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DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF TEXAS AT EL PASO, EL PASO, TEXAS 79968-0555
DEPARTMENT OF GEOLOGY AND GEOPHYSICS UNIVERSITY OF UTAH, SALT LAKE CITY, UTAH 84112
Abstract
This study reevaluates seismicity recorded in the War-Wink gas field, Ward County, Texas, between 1976 and 1979. Earthquakes were relocated using a velocity model from sonic well logs in the vicinity of the field. Well-located earthquakes were compared with a geologic model built from well control and with oil field operations. The new locations reveal that the seismicity occur in three depth intervals. Earthquakes in the lowest zone occur in early Paleozoic carbonates (4.3 to 6.2 km below sea level) and have no apparent relation to oil field operations. In the middle zone of overpressured shales and limestones (2.4 to 4.3 km below sea level), earthquakes have predominantly normal composite mechanisms with north-south oriented extension and may be related to differential compaction over an underlying anticline and fluid pressure reduction from oil production. Earthquakes in the upper zone of Permian aged carbonates and clastics have predominantly strike-slip composite mechanisms and appear to be related to compaction of, and to high fluid pressures leaking upwards from the underlying overpressure zone. An inversion for stress orientation using the nine composite mechanisms determined for the field suggests that
3 is oriented N10°E and is in good agreement with
3 orientations determined in other studies of oil fields within the Permian Basin.
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S. D. Davis, P. A. Nyffenegger, and C. Frohlich The 9 April 1993 earthquake in south-central Texas: Was it induced by fluid withdrawal? Bulletin of the Seismological Society of America, December 1, 1995; 85(6): 1888 - 1895. [Abstract] [PDF] |
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