The 7 May 2001 induced seismic event in the Ekofisk oil field, North Sea

A moderate size seismic event on 7 May 2001 was strongly felt on platforms in the Ekofisk oil field, in the southern North Sea, but did not cause damage to platforms or wells. We combined near- and far-field observations to develop a consistent source model and to determine whether the event was induced. Seismic data placed the epicenter inside the Ekofisk field and suggested a shallow source depth based on spectral and moment tensor analysis. GPS data from the Ekofisk platforms displayed permanent vertical and horizontal movement due to the event. A topographic bulge in the sea bottom, revealed by differential bathymetry data, and overpressure in the overburden in the northeastern part of the field, detected only after the event, had been caused by unintentional water injection that started in 1999. The injection pressure and rate were sufficient to raise the overburden. Pressure gauge and compaction data ruled out that the event occurred at reservoir level, which was further supported by unaffected production rates and absence of well failure. We therefore conclude that the event occurred in the overburden, at less than 3 km depth. Initially, this appeared unlikely on account of very low shear strength of the overburden clay-rich shale and mud rocks. The seismic event was induced owing to stress changes caused by water injection. The event possibly initiated on the northern flank of the field near the water injector and may have involved flexure of the overburden into the depression bowl in the rest of the field. Moment tensor analysis is consistent with a pure double-couple source. We suggest that slip occurred on the near-horizontal rather than along the near-vertical nodal plane. Stress drop was low, and owing to the low overburden shear strength, the event released less energy than a typical stress drop event with similar source dimensions.