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Abstract: Are the Shallow Oil Reservoirs of South-Central Kentucky Undergoing Natural, Microseismic Deformation?

RUTLEDGE, JAMES

In a series of downhole, seismic-monitoring tests conducted from 1993-1995 in Clinton County, Kentucky, it was shown that microearthquakes are temporally and spatially associated with oil production from low-porosity, fractured, carbonate rocks at <600 m depth. Gross changes in production rate correlate with microearthquake event rate changes. Hypocenters and first- motion data have revealed previously undetected, low,angle thrust faults above and below the currently-drained depth intervals. Production history, well logs and drill tests indicate the seismically-active faults or fractures are previously-drained intervals that have subsequently recovered to hydrostatic pressure via brine invasion. Computed storage volumes and correlations of production intervals with the seismically active faults indicate the oil reservoir in the study area is primarily a set of compartmentalized, low-angle thrust faults.

State of stress determined from composite focal mechanisms indicates a near-surface thrust regime. The seismic behavior is consistent with poroelastic models that predict slight increases in compressive stress above and below currently-drained volumes. Estimated extraction-induced stress changes outside currently drained volumes are very small (<0.008 MPa). The pressure cycling and partial replacement of oil with denser brine along the seismically-active faults preceding current, adjacent production may have driven the faults closer to failure (shear slip). The small magnitudes of production-induced stress changes implies the productive fractures are critically stressed for shear failure within the pre-existing, background state of stress and, further, that similar, potentially-productive fractures may be naturally active at lower rates with no production activity.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky