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 σ3 is oriented N10°E and is in good agreement with σ3 orientations determined in other studies of oil fields within the Permian Basin.

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