, which is at the headwaters of the Danvers buried bedrock valley, locally occurs the base of Quaternary glacial sediments, and may provide a conduit for gas migration. A detailed study geological and geophysical study of the shallow bedrock (Pennsylvanian) and Quaternary units was completed as part of this investigation. An exploratory seismic reflection survey using both P and SH-wave techniques was performed for the purpose of defining bedrock topography and preglacial drainage, quaternary stratigraphy, and delineating possible gas accumulations in the shallow subsurface. Quaternary thickness ranges from 30 to 100 ft, and tapers to the west. Surficial geology is dominated by lake deposits of the Equality Formation. Quaternary stratigraphy includes several Wisconsinan till units that interfinger with lenses of sand, gravel, silt, and clay as well as Illinoian till and lacustrine deposits. A thin base of sand and gravel overlies the Pennsylvanian bedrock. Bedrock valley walls begin further to the west than previously thought. Gas accumulations are interpreted in some sand and gravel lenses within Quaternary units.