DEPOSITIONAL CHARACTERIZATION OF THE EAU CLAIRE FORMATION: FACIES, MINERALOGY AND GEOCHEMISTRY

The Upper Cambrian-age Eau Claire Formation serves as the primary seal at the Illinois Basin–Decatur Project (IBDP), a one million tonne carbon capture and storage demonstration project located in Decatur, Illinois. The Eau Claire Formation conformably overlies the Mt. Simon Sandstone, the reservoir for the IBDP. At the IBDP site, analysis of drill core and geophysical logs reveals the Eau Claire consists of two major lithostratigraphic units: a siliciclastic dominant lower unit and a carbonate dominant upper unit. Within the Eau Claire, four major depositional facies exist within a tidally influenced, shallow marine depositional environment. These include (Unit A) intertidal mixed sand/mud flats and tidal channels of the foreshore environment; (Unit B) subtidal deposits of the shoreface environment; (Unit C) subtidal deposits of the offshore transition zone; and (Unit D) subtidal carbonate shelf deposits. An approximately +8‰ δ¹³C excursion (R² = 0.97) defines the Eau Claire at the IBDP site, possibly correlative with the Steptoean positive carbon isotope excursion (SPICE) found worldwide 500–495 Ma ago.

The mineralogy of the four depositional facies varies widely throughout the Eau Claire, with a persistently high K-feldspar content. The K-feldspar mode averages 8.7% for the formation and reaches an average of 20.3% in Unit C. Unit A is approximately 30 ft. (9.1 m) thick and exhibits intense bioturbation, ripple beds, and flaser/lenticular bedding, with an average core analysis porosity and permeability (n = 20) of 6.7% and 8.6 mD. Unit B is an approximately 40 ft. (12.2 m) thick tight shale sequence, with thin siltstones interbedded throughout. Unit B represents the most desirable seal facies in the formation with an average point count porosity of 0.1% and no visible pore throat connectivity. Unit C is the largest siliciclastic unit in the formation, spanning an average of 95 ft. (29 m) in all three wells at the IBDP. Unit C exhibits the classic tidalite signature of rhythmically interbedded siltstones and shales, with an average core analysis porosity and permeability (n = 50) of 9.4% and 0.9 mD, respectively. Unit D spans approximately 345 ft. (105.2 m) and begins as a mixed siliciclastic/carbonate environment, slowly becoming a dolomitized carbonate shelf dominated by oolitic packstone and grainstone.