The Midwest Geological Sequestration Consortium (MGSC) carried out a small-scale carbon dioxide (CO 2) injection test in a sandstone within the Clore Formation (Mississippian System, Chesterian Series) in order to gauge the large-scale CO 2 storage that might be realized from enhanced oil recovery (EOR) of mature Illinois Basin oil fields via miscible liquid CO 2 flooding. As part of the MGSC's Validation Phase (Phase II) studies, the small injection pilot test was conducted at the Bald Unit site within the Mumford Hills Field in Posey County, southwestern Indiana, which was chosen for the project on the basis of site infrastructure and reservoir conditions. Geologic data on the target formation were extensive. Core analyses, porosity and permeability data, and geophysical logs from 40 wells were used to construct cross sections and structure contour and isopach maps in order to characterize and define the reservoir architecture of the target formation. A geocellular model of the reservoir was constructed to improve understanding of CO 2 behavior in the subsurface. At the time of site selection, the Field was under secondary recovery through edge-water injection, but the wells selected for the pilot in the Bald Unit had been temporarily shut-in for several years. The most recently shut-in production well, which was surrounded by four nearby shut-in production wells in a five-spot pattern, was converted to CO 2 injection for this pilot. Two additional wells outside the immediate fivespot pattern, one of which was an active producer, were instrumented to measure surface temperature and pressure. The CO 2 injection period lasted from September 3, 2009, through December 14, 2010, with one three-month interruption caused by cessation of CO 2 deliveries due to winter weather. Water was injected into the CO 2 injection well during this period. A total of 6,300 tonnes (6,950 tons) of CO 2 were injected into the reservoir at rates that generally ranged from 18 to 32 tonnes (20 to 35 tons) per day. The CO 2 injection bottomhole pressure generally remained at 8.3 to 9.0 MPag (1,200 to 1,300 psig). The CO 2 injection was followed by continued monitoring for nine months during post-CO 2 water injection. A monitoring, verification, and accounting (MVA) program was designed to determine the fate of injected CO 2. Extensive periodic sampling and analysis of brine, groundwater, and produced gases began before CO 2 injection and continued through the monitored waterflood periods. Samples were gathered from production wells and three newly installed groundwater monitoring wells. Samples underwent geochemical and isotopic analyses to reveal any CO 2-related changes. Groundwater and kinetic modeling and mineralogical analysis were also employed to better understand the long-term dynamics of CO 2 in the reservoir. No CO 2 leakage into groundwater was detected, and analysis of brine and gas chemistry made it possible to track the path of plume migration and infer geochemical reactions and trapping of CO 2. Cased-hole logging did not de...