Abstract
Abstract Microseismic monitoring tests were conducted from 1993 to 1995 in the Seventy-Six oil field, Clinton County, Kentucky. Oil is produced from low-porosity, fractured carbonate rocks at 3 ; no injection operations were conducted. Gross changes in production rate correlate with event rate: event rate lags changes in production rate by 2 to 3 weeks. 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 that the seismically active faults or fractures are previously drained intervals that have subsequently recovered to hydrostatic pressure via brine invasion. Storage capacity computed for one of these drained fractures implies that total oil production represents about 20% of total pore volume. Correlation of older production intervals and well-log porosity anomalies with the seismically active faults indicate that the oil reservoir in the study area is primarily a set of compartmentalized, low-angle thrust faults. Although low-angle fracture sets have not previously been considered in the exploration and development of the area, the mapped thrust faults are consistent with other investigators' interpretations of oil associated with secondary fracture sets occurring along deeper-seated, wrench-fault structures. Stress determined from composite focal mechanisms indicates a near-surface (
Published Version
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