--> State of Stress in the Illinois Basin and Constraints on Inducing Failure

AAPG Eastern Section Meeting

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State of Stress in the Illinois Basin and Constraints on Inducing Failure

Abstract

The stress regime in the Illinois Basin was investigated to assess how the rock column might respond to the injection of fluids, including co-produced formation brines and supercritical CO2. This response is a concern as injection practices could increase pore pressure and potentially induce seismicity.

Data were collected to determine the magnitude and orientation of a three-component stress field: vertical (Sv), minimum (Sh), and maximum (SH) horizontal stresses. Sv was evaluated with a six-layer lithostratigraphic column. A two-layer pressure-depth Sv model for the central portion of the basin and a single pressure gradient model for the surrounding region were generated. In the central portion of the basin, the Sv gradient is 1.11 psi/ft to a depth of 7000 ft, followed by a gradient of 1.20 psi/ft below 7,000 ft. In the area surrounding the deep basin, the Sv gradient was calculated as 1.13 psi/ft. Sh was evaluated from multiple data sources, primarily fracture closure values from either hydraulic fracture records or extended leak-off tests. Sh gradient calculations ranged from 1.07–1.21 psi/ft. The Sh values for the basal clastic units that directly overlie the crystalline basement complex are lower than those for units in the overlying horizons. SH was based on a critically stressed model yielding values between 1.77 to 2.65 psi/ft, which is significantly greater than the gradient values for Sv or Sh.

Stress orientation data for the Illinois Basin were collected from multiple sources. The orientation of the principal stress, SH, across the study area relatively uniform in strike at approximately N 60 E but, has marked deviations. These deviations result from localized structural discontinuities in the crust.