HYDROLOGICAL ANALYSIS OF NONCONFORMITIES: IMPLICATIONS FOR INJECTION-INDUCED SEISMICITY IN THE MIDCONTINENT UNITED STATES

Seismic activity related to deep wastewater injection affects the mid-continent United States and the Rocky Mountain west, where injection of wastewater at depth increases pore-fluid pressure (P_f), triggering slip on pre-existing faults. This produces seismic activity which in many cases occurs kms away from the injection point. The transmission of P_f is likely geologically controlled, and knowledge of the at-depth geological environment is essential to determining how to mitigate the seismic hazard involved with deep injection. We examine nonconformities involving Precambrian crystalline basement rock in field sites in Michigan and Wyoming, and core from Michigan, Wisconsin, and Minnesota to define the nature of the nonconformity/regolith zone, and to examine deformation in the basement rocks.

Outcrops along the southern shore of Lake Superior expose the contact of the Late Proterozoic Jacobsville Sandstone on Archean granitic gneiss, greenstone, and serpentinized mafic rocks. The contact ranges from an unaltered sharp contact zone to a weathered zone of clay-rich regolith up to 3 m thick. Very thin vertical fractures with reduced (bleached) zones up to 1 m wide commonly parallel and lie above epidote-mineralized faults in Archean rocks. The outcrop in Cody, WY exposes the contact of the Cambrian Flathead Sandstone on Precambrian granite. The contact here is undulating and ranges from a sharp unaltered contact to a weathered zone up to 2 m thick. Vertical fractures both truncate at and cut through the contact. Core from the MI basin and WI show the contact between the Cambrian Mt. Simon Sandstone and Precambrian rocks. The basement rocks in the Michigan basin core consist of altered granite and granitic gneiss. Textures in thin section suggest that feldspar alteration and remineralization is concentrated in the basement rocks within 3 m of the contact. Macro- and micro-scale veins indicate fluid-rock interactions. The contact zone captured in the WI core grade from a mature quartz arenite with shaley intervals, to a granitic wash, into an altered granite. We observe great variations in the nature of the contact zone in the different localities. Alteration is concentrated in basement, and at the contact, decreasing with depth, which likely produces significant variations in hydrologic properties of the contact zone.