Results of electromagnetic exploration of the Mutnovsky heat-carrier deposit (Kamchatka)

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MUTNOVKA - this is an informal name of the area in the vicinity of Mutnovsky volcano in Kamchatka. In 2005 NordWest Ltd togeteher with Kamchatgeologia Company carried out a series of EM measurements. Official name of the expedition place is Mutnovskoe heat-carrier deposit.

Mutnovsky volcano is a dormant volcano. It had become extinct about 100 million years ago. But this volcano is quite useful since it is still heating the Earth's interior with its hot breath: volcanic activity here is reduced to the production of steam and hot water that is utilized for electric power generation (Mutnovskaya Geothermal Power Plant).

Geothermal plants provide 25% of power consumed in the Kamchatka region. RAO UES of Russia is exploring the possibility to raise a series of geothermal power plants with the purpose of further development of Mutnovsky volcano heat-carrier reserves utilization. Expected total power output of these plants is up to 300 MW.

EM prospecting within this region involved AMT, MT and FS (frequency sounding) using the MERI and Phoenix instrumentation. Field measurements were carried out in a hard-to-reach highland (altitude about 1100 m) in August-September of 2004, 2005 and 2006.

The works included three stages:

  • AMT and MT sounding along a regional profile
  • detailed FS in the center of the region
  • detailed 3D MT survey

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The works resulted in construction of a resistivity map of the studied region.

Shown below is resistivity cross-section along the regional profile covered at the first stage of works. Within the upper part of the cross-section (at depths down to 2 km), an anomalous low-resistivity horizon is revealed associated presumably with water-saturated rocks of tuff origin. Detailed information is obtained about the relief of the horizon's top and bottom and about the distribution of lateral inhomogeneities of conductivity within its region.

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In the central part of the area, most prominent anomalous area in the low resistivity complex was detected. This area is associated with accumulation of hot mineralized water.

The deep part of the constructed model speaks for inhomogeneous geoelectric structure of Mesozoic basement of the cross-section, although high-resistive on the whole. In particular, a relatively conductive subvertical channel is revealed that links the upper-crustal conductive region with the subsurface conductivity anomaly described above.

Shown below is the conductance map down to the high-resistive basement, within which a prominent isometric anomaly is reliably detected. Such high-contrast anomalies are supposed to be surface indications of deep unloading zones formed in hydrothermal systems.

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Along one of the profiles, a geological-geophysical model is constructed. It is seen from the model that at a depth of 4.5 km and deeper an anomalously low-resistive (1-10 OhmВ·m) structure is revealed. By a series of geological and geophysical manifestations this unit is interpreted as an upper-crustal center of melting or partial melting.

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In the center of the model, adjoining to the high-temperature center is a subvertical low-resistivity zone (10-20 OhmВ·m) identified as a heat-carrier unloading zone. To the left of this zone a high resistivity (60-600 OhmВ·m) Neogene-Paleogene strata is detected, and to the right - a granodiorite intrusion is apparent (400-1000 OhmВ·m) rooting down to large depths.

Revealed in the upper part of the unloading zone within the depth interval 300-1200 m is a segment of the cross-section with rather steep increase of resistivity up to 40-100 OhmВ·m (against the background of 1-40 OhmВ·m). Basing on the drilling data, this segment is referred to the zone of thermal alteration of rocks - to the silification zone. This zone is formed within the volcanic-sedimentary complex during the process of thermal activity. The zone can be considered as a kind of cork closing up the unloading zone at its uppermost part. Evidently, it is not accidental that no natural heat-carrier releases were revealed in the vicinity of this zone. Note that the productive intervals (inflow zones) in well 047 are penetrated at depths 1600 to 2200 m, that is, deeper than the silification zone (the "kork").

See the well 048 drilled 650m to the south of well 047. According to the model described, its shaft goes aside of the silification zone. Here the productive intervals are penetrated at relatively small depths - 700 to 1200 m. At the same depths in the model a lens-like body with extremely low resistivity (a few fraction - a few unit OhmВ·m) is distinguished in the model. Maximum thickness of this structure (according to modeling) amounts up to 250 m, its width is 1200 m. This structure can be considered as a specific natural reservoir filled with overheated water.

In the upper part of the cross-section, a low-resistivity (3-40 OhmВ·m) horizon is revealed. Its thickness amounting to 500-1200 m is considerably large. Within these strata, a thin (150-250 m) horizon with anomalously low resistivity (1-6 OhmВ·m) is distinguished. This horizon reflects the zone of spreading of chilled water formed during the thermal activity.


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