Hydrogeologic Controls on Induced Seismicity in Crystalline Basement Rocks Due to Fluid Injection into Basal Reservoirs

A series of M_b 3.8–5.5 induced seismic events in the midcontinent region, United States, resulted from injection of fluid either into a basal sedimentary reservoir with no underlying confining unit or directly into the underlying crystalline basement complex. The earthquakes probably occurred along faults that were likely critically stressed within the crystalline basement. These faults were located at a considerable distance (up to 10 km) from the injection wells and head increases at the hypocenters were likely relatively small (～70–150 m). We present a suite of simulations that use a simple hydrogeologic‐geomechanical model to assess what hydrogeologic conditions promote or deter induced seismic events within the crystalline basement across the midcontinent. The presence of a confining unit beneath the injection reservoir horizon had the single largest effect in preventing induced seismicity within the underlying crystalline basement. For a crystalline basement having a permeability of 2 × 10^?17 m² and specific storage coefficient of 10^?7/m, injection at a rate of 5455 m³/d into the basal aquifer with no underlying basal seal over 10 years resulted in probable brittle failure to depths of about 0.6 km below the injection reservoir. Including a permeable (k_z = 10^?13 m²) Precambrian normal fault, located 20 m from the injection well, increased the depth of the failure region below the reservoir to 3 km. For a large permeability contrast between a Precambrian thrust fault (10^?12 m²) and the surrounding crystalline basement (10^?18 m²), the failure region can extend laterally 10 km away from the injection well.     

Solubility of excess alumina in hydrous granitic melts in equilibrium with peraluminous minerals at 700–800 °C and 200 MPa,and applications of the aluminum saturation index

We have studied the controls on the Aluminum Saturation Index (ASI = molec. Al₂O₃/[(CaO)+(Na₂O)+(K₂O)]) and the concentration of normative corundum of granitic liquids saturated in alumina by equilibrating peraluminous minerals with initially metaluminous haplogranitic minimum composition liquids at 700–800 °C and 200 MPa, at, and below H₂O saturation. The ASI and normative corundum increase with increasing H₂O concentration in the melt (0.04 to 0.10 moles excess Al₂O₃ per mole of H₂O), temperature, and with addition of the non-haplogranitic components Fe, Mg, and B. The ASI parameter and concentration of normative corundum cannot be used to monitor aAl₂O₃ between different mineral assemblages and melt because other components that affect the solubility of alumina, including H₂O, Fe, Mg, and B, do not appear in their formulations. ASI and normative corundum, however, provide petrogenetic information about magmas generated by partial melting of strongly peraluminous protoliths by virtue of their regular and predictable variation with melt composition (e.g., H₂O concentration) and temperature. For the application of these data to natural rocks it is necessary to choose as an analogue system the ASI-solubility or normative corundum-solubility relations of the most chemically complex peraluminous mineral present in the rock. Comparison of ASI values of anatectic leucosomes and allochthonous leucogranites with experimentally predicted values suggests low H₂O concentrations in melt during crustal partial melting. Rapid melt segregation before equilibration with restitic peraluminous phases is also suggested in some cases.Editorial responsibility: I. Carmichael