Chapter 3 - The Geology, Seismicity & Exploitation Of The Geysers Geothermal Area

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Geysers Geology
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The Geysers geothermal area is the world's largest development of geothermal steam for electrical production, currently supplying 6% of northern California's power. The rocks of The Geysers form three geological units: (1) a serpentinite caprock, underlain by (2) a fractured greywacke vapour-dominated reservoir rock, which is heated from below by a (3) heat source thought to be magma. The Geysers area consists of two Jurassic-Cretaceous units assigned to the Franciscan and Great Valley sequences, partially capped by Quaternary volcanics. The Franciscan assemblage was intruded during the Pleistocene by a composite batholith known as the felsite. The reservoir, bounded by major fault zones to the north-east and south-west, can be divided into two distinct parts - a field-wide 'normal' reservoir and a high temperature reservoir lying under the normal field to the north-west. Porosity within the reservoir is fracture related, enhanced by repeated episodes of intrusion by the felsite. Compared to the amount of commercially injected fluid, the amount of natural reservoir recharge is essentially nil.

The Geysers is one of the most seismically active areas in California. This seismicity is almost exclusively confined to the commercially developed area and has been monitored by a succession of temporary and permanent networks. The NCSN network, expanded into The Geysers in 1975, shows that the seismicity in the most heavily exploited areas increased dramatically during the rapid development of the resource during the 1980s. Seismicity is thought to be a result of commercial activity.

In the late 1980s, pressure decline occurred in the reservoir, indicating that The Geysers had become over-developed. Condensate injection is currently used to delay the decline of the geothermal field, using former production wells and pipelines carrying treated sewage from neighbouring towns. This method has limited success, but has induced more seismicity. 50% of Geysers seismicity is thought to be caused by fluid injection, with the remainder caused by steam extraction and tectonic processes. Since September 1997, sewage has been injected into south-east Geysers by two pipelines. Although there has been an increase in seismicity since then, most of this is Central Geysers and may not be linked to the pipeline activity. Several mechanisms have been suggested for induced seismicity. Currently The Geysers is generating only 66% of its total installed capacity of 2043 MW.