Seismicity induced by fluid injection in the deep subsurface often occurs in low-permeability basement rocks. At the Illinois Basin – Decatur Project, the target storage reservoir a high-porosity and permeability sedimentary unit, and more than 90% of the seismic events occur in the very low permeability grano-rhyolite rocks underlying the Mt. Simon sandstone reservoir indicating that perturbations in fluid pressure, reservoir stress, or both were transmitted to the underlying basement rocks. Geological models built for reservoir simulation are well constrained by logs, spinner survey, and pressure data, except for flow into and through the igneous basement, and additional model permeability was required to achieve good history match. Image logs acquired through the top of the basement indicate that it is highly fractured. A discrete fracture network model was constrained by the induced seismicity and the fracture observations from the wellbore to develop a basement permeability model. The workflow resulted in a realistic characterization of fracture distribution in the basement that led to an excellent match of reservoir behavior.

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