RELEVANCE OF INTEGRATED AND HIGH-RESOLUTION FACIES MODELS IN GEOLOGICAL SEQUESTRATION OF CO2 IN THE ILLINOIS BASIN

Geological sequestration of CO₂ is crucial in the Illinois Basin to reduce carbon footprint by many coal-fired plants. Formations with qualities to be reservoirs for storing industrial CO₂ are the Cambrian Mt. Simon and the Ordovician St. Peter Sandstones for their porosities, permeabilities, depths and presence of cap rocks. The Mt. Simon Sandstone (MSS) extend from western Kentucky to Wisconsin with maximum thickness in the northeastern Illinois Basin. It is underlain by the Precambrian crystalline rocks and overlain by the Eau Claire Formation consisting of shale, dolomite and sandstone. The MSS is interpreted as a tidally influenced quartz arenite, deposited in a shallow marine setting with occurrences of conglomerate, shale, dolomite and sandstone-mudstone rhythmites. The St. Peter Sandstone (SPS) is interpreted as a predominantly quartz sandstone, acting as a secondary storage reservoir at a shallower depth, capped by the Maquoketa Shale acting as a secondary seal.

In spite of previous studies on the MSS and the SPS, the challenges in storing CO₂ are significant, due to limited availability of core data and well logs for predicting parameters in between the wells. Analyses of cores from different wells spread across the Illinois Basin indicate variations in litho-facies within the reservoirs and the cap rocks. The small scale variations in facies and structures and variations in depth related diagenesis in between the wells, may contribute to heterogeneities, which may result over- or under-estimation of the porosities and the permeabilities used in numerical simulations. Consequently, migration of the CO₂ plume and the brines through the formations can be different from the model estimates.

Improved reservoir characterization utilizing high-resolution facies models can reduce the uncertainties arising out of limited core and well log data in the Illinois Basin. An integrated approach involving detailed paleo-environment and sediment transport analysis using modern and ancient analogues together with available cores, seismic, well logs and flume data can enhance understanding of the distribution of lithologies. Further investigations of burial of the sediments with time and their diagenesis can reduce uncertainties in estimating net-to-gross reservoir area and distribution of porosities.