GEOLOGIC CHARACTERIZATION OF THE GREAT UNCONFORMITY INJECTION INTERFACE REGION FROM FIELD AND DRILLCORE ANALOG STUDIES: IMPLICATIONS FOR MIDCONTINENT INDUCED SEISMICITY

We aim to understand how relationships between structural features and alteration (structural diagenesis) affect fluid pressure migration related to high-volume wastewater injection and how these relationships potentially influence induced seismicity. Over the past decade, there has been a significant increase in the number and intensity of earthquakes in the mid-continent region of the United States. These earthquakes have been linked to the deep injection of wastewater fluids produced in association with the hydraulic fracturing process. The fluids are injected primarily into basal sedimentary bedrock reservoirs overlying the Great Unconformity and Precambrian crystalline basement rocks across this region. Variations in rock properties due to diagenesis, in combination with the variable structure of the unconformity, can influence fluid migration and potentially lead to reactivation of basement and/or reservoir faults.

We examine the spatial distribution of physical and chemical heterogeneities in drillcore and outcrop nonconformities in Minnesota and Colorado as analogs for injection horizons. We characterize the degree of historical fluid communication and the potential for future communication using petrography, geochemical analyses, and relative permeability testing. The drillcore samples display mineral dissolution and carbonate, oxide, and clay alteration ranging from ~6 m above the nonconformity contact to 95-120 m of alteration below the contact. Intensity of alteration is strongest closer to the nonconformity contact. The basement is faulted as close as 3.7 cm from the contact and as deep as the lowest part of the extracted core at 121 m below the nonconformity contact. Past fluid communication in the cores is evident in layered veins. Basement rocks have lower porosity than the overlying strata, initially promoting fluid migration along the contact. Fractures and microporosity also provide channels for fluid migration. These observations support past research on nonconformity analogs and suggest 1) the altered region below nonconformity injection sites should be modeled as a separate hydro-lithologic unit and not assumed to be homogenous or impermeable and 2) the suitability of injection sites should be evaluated locally to examine the impact of these altered basement horizons.