Surface structure analysis for the Bulalo Geothermal Field

1
Surface structure analysis for the Bulalo
Geothermal Field

• Possible implications to local
• and regional tectonics
• By Dennis Jerome P. Aquino
• Slide Presentation for Masters Defense
• UP-NIGS, 15 March 2004

2
Presentation Outline

• Study Overview
• Results
• Discussion
• Recap of Study

3
Six geothermal fields ? Total installed capacity
of 1,910 megawatts (MWe)
4
Bulalo Geothermal Field

• Location Makiling-Banahaw (Mak-Ban)
• Geothermal Reservation in SW Luzon
• Production area SE slopes of Mt. Makiling

(Photo credit for all pics J. Stimac)
5
Why study surface structures?

• Structures influence permeability
• Logical drilling targets for exploration
• initial phase
• developmental phase

6
Regional Tectonic Map of Luzon Island
Illustrations are adapted from Oles and others
(1991).
7
N-S and E-W trending structures in SW Luzon
Drawing is modified from Oles and others (1991).
8
NE and NW trending structures in SW Luzon
MC
Drawing is modified from Oles and others (1991).
9
Map is modified from Stimac and Golla (1999).
10
Current Structural Map of the Bulalo Geothermal
Field
Map is modified from the Bulalo Resource
Assessment Team (1999).
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Coulomb-Anderson Theory of Faulting

• Premises
• Principal stress axes (s1, s2, s3)
• ?mutually perpendicular
• 2 axes horizontal plane
• 1 axis perpendicular to surface
• 2. Planes of conjugate faults -- 30 from s1

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Fault Mechanics

• The s2 is oriented to the vertical
• s1 and s3 are in the horizontal
• Dip of strike-slip faults should be 90or
  vertical

• The s1 is oriented to the vertical
• s2 and s3 are in the horizontal
• Dip of normal faults should be about 60

• The s3 is oriented to the vertical
• s1 and s2 are in the horizontal
• Dip of thrust faults should be about 30

Drawings are modified from Davis and Reynolds
(1996).
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Issues

• Inherent characteristics of the surface
  structures
• Effect on the underlying geothermal system
• Relationship with Macolod Corridor

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Objectives

• Verify surface faults and other structures
• Characterize the stress dynamics
• Relate the local structural setting with the
  regional picture

15
Methodology

• Image processing
• Fieldwork
• Data synthesis

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Image Processing

• AIRSAR DEM ? 10 m resolution
• Topo Map DEM ? 2 m contour interval
• Sun shading angles highlight lineaments

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AIRSAR DEM with sun shading at 45
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AIRSAR DEM with sun shading at 135
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Topographic Map DEM with sun shading at 45
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Topographic Map DEM with sun shading at 315
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Summary of Interpretations
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Modified Structure Map of the Bulalo Geothermal
Field
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Conclusions from Image Processing

• Almost all of the faults were identified from the
  DEMs
• Nine additional lineaments were likewise observed
• NE-striking crease structure on top of Bulalo
  dome

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Field Mapping of Structures

• Surface lithology was subdivided into five units
  based on the results of the fieldwork
• ? Altered Ground
• ? Alluvial Deposits
• ? Taal Tuff (5.6-6.8 years Ka)
• ? Bulalo Dacite (40Ar/39Ar 3-22 Ka)
• ? Cabulugan Andesite (0.250.14 Ma)

25
Generalized Geologic Map of the Bulalo Geothermal
Field
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Data Synthesis

• To determine dominant trend or preferred
  orientation of structures
• Stereographic plots were prepared

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Fractures
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Fractures of the Bulalo Dacite
Left Steam-emanating fractures oriented to NW
Below Gaping fracture atop Bulalo dome that is
trending NE
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Fractures of the Alluvial Deposits

• Large, gaping NE fractures with hydrothermally
  altered material
• Proximal to the trace of the Makiling-Arcuate
  Fault

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Fractures of the Alluvial Deposits
NS fractures that steepen upwards are aligned
with the Tigsa Fault I.
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Fractures of the Altered Ground
Fumarole is closely associated with the NE
fracture plane (yellow)
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Fractures of the Altered Ground
N
s1
Dextral movement of N-S structures is deduced by
the orientation of deformed accretionary lapilli.
s1 is in the NE.
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Stereographic plots of joints

34
Normal Faults
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Small-scale Graben of the Taal Tuff
Top Graben with N30E strike coincides with the
Olila-Cumbantog Fault.
Bottom NS-trending fractures, oblique to the
graben above
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Normal Fault of the Bulalo Dacite

• Slickenlines and chatter marks
• normal sinistral displacement

LL

• NW-striking structure approximates
• The Bulalo Fault I

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Stereographic plots of normal faults
All Normal Faults NE

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Thrust Faults
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Conjugate Thrust Faults of the Alluvial Deposits
Alluvial II
Reworked Tuff
Alluvial I

• Conjugate thrust faults strike N65W
• s1 is N25E

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Reverse Fault (?) of the Cabulugan Andesite

• N62W-striking reverse fault based on

• Disrupted lava sheeting presence of drag
  folding?
• (b) Occurrence of broken blocks along the fault
  trace

41
Stereographic plots of thrust faults
All Thrust Faults N65W
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Strike-slip Faults
43
The concept of the Riedel shears
R shears synthetic 15 from main shear R
shears antithetic 75 from main shear
s1 is the bisector between the R and R shears.
Drawing is modified from Woodcock and Schubert
(1994).
44
Strike-slip Faults of the Taal Tuff
N
Dextral movement of the NS fault (white) is shown
by the NE antithetic R-shears (yellow).
R
U
R
D
Notice that the N-S dextral fault has a normal
component.
R
R
The direction of s1 (pink) is N20E.
s1
This is a textbook case of TRANSTENSION.
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What is transtension?
s1
s1
Drawing is adapted from Sanderson and Marchini
(1984).
46
Strike-slip Faults of the Taal Tuff
Sinistral movement of NE fault (yellow) is shown
by its R shears (green).
E-W fracture (red) is displaced right-laterally
by a N-S fault (white) and left-laterally by a NE
fault (yellow).
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Strike-slip Faults of the Taal Tuff
N
s1
s3
Yellow NE, Sinistral Red N-S, Dextral
Interpretation Conjugate faults resulting from a
horizontal s1 oriented to N20E.
Drawing is modified from Davis and Reynolds
(1996).
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Strike-slip Faults of the Taal Tuff
Tulip
Palm Tree
Drawings are modified from Twiss and Moores
(1991).
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Stereographic plots of strike-slip faults
Taal Tuff Sinistral
NE

• Antithetic R-shears or conjugate sinistral
  shears of NS dextral faults

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Conclusions From Field Mapping
1. Outcrop-level evidence for structures based on

• Alignment of Faults
• e.g. Olila-Cumbantog Fault (small-scale graben)
• B. Alignment of Fractures
• e.g. Makiling-Arcuate Fault
• C. Alignment of Altered Ground Patches
• e.g. Tigsa Fault I

2. NS-striking structures are dominant ? provided
crucial evidence for transtension
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Conclusions From Field Mapping
4. NE-trending structures are abundant and
tensional
5. NW-oriented structures are not as prominent
6. Transtensional stress regime horizontal s1 in
the NE and a horizontal s3 in the NW
7. Structures cutting very young lithology
Active faulting?
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Conclusions from Data Synthesis

• The general orientation of structures are as
  follows

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Discussion 1a Morphology of Bulalo Structures

• Q Do normal faults in the Bulalo field have
  unusually higher dip values?

Olila-Cumbantog Fault 70
Bulalo Fault I 80
Normal faults are steeper than usual.
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Possible Explanations for Steep Dips

• Not steep throughout?
• Oblique movement?
• Horizontal s1?

s3
s3
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Discussion 1b Morphology of Bulalo Structures

• Q Are NS structures really strike-slip faults?

• Strike-slip Checklist
• Vertical dip angles?
• 2. Riedel shearing?
• 3. Duplex structures?
• 4. Structures in bends?
• 5. Conjugate pairs?

Ok!
s1
Ok!
s3
Yes!
Ok!
Ok!
Ok!
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Discussion 2 The Transtension Test

• Does transtension really occur along N-S dextral
  faults?
• 1. s1 and s3 in the horizontal plane
• 2. Thrust faults striking NW
• 3. Normal faults striking NE
• 4. R-shears striking N-S and R-shears trending
  NE

s3
Yes!
Yes!
Yes!
s3
Yes!
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Discussion 3 Local Implications of Transtension

• Q How would a transtensional stress regime
  affect the geothermal system of the Bulalo field?

s3
NE and NS structures are extensional in nature ?
favorable targets for reservoir permeability
s3
NW structures are compressional in nature ? not
favorable targets?
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How come NW structures also enhance sweet spot
permeability?
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NS zone
NE zone
NW zone
? Defines sweet spot of Bulalo
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Discussion 4 Regional Implications of
Transtension
s1
s3
s3
s1
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Recap of the Study
Surface structure analysis was performed
using ? Image Processing ? Field Mapping ?
Data Synthesis
Transtension in the Bulalo field is evidenced
by ? dominant NS dextral shearing ? NE normal
structures ? NW compressional faults
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Recap of the Study
Local implications of transtension are as
follows ? NEs1 and NWs3 in the horizontal
plane ? NE and NS structures permeablity
targets ? Sweet Spot is defined
Transtension, in a regional context, accounts
for ? steep graben faults of the Macolod
Corridor ? NW-striking regional folds and thrust
zones ? present-day dextral displacement of NS
faults
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Acknowledgements
UP-NIGS PGI VTEC

• A. M. F. Lagmay
• M. Aurelio
• J. Stimac
• V. Maglambayan
• A. Peleo-Alampay
• X. Anacio
• C. Arcilla
• R. Balangue-Tarriela
• C. Bastero
• R. Bautista
• J. Bingcang
• L. Blay
• P. Burris
• H. Cabria
• M. del Rosario
• A. Dimabuyu, Jr.
• E. Francisco

• N. Fernandez
• G. Golla
• A. Jimenez
• J. Latayan
• Libay
• P. Malicsi
• M. Montes
• G. Norini
• Quiatchon brothers
• C. Satioquia
• E. Sunio
• W. Tamayo
• A. Tengonciang
• H. Uy
• R. Vicedo
• Immediate family
• Close friends

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Focal Points of Earthquakes with M5 from
1976-2004
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Focal Points of Earthquakes with M5 from
1976-2004
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Tectonic Models for the Macolod Corridor

• NW-SE tension due to
• Diffuse movement of
• major sinistral fault zones
• PFZ WLSZ
• Defant and others (1990)
• Defant and Ragland (1988)
• (b) PFZ VIPFZ
• Forster and others (1990)
• Pubellier and others (1990)

Illustrations are adapted from Oles and others
(1991).
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Tectonic Models for the Macolod Corridor

• NW-SE tension due to
• 2. Block Rotation
• Occurs between two
• subparallel strike-slip zones
• In case of NW sinistral zones, dextral NE faults
  will form
• Extension and shortening occur along the major
  strike-slips

PFZ
Illustration is modified from Nicholson and
others (1986).
VIPFZ
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Coulomb-Anderson Theory of Faulting
? 90 - 2F
F
2?
Cohesive Strength
Since average angle of internal friction (F) of
rocks in nature is 30, then ? 30
Drawings are modified from Davis and Reynolds
(1996).
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Typical Values of Angle of Internal Friction
(After Hancock, 2002)
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Generalized Geologic Map of the Bulalo Geothermal
Field
Map is modified from Stimac and Golla (1999).
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Transpression s1 gt 45
U
Pure Strike-slip s1 45
Transtension s1 lt 45
Drawing above is adapted from Sanderson and
Marchini (1984).
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Structure Readings

• Joints (135 pts.)
• Cabulugan Andesite 3 pts.
• Bulalo Dacite 33 pts.
• Taal Tuff 49 pts.
• Alluvial Deposits 20 pts
• Altered Ground 30 pts.

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Structure Readings

• Normal Faults (34 pts.)
• Cabulugan Andesite 3 pts.
• Bulalo Dacite 7 pts.
• Taal Tuff 12 pts.
• Alluvial Deposits 12 pts.
• Thrust Faults (3 pts.)
• Cabulugan Andesite 1 pt.
• Alluvial Deposits 2 pts.

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Structure Readings

• Dextral Faults (20 pts.)
• Taal Tuff 19 pts.
• Bulalp Dacite 1 pt.
• Sinistral Faults (3 pts.)
• Taal Tuff 3 pts.

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Map is modified from Lagmay and co-workers (2004).