a b s t r a c tThe Illinois Basin-Decatur Project safely and successfully injected, over three years, nearly 1.1 million tons (1 million tonnes) of supercritical carbon dioxide (CO 2 ) into the base of a 1640 ft (500 m) thick saline sandstone reservoir at a depth of 7025 ft (2.14 km). The injection interval, with its high porosity and permeability, allowed for injection pressures to be far below fracture pressures during the daily 1102 tons (1000 tonnes) injection rate. Microseismicity was monitored 1.5 years before injection, through the 3 years of injection and now during permanent shut-in which began in November 2014. The overall average of locatable events per day, during injection, was a little over 4, and events appear to be related to development on previously undetected planes of weakness. Some of these planes and active areas may be related to features developed during diagenetic or compactional processes associated with the Precambrian surface topography. Microseismicity during transient shut-in did not show a rate of decrease, large changes in magnitude, distance from the injection well, or depth.

microseismicity induced by CO2 injection at Decatur, Illinois, occurs in distinct clusters and shows no obvious correlation with the proceeding pressure front. We analyze some of these clusters in more depth by using a waveform cross‐correlation approach. With this approach we can associate about 1400 events from two clusters, with moment magnitudes between 1.1 and −1.7, with specific formations of much smaller vertical dimensions (tens of meters) than the depth resolution of traveltime‐based event locations. The differentiation of reservoir and basement events, and the definition of subclusters by waveform correlation, rather than by location, helps to better analyze the spatiotemporal evolution of the events within a cluster. In the Decatur case, this is characterized by event migration from the reservoir into the adjacent basement. The spatial variation of Brune stress drop and Gutenberg b value exhibits signs of a fluid‐driven triggering mechanism at the cluster level, revealing a punctual hydraulic connection between reservoir and basement, most likely associated with basement faults cutting into the reservoir. The observed clustering of microseismicity can thus be explained by the lateral heterogeneity of permeability and crustal strength and is overall consistent with a pressure‐induced triggering mechanism. Hence, proper long‐term risk mitigation for large‐scale fluid injection close to the basement requires prior mapping of small subseismic basement‐connected faults.