Geology, Structure and Hydrothermal Alteration of Geothermal Systems

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Geology, Structure and Hydrothermal Alteration of
Geothermal Systems
Joe Moore Energy Geoscience Institute
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Outline

• 1. Hydrologic structure of geothermal systems
• Deep circulation systems
• Silicic volcanic terrains with little topographic
  relief
• Andesitic volcanoes with high topographic relief
• Hydrothermal alteration
• Distribution of alteration phases
• Relationship to fluid flow patterns
• Influence of rock type
• Relationship to permeability
• Duration of hydrothermal activity

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The Challenge

• Locate the upflow zones
• Determine the characteristics of the geothermal
  aquifers
• Produce the resource in a sustainable manner

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• All geothermal systems are characterized by
• A heat source (magmatic or nonmagmatic)
• Convective upflow
• Recharge by meteoric waters
• Deep mixing with meteoric waters and/or
  condensate
• Boiling and steam migration
• Outflow of the deep fluids to the surface or
  other hydraulic base level

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Geothermal Environments
Consider geothermal systems related to Deep
circulation of groundwaters (frequently
associated with fault bounded basins Dixie
Valley, Beowawe) Silicic volcanic terrains
with low topographic relief Taupo volcanic zone,
Coso, Roosevelt Hot Springs) Andesitic
volcanoes with high topographic relief
Ahuachapan, Zunil Other environments include
rift basins (Salton Sea, Cerro Prieto), and
calderas (Yellowstone, Los Humeros)
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Deep Circulation Hydrothermal Resource
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Temperatures at 3 km Depth
(INL Website data from SMU)
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Geothermal Resources In Utah
(INL Website)
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Formation of Geothermal Aquifers
(G. Culver, Geo-Heat Center)
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Temperatures in the Ellidaar Geothermal Field,
Iceland
(Tomasson, 1993)
Cross section 3 km
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HEAT SOURCE Circles and Squares
Magmatic Triangles Extensional Diamonds not sure
Cyan gt 3.0 Ra Yellow 2 3 Ra Red 1 2
Ra Orange 0.6 1 Ra Green 0. 3 0. 6 Ra Blue
lt 0.3 Ra
Mantle Helium Evidence (Kennedy and van Soest,
2005)
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Sulphurdale
N
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Conceptual Model of a Silica Volcanic System
(Henley and Ellis, 1985)
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Coso Geothermal Field, CA
CHS
DK
WP
1 mile
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Thermal Structure Along N-S Profile, West Side of
Coso
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Roosevelt Hot Springs, Utah
Silica (Sinter) Deposits
Yellowstone
Yellowstone
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Zunil Geothermal System, Guatemala
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Distribution of Lithospheric Plates and Active
Volcanoes
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Conceptual Model of an Andesitic Volcano
(Henley and Ellis, 1985)
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Surface Manifestations Associated With Boiling
Fluids
Photos by J. LaFleur and D. Foley
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Travertine (CaCO3 ) Deposits
Zunil, Guatemala
Yellowstone
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Hot Spring Mound at Midway
(Geo-Heat Center, 2004)
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Vapor-Dominated Geothermal Systems

• Pluton related volumes of 100s of km3
• The Geysers (CA), Larderello (Italy)
• Volcanic Hosted volumes of 10s of km3
• Darajat, Kamojang and Karaha-Telaga Bodas
  (Indonesia), Puna (HI), Matsukawa (Japan)
• Shallow vapor-caps volumes of a few km3
• Cove Fort-Sulphurdale (UT)

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View of Kawah Saat and Telaga Bodas
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Karaha-Telaga Bodas
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Hydrothermal Alteration(WHY BOTHER?)

• As the fluids circulate, they react with the
  rocks. The hydrothermal minerals they produce
• Influence the geophysical signatures of the rocks
  through changes in their densities, porosities,
  permeabilities, and electrical properties.
• Provide information needed during drilling
  operations (casing points).
• Can be utilized to guide exploration and
  development by providing information on
  temperature distributions, thermal gradients,
  fluid compositions, permeable zones.
• Provide spatial information with respect to the
  location of the caprock and zones of discharge
  and recharge.

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Useful Tools

• Binocular microscope and chipboards
• Thin sections
• X-ray diffraction analyses (clays)
• Fluid inclusions (temperature and fluid
  composition)
• Scanning electron microscopy (textural
  relationships)
• Dating techniques (14C, 40Ar/39Ar, K-Ar)

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Factors Influencing Hydrothermal Alteration
(after Browne, 1978)

• Temperature
• Pressure
• Rock Type
• Permeability
• Fluid Composition
• Duration of Activity

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Thermal Stabilities of Common Geothermal Minerals
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Argillic/Phyllic Alteration
K-33 3965 ft.
Ser
Qtz
Py
Argillic alteration (lt225oC) clays smectite and
interlayered illite-smectite and
chlorite-smectite Phyllic alteration illite
(gt225 to 250oC)
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Bulalo Geothermal Field
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(Raharjo et al, 2002)
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Propylitic Zone
Epidote
Actinolite and Pyrite
gt250oC
gt300oC
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Potassic Zone (gt320oC)
Biotite
Garnet
Pyroxene
Tour
Garnet
Biotite
KRH 2-1 9500 ft.
KRH 2-1 9850 ft.
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Distribution of Mineral Surfaces at Karaha-Telaga
Bodas
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Bulalo Geothermal Field
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Advanced Argillic Alteration, Cove
Fort-Sulphurdale, Utah
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Predicted Mineralogy
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Anhydrite After Actinolite
T-21001.3 m
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Calcite Vein
64-16
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Wairakite After Anhydrite and Calcite
K-21 1546.9 m
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Calc-silicate Stability Diagram
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General Characteristics of Alteration Assemblages
(Hedenquist, 1988)
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Rock Types

• Influences alteration mainly through control on
  permeability by texture, porosity and strength
• Volcanic Systems
• Dominated by
• Tuffaceous deposits (pyroclastic and epiclastic
  deposits, lahars)
• Lava flows
• Sediments (minor, in local basins)
• Intrusions (commonly form thin dikes and sills,
  rarely large stocks)
• Basement rocks (regional sedimentary and
  metamorphic sequences and intrusive complexes)

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Facies Model of the Bulalo Reservoir
DEPTH RANGE FT. BSL
Shallow And-Dac Volcanic Sequence (lavas and
tuffs)
1000-2500
Rhyolitic ash-flow tuffs and lavas (SR1 SR2)
with and epiclastic deposits
2000-4000
And Lava Marker w/ related tuff deposits
3500-4500
Dacite Dome Complex w/ dac lava domes tuffs
4000-6000
5500-7000
Bas-And Volcanic Sequence
6500-10000
Deep Andesite Sequence - and lava dominated with
intrusions
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Rock Types
Tuffaceous deposits alteration to clays can
begin shortly after deposition at very low
temperatures resulting in rocks with low
permeabilities at low to moderate
temperatures (below propylitic zone) rocks
commonly behave in a ductile fashion poor
reservoir rocks until they become brittle when
affected by high temperature alteration Lava
flows can behave in a brittle fashion even at
relatively low temperatures rubbleized flow
tops and bottoms may have high porosities but
be of limited extent and be poorly connected to
major through going fractures
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Relationship of Rock Type to Fracturing
Fractured Lava Flows
Unfractured Pyroclastics
Mineralized fracture
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Fracture Orientations in Medicine Lake Well 88-28
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Litho/Structural Facies Model of Bulalo Reservoir
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Fault Kinematics in Karaha-Telaga Bodas Well K-33
Abbreviations cr caprock d dextral n
normal o oblique r reservoir s sinstral
t tensile
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Bulalo Sulfonate Tracer Summary
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Permeability Controls on Alteration Mineralogy

• Rocks with low permeabilities
• The rocks may remain relatively unaltered (lava
  flows) even at relatively high temperatures
• Equilibrium between the rocks and fluids may not
  be reached and relict phases may persist
• High permeability channels (past and present) are
  frequently associated with
• Hydrothermal breccias
• Boiling
• Repetitive fracturing and multiple vein sets

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The Effects of Boiling

• Boiling effects
• Gas contents of the fluid
• Mineral precipitation
• Temperature-pressure relationships
• The formation and distribution of condensates

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Calcite Cemented Hydrothermal Breccia
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Mineral Deposition Resulting from Boiling
Bladed calcite
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Effect of Boiling on Mineral Stabilities
Browne and Ellis, 1970
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Duration of Geothermal Activity

• Life spans of geothermal systems are poorly
  known
• Few geothermal systems have been dated directly
• Hochstein and Browne suggest that
• Ohaaki-Broadlands (NZ) has been active for at
  least 300,000 y
• Kawerau (NZ) for at least 280,000 y
• Icelandic geothermal systems for lt 250,000 y
• Individual pulses in volcanic systems may be very
  short lived, with long periods of quiescence.
• Dating of hot spring deposits and altered rocks
  at Karaha-Telaga Bodas, Tiwi and Coso suggest
  these pulses may last less than several tens of
  thousands of years.

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An Example from Matalibong-25,Tiwi Philippines
Determination of mineral distributions and
paragenetic relationships Measurement of fluid
inclusion temperatures and salinities 40Ar/39Ar
spectrum dating of adularia from three
depths
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Mineral and Fracture Distributions
Spinner Log
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Fluid Inclusion Measurements
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40Ar/39Ar Spectrum dating
Adularia ages 5932 ft (303 ka 6 ka), 6065 ft
(314 ka 5 ka), 6075 ft (272 ka 22 ka)
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Conclusions

• Most geothermal systems can be described in
  general terms using relatively few simple
  conceptual models
• These models can provide the basis for
  prioritizing exploration activities and initial
  interpretations of geophysical and geochemical
  data.
• The primary factors controlling hydrothermal
  alteration are temperature, rock type and
  permeability.
• Faults and fractures become increasingly
  important as fluuid conduits with increasing
  temperature and depth.
• Fluid conduits often display evidence of
  reactivation, even after long periods of
  quiescence.

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THE END