Monitoring Microseismicity in a Carbonate Oil Field, North Oman

Microseismicity was monitored continuously for 2003 and 2004 years using shallow downhole seismic network in a carbonate oil field in Northern Oman. A total of 406 microearthquake events were analyzed to assess events location relative to producing horizons. The depth of the microearthquakes ranges from 0 to 2.95 km below the ground surface. The events location is confined to the carbonate reservoir boundary and temporally correlates well with the gas production, total fluid productions and water injection for the 2003 year. There is no temporal correlation between oil production and seismic activity for the 2003. Direct month to month temporal correlation is not apparent between any of oil, gas, fluid productions/water injections and the microseismic activity for the 2004 year. However, a strong temporal correlation between gas production and the number of events and an improved correlation between the oil production and the number of events were obtained when applying a time lag of one month. The focal plane solutions for the largest events in the 2003 indicate normal faulting with extensional stress is in the NW-SE directions. The spatial and temporal distribution of seismic events in the carbonate field fit the characteristics of reservoir induced seismicity and the triggering mechanism can be explained by the Mohr envelope criterion. This indicates that the reservoir layers are critically stressed and the pore pressure is changing at variable rate.