Near-surface Microearthquakes at The Geysers Geothermal Field, California

 — A 150-m length, 6-level, three-component, vertical geophone array was cemented into the 67- to 219-m depth interval (220 to 720 ft) of Unocal's well GDCF 63-29 during a plug and abandonment operation on April 7, 1998. Casing deformation has been observed in wells of the study area including the GDCF 63-29 well. An objective of the study was to determine if shallow deformation at The Geysers is manifested seismically. Near-surface microearthquake activity was monitored for a period of one year; during the latter four months, monitoring was supplemented with four surface stations to help constrain locations of shallow seismicity. Event locations occurring within about 750 m of the array bottom have been determined for the 10-week period January 6 to March 16, 1999. These events are distinct from surface-monitored seismicity at The Geysers in that they occur predominantly above the producing reservoir, at depths ranging from about 220 to 1000 m (600 to −180 m elevation). The shallow events tend to be episodic, with relatively quiescent periods of up to three weeks occurring between swarms. Event locations show a northeast-striking trend, similar to seismicity trends mapped deeper in the reservoir, and parallel to the strike of a major surface lineaments observed over the productive field. However, clear fault or fracture planes are not resolved from the hypocenters. Composite fault-plane solutions suggest oblique reverse faulting in the overburden. The shallowest seismicity terminates near the base of a serpentine unit, a contact which is the locus of most of the well casing deformations logged in the area, suggesting that reservoir contraction is accommodated along numerous discrete faults below the serpentine, but as continuous plastic deformation in the serpentine. It is hypothesized that the resulting strain discontinuity at the base of the serpentine explains the prevalence of wellbore deformation there. The shallow, above-reservoir microseismicity is strongly correlated in time with the injection and the deeper injection-induced seismic activity occurring in the reservoir immediately below. This suggests that deep injection-induced events trigger shallower events, by a remote triggering mechanism which has been observed at a larger scale at The Geysers and elsewhere.