STRUCTURAL CHARACTERIZATION OF FAULTING AND FOLDING OF DEEP RESERVOIRS BENEATH THE ILLINOIS BASIN AND RELATION TO CONTEMPORARY SEISMICITY

In areas of high-population density, problems related to natural gas storage and CO2 sequestration require integrated geophysical solutions. This is especially critical for areas of possible seismic risk where reactivation of faults that are deep-seated beneath areas of structural closure may occur. In order to evaluate a “pilot” area in the Wabash Valley seismic zone (WVSZ) of southern Illinois, we have integrated information from hundreds of petroleum-industry drill holes and available seismic reflection data. Our studies of reflection data indicate that many of the major Paleozoic folds and monoclines are cored by high-angle reverse faults in a Laramide-style of deformation. This observation is supported by “slope” maps constructed from drill hole logs for prominent stratal markers that suggest that areas of steep dip (e.g., monoclinal flexure) are frequently underlain by a facilitating fault. Such faults have propagated up from Precambrian basement and thus seem to provide “pre-existing zones of weakness” available for reactivation. However, the study of seismogenesis in the region has been hampered by the fact that earthquake epicenters frequently do not correlate with such Paleozoic structures and that the larger (mb > 3.0) earthquakes typically have focal depths well into Precambrian crust. We accordingly have focused on three instrumentally recorded earthquakes in the WVSZ, which are relatively well-characterized and for which we have nearby seismic reflection profiles. Reprocessing of some of these profiles reveals that the hypocenter of a thrust-fault mechanism event (1968) correlates spatially to an isolated dipping reflector sequence in the middle crust that apparently has affected folding in the overlying Paleozoic sediments. The foci of two strike-slip mechanism events (1974 and 1987) match a vertical reflector-defined fault zone in shallow Precambrian crust. In these cases, the epicenters do not obviously correlate with structures expressed in the Paleozoic section, suggesting “blind” reactivation of Precambrian structures. In none of these cases can propagation of seismic deformation into Paleozoic sediments be unequivocally documented. Thus, based on available data, we may conclude that gas storage would not be affected by an earthquake such as has happened in the last half-century.