At the Decatur carbon capture and storage site (IL, USA) CO₂ has been injected from 2011–2014 and from 2017 to present near the base of the Lower Mt. Simon Sandstone saline reservoir, resulting in microseismicity. Microseismicity is mainly located in the basement and distributed in distinct spatial clusters. The lack of significant impedance contrasts within the basement makes the interpretation of active-source seismic reflection data challenging, however, recent reprocessing allowed to resolve faults above and at the top of the basement. These faults generally do not coincide with the location of microseismic events and their continuation to the general depth of the seismic events cannot be assumed. This paper shows how the interpretation of the microseismicity can complement structural interpretations of active-source seismic reflection data. In particular, we analyze clusters and bursts (abrupt increases) of microseismicity, identify unresolved, smaller-scale weaknesses and extract statistical parameters. These parameters allow comparisons with the interpreted faults, and with fracture sets intercepted by boreholes. During injection at the Decatur site, the injection pressure was kept far below fracture pressure, nevertheless, seismic events were induced and spread far beyond the expected extent of the CO₂ plume. We argue that local stress transfers related to the CO₂ injection reactivated pre-existing fractures within the critically stressed basement. Finally, we conducted a slip tendency analysis for faults interpreted from active seismic, selected cluster, bursts and nodal planes from focal mechanisms to determine if the interpreted structures are optimally oriented with respect to the stress regime. Our results suggest that the orientation of fractures close to the injection well, generally shows slight deviations from the optimal orientation for slip. This might indicate either slight local deviations of the maximum horizontal stress azimuth from the average direction used in the analysis, or the lack of optimally oriented fractures at this location.

在迪凯特碳捕集和封存场（美国伊利诺伊州）CO ₂2011-2014 年和 2017 年注入到下西蒙山砂岩盐储层底部附近，导致微地震。微震主要位于基底，呈明显的空间群分布。基底内缺乏显着的阻抗对比使得对有源地震反射数据的解释具有挑战性，然而，最近的再处理允许解决基底上方和顶部的断层。这些断层通常与微地震事件的位置不一致，并且不能假设它们延续到地震事件的一般深度。本文展示了微震解释如何补充对活动源地震反射数据的结构解释。特别是，我们分析微地震的集群和爆发（突然增加），识别未解决的小规模弱点并提取统计参数。这些参数允许与解释的断层和钻孔截获的裂缝组进行比较。在迪凯特现场注入期间，注入压力保持在远低于破裂压力，然而，地震事件的诱发和传播远远超出了 CO 的预期范围₂羽。我们认为与 CO ₂注入相关的局部应力转移重新激活了临界应力基底内预先存在的裂缝。最后，我们对从活动地震、选定集群、爆发和来自震源机制的节点平面解释的断层进行了滑动趋势分析，以确定解释的结构是否相对于应力状态进行了最佳定向。我们的结果表明，靠近注入井的裂缝方向通常与滑动的最佳方向略有偏差。这可能表明最大水平应力方位与分析中使用的平均方向有轻微的局部偏差，或者在该位置缺乏最佳定向的裂缝。