GEOLOGIC CHARACTERIZATION OF A DRILLCORE ANALOG FOR AREAS WITH INJECTION-INDUCED SEISMICITY

Induced seismicity in the midcontinent region of the US is a geologic hazard identified by GSA to be a Critical Issues topic. The increase in both intensity and occurrence of earthquakes has been directly linked to reactivation of subsurface faults due to high rates of wastewater fluid injection at ~ 1–5 kilometers depth, near the Paleozoic sedimentary - Precambrian crystalline basement rock nonconformity interface. While much is known about the potential causes of induced seismicity, comparatively little is known about the physical and chemical rock properties as they influence permeability architecture because the nonconformity contact is only locally exposed in the midcontinent region.

We characterize the degree of fluid-rock interactions observed to infer how high-pressure injection may influence fluid migration across the nonconformity interface. We use a drillcore analog from SE Minnesota representative of subsurface geology within the midcontinent region. The major goals of this research are to: 1) examine the spatial distribution of structural features, mineralogy, and alteration in the drillcore using petrography, whole-rock X-ray diffraction and X-ray fluorescence analyses; and 2) provide a comparative set of detailed rock properties and lab permeability measurements that can be directly correlated to microscale textural variations.

The drillcore samples display a variety of mineralogical alteration ranging from ~6 m above to ~ 95 m below the nonconformity contact. Intensity of alteration is strongest closest to the contact and permeability is highly variable to ~60 m below the contact. Fractures and slip surfaces occur parallel and at high-angles to basement foliation fabric and exhibit significant porosity or clay alteration. Vein textures and mineralogical compositions suggest complicated deformation and fluid-rock interaction histories within the crystalline basement rock. These observations support past research on nonconformity interface analogs and suggest: 1) alteration should be modeled as a separate hydro-lithologic unit and not assumed to be homogenous or impermeable; and 2) pre-existing faults, fractures, and vein systems are potentially more susceptible to alteration and may lead to fault zone weakening and reactivation.