Geothermal Resources Council Transactions, Vol. 28, August 29 – September 1, 2004
PICO ALTO, TERCEIRA: A NEW GEOTHERMAL FIELD IN THE AZORES
Roger Henneberger1, Rui Cabeças2, Rita Martins3, and Eduardo Granados1
1
GeothermEx, Inc., Richmond, CA
SOGEO – Sociedade Geotérmica dos Açores, Ponta Delgada, Açores (Portugal)
3
GeoTerceira – Sociedade Geoeléctrica da Terceira, S.A., Angra do Heroísmo, Açores (Portugal)
2
Keywords
energy resources (such as wind and hydropower) is limited
by their volatility and the small scale of the systems they
would serve.
Geothermal power generation has been developed
successfully on the largest and most populous island, São
Miguel, since the early 1980s. Expansion of the generation
capacity by Sociedade Geotérmica dos Açores (SOGEO) in
the second half of the 1990s has led to the present situation,
in which geothermal supplies about 25% of São Miguel’s
electrical energy needs as a base-load source of power.
This success has encouraged interest in geothermal
development on other islands.
The most advanced
exploration outside of São Miguel has taken place on
Terceira, the second most populous island, where
Sociedade Geoeléctrica da Terceira, S.A. (GeoTerceira), a
sister-company of SOGEO, holds a concession and is
actively pursuing geothermal development. GeoTerceira’s
objective is the installation of a geothermal power station,
of about 12 MW capacity, to supply a significant fraction
Pico Alto, Terceira, Azores, temperature gradient drilling,
audiomagnetotelluric surveys, exploration
ABSTRACT
A new high-temperature geothermal resource has been
identified on the island of Terceira in the Azores, through
an exploration program undertaken by GeoTerceira with
the assistance of GeothermEx. Previous exploration on the
island did not provide enough information about the
location and extent of the geothermal system to support
deep drilling; the new program was designed specifically to
reduce confirmation-drilling risk.
A 624-station
audiomagnetotelluric survey determined the resistivity
structure of the area of interest with sufficient accuracy to
develop a working model of the system, which was tested
by drilling 4 temperature observation wells up to 600 m
deep. Measured temperatures revealed a convective zone
of at least 233°C, and a broad area in which temperatures
greater than 200°C can be projected. A confirmationdrilling program is being planned in light of the positive
results of the exploration work.
Introduction
The Azores archipelago is located in the North Atlantic
Ocean between latitude 37°-40° north and longitude 25°31° west. It consists of 9 inhabited islands that straddle the
mid-Atlantic ridge off the coast of mainland Portugal, in a
complex geotectonic setting associated with the triple
junction point of the North American, Euro-Asian and
African plates (Figure 1). All of the islands are volcanic in
origin, and, on at least several of them, there is potentially
exploitable geothermal energy. The Azores constitute an
ultra-peripheral region of Europe, in which the strategy for
the development and use of renewable energy favors
indigenous sources. Geothermal energy has first-priority
status among renewable sources due to its abundance and
reliability. The development potential for other renewable
Figure 1. Geographic and tectonic setting of Terceira island
(from Nunes et al., 1990). AM = American plate; EU =
European plate; AF = African plate; CMA = mid-Atlantic
ridge; RT = Terceira rift; TSJ = São Jorge transform; ZFNA =
north Azores fracture zone; ZFOA = west Azores fracture
zone; ZFEA = east Azores fracture zone; FG = Gloria fault.
345
Henneberger et. al.
several dipole-dipole surveys and at least one vertical
electric sounding) and drilled 9 shallow temperatureobservation wells ranging in depth from 85 to 222 meters.
This work was complemented by studies of surface
geology, volcanology and tectonics, as well as a limited
seismic reflection survey performed by the Instituto de
Geociências dos Açores.
In 1980 the United States Geological Survey included
Terceira in its inventory of the geothermal resources of
Portugal, making a resource estimate based mainly on the
characteristics of the volcanoes and surface thermal
manifestations (Johnston, 1980). At about the same time,
LGT contracted exploration and development services to be
performed on the islands of São Miguel, Terceira and Faial.
On Terceira, the contracted work included a two-phase
program of basic geothermal exploration, which was
carried out by Geothermal Energy New Zealand Ltd. in
association with Mitsubishi Corporation (1981, 1982). The
first phase consisted of geologic and geochemical studies;
the second was a geophysical program that included
gravimetry, DC resistivity and a 100-station MT survey.
Little new exploration occurred on Terceira during
1983-1992, while geothermal development work in the
Azores was focused on São Miguel. Around 1993, the
Commission of the European Community authorized a new
program of exploratory studies. This work included
additional tectonic mapping (based mainly on remote
sensing imagery), an 18-station MT survey, and new
geochemical studies that included surveys of soil mercury,
radon and helium (IDROGEO and 3R Research, 1994).
Another hiatus occurred until 1999-2000, when EDA,
through SOGEO, renewed exploration activities on the
island. In September 2000, the partners that eventually
formed GeoTerceira sought a geothermal concession with
the aim of commercially developing the resource.
of Terceira’s power demand, which includes the needs of a
United States Air Force base as well as the island’s more
than 65,000 inhabitants.
GeoTerceira, which is owned jointly by Electricidade
dos Açores (EDA) and Electricidade de Portugal (EDP), is
a private commercial enterprise, and therefore must follow
a development approach that minimizes the time, cost and
risk associated with bringing geothermal power on line.
The remote location of the Azores, which leads to high
costs and lead times for mobilizing heavy equipment, adds
to the need for efficient planning. The following sections
describe the successful critical-path program that
GeoTerceira has used to identify a new high-temperature
resource and prepare for confirmation well drilling.
Previous Exploration
Terceira island consists of 5 volcanic complexes: Cinco
Picos, the caldera of Guilherme Moniz, the Pico Alto
volcanic field, the shield volcano of Santa Bárbara, and the
Central Rift Zone between Santa Bárbara and Pico Alto
(Figure 2). The last three have erupted repeatedly within
the last 20,000 years, and historic eruptions have occurred
in the Central Rift Zone near the Pico Alto Field (in 1761),
and off the west coast of the island (as recently as 1999).
There are few surface manifestations of hydrothermal
activity on the island, but an area of fumaroles is present at
Furnas do Enxofre, within the Pico Alto volcanic field.
Design of Exploration Program
A review of the exploration results through 1999,
carried out by SOGEO and GeothermEx, Inc., showed that
further exploration work was needed in order proceed to
confirm the resource by deep drilling without excessive
risk. The available data were sufficient to define an area of
interest, in the vicinity of Furnas do Enxofre and the Pico
Alto volcanic field (Figure 2), but, apart from the
fumaroles, there was little evidence from which to assess
the size, shape and location of the inferred geothermal
system.
Geoelectrical surveys had confirmed the presence of a
resistivity low, but did not present a consistent enough
picture of the resistivity structure to infer much about
subsurface conditions. Five of the 9 temperature-
Figure 2. Map of Terceira island, showing areas of interest
determined from pre-1999 exploration
Apart from basic studies of the thermal features of the
island (e.g., Zbyzewski et al., 1971; Self, 1973), there was
very little investigation of the geothermal potential of
Terceira before the late 1970s. During the late 1970s, the
Laboratório de Geociências e Tecnologia (LGT) of the
Regional Government of the Azores carried out
geoelectrical surveys of limited extent (consisting of
346
Henneberger et. al.
perform the survey under the supervision of GeothermEx.
Survey techniques and coverage were modified during the
planning and execution of the survey, as opportunities to
improve the results were identified. A total of 624 AMT
stations were occupied and data collected, using a
frequency range of 0.1 Hz to 10 kHz. In addition, 9 MT
stations were acquired.
The AMT survey succeeded in clarifying most of the
uncertainties left by earlier surveys about the resistivity
structure. As expected, a conductive zone was revealed at
shallow levels (near 250 meters above mean sea level) in
the vicinity of Furnas do Enxofre (Figure 4). At deeper
levels, however, the conductive zone was found to be
located further to the southwest, and to extend along an
arcing NW-SE trend (Figure 5). Based on the hypothesis
that the conductive zone is a result of hydrothermal
alteration of original rocks to hydrated clay minerals near
the top and lateral margins of the geothermal reservoir, the
resistivity structure led to a working model in which the
reservoir is located mainly to the northeast of the fumarole
area, and extends northwest and, to a lesser degree,
southeast of the fumaroles. With this model in mind,
much-reduced primary and secondary areas of interest for
further investigation could be defined (Figure 5).
A second significant conductive anomaly, near Agualva,
to the northeast of the Pico Alto area, was also identified
(Figure 5). It represents another area of interest for further
investigation, but, lacking direct evidence of high
temperatures, it merits a lower priority than the Pico Alto
area.
The high quality of the AMT results, in combination
with other data, provided a sound basis for selecting
locations for temperature observation wells. Ten potential
well sites were identified and assigned priorities for the
next phase of work.
Figure 3. Ground radon and soil mercury anomalies (data
from IDROGEO and 3R Research, 1994)
observation wells were clustered around Furnas do
Enxofre, revealing mainly the effects of the fumaroles. The
others were drilled at scattered locations in the central part
of the island, and showed low temperatures with profiles
indicating that they had not penetrated below the zone of
shallow, cold groundwater. Soil mercury and radon
surveys showed scattered anomalies that clouded more than
clarified the geothermal picture (Figure 3).
A program of additional exploration was designed to
meet the specific objective of being able to site deep
confirmation wells with an acceptable level of risk.
Reducing risk meant obtaining better information about the
extent and location of the geothermal system that feeds the
fumaroles at Furnas do Enxofre. Taking into account the
geologic setting, project objectives, and logistical situation,
a two-stage program was established. The program
included:
1) A detailed geoelectrical survey, using up-to-date
methods, in and around the area of interest, to
define the subsurface resistivity structure as
accurately as possible.
2) A series of intermediate-depth temperature
observation wells, to observe directly the
temperature gradients produced by the geothermal
system.
Various electric survey methods were considered, and it
was determined that the audiomagnetotelluric (AMT)
method would be the most appropriate for the geologic
terrane and logistical conditions. A depth range of 500 to
600 meters was judged necessary for the temperature
observation wells. The exploration program was initiated
in 2000.
AMT Survey
An AMT survey with a relatively dense nominal station
spacing of 200 meters was planned to cover as much of the
primary area of interest (about 40 km2) as logistically
possible, with coverage also of selected parts of the
extended area of interest. Geosystem srl was contracted to
Figure 4. Area of AMT survey, and contour of 6-ohm
resistivity at +250 m elevation
347
Henneberger et. al.
development program (Figure 6).
The last of the
temperature observation wells was completed in early April
2004.
Figure 5. Contour of 6-ohm resistivity at +100 m elevation,
and updated area of interest based on AMT survey results
Temperature Observation Wells
The temperature gradient drilling program needed to be
designed carefully to meet its objectives. The relatively
deep holes would need to be drilled in a difficult
environment of mixed lithologies and variable
permeability. In addition, to prevent downflows that might
obscure temperature profiles, the wells would need to be
completed with tubing cemented securely over the entire
length of the hole. Problems of site access had to be faced,
and a more-rigorous-than-expected environmental review
process was required, because of the sensitive nature of the
project area. The central part of Terceira contains various
habitats and species (both native and endemic) that are
protected within the European Natura 2000 network. It is
also considered to be an important recharge area for freshwater aquifers that supply a substantial fraction of the
island’s population. These factors led to delays in the startup of the program, and required modifications to some hole
locations and drilling methods.
Drilling began in the summer of 2003. The original plan
was to use a rotary drilling rig to drill the top-hole portion
of each well and a coring rig for the deeper interval.
Experience in the first well showed that coring was slow in
many intervals, while the rotary rig performed better in
most rock types. After the first well, therefore, the rotary
rig was used exclusively for drilling all intervals. The well
design consisted of a first section drilled to about 200
meters and cased with a 7-inch casing; the second and final
section was completed with a cemented string of 1.66-inch
tubing. The upper section of each hole was drilled as much
as possible with an air hammer in order to protect the water
quality of the shallow aquifers.
The drilling program was adjusted as it proceeded, in
light of initial results, costs and project objectives.
Although a program of 5 to 6 holes had been envisioned, it
was eventually determined that 4 holes would provide
sufficient information while avoiding delays in the overall
Figure 6. Locations of temperature observation wells, and
inferred temperature distribution at mean sea level
Temperature profiles measured in the wells confirmed
that both their design and completion were satisfactory.
Except in well TG-J, which is nearest to the fumaroles at
Furnas do Enxofre, near-isothermal conditions are present
in the first several hundred meters, reflecting the influence
of shallow groundwater aquifers or other non-geothermal
phenomena (Figure 7). Beginning near about 300 meters
depth, a change to very high temperature gradients occurs,
providing a direct indication of geothermal conditions
below. The temperature profiles indicate little, if any,
impact from flow of water behind the tubing, and are
reliable for extrapolating temperatures. Deep temperatures
can confidently be estimated to exceed 200°C over a
substantial area (Figure 6). In well TG-H, a convective
profile in the bottom interval and a maximum measured
temperature of 233°C demonstrate the presence of a hightemperature reservoir (Figure 7).
The temperature gradient data have confirmed and
strengthened the conceptual model suggested by the AMT
and other exploration results, and have provided the
information needed to proceed with confirmation drilling,
having reduced exploration risk to an acceptable level.
GeoTerceira is currently undertaking the preparations
needed to drill the first deep, full-diameter wells on
Terceira. It is anticipated that drilling activities will start in
the second half of 2005, after executing the necessary civil
works for drilling sites, the tender process for the drilling
services, and the preparation and submission of the reports
demonstrating
compliance
with
environmental
requirements.
348
Henneberger et. al.

adapting the selected methods to the logistical,
regulatory, budgetary and other constraints under
which the project must be carried out; and

providing the flexibility to make adjustments as
work proceeds, orienting the work at all times to
the objectives of the program.
References
Geothermal Energy New Zealand Ltd. and Mitsubishi
Corporation, 1981.
“Geothermal Prospection, Ilha
Terceira, Açores: Geological Report.” Report prepared for
Região Autónoma dos Açores, Laboratório de Geociências
e Tecnologia, July 1981.
Geothermal Energy New Zealand Ltd. and Mitsubishi
Corporation, 1982.
“Geothermal Prospection, Ilha
Terceira, Açores: Geophysics Survey.” Report prepared
for Região Autónoma dos Açores, Laboratório de
Geociências e Tecnologia, May 1982.
IDROGEO srl and 3R Research snc, 1994. “Terceira
Geothermal: Identification and Characterisation of a New
Deep Seated High Temperature Geothermal Reservoir –
Phase I – Final Repot. Report for Commission of the
European Communities, D.G. XII, Non Nuclear Energy
Programme 1991-94 (Joule II), July 1994.
Figure 7. Stabilized temperature profiles measured in
observation wells
Conclusions
Johnston, D. A., 1980. “Geothermal Resources in Portugal
and the Azores.” Unpublished report prepared for the U.S.
Department of Energy, 51 p.
GeoTerceira’s exploration program on the island of
Terceira has succeeded in identifying a high-temperature
geothermal system in the Pico Alto area, and has provided
enough information about the position, extent and depth of
the system to design a confirmation-drilling program with a
relatively low level of exploration risk. The program to
date serves as an example of how an exploration effort can
be tailored to meet the technical, economic and scheduling
requirements of a commercial geothermal development
project. Essential elements of such a program are:


Nunes, J. C., J. L. Alves and V. H. Forjaz, 1990.
“Sismicidade instrumental dos Açores no período 1980-89:
implicações neotectónicas.” In Encontro 10 anos após o
sismo dos Açores de 1 Jan 80. Angra do Heroismo,
October 1990.
Self, S., 1973. “Recent Volcanism on Terceira, Azores.”
PhD thesis, Imperial College, London, December 1973.
clearly defining of the objectives of the program,
with an eye toward risk reduction in successive
stages;
Zbyzewski, G., A. Cândido de Medeiros, O. da Veiga
Ferreira, and C. Torre de Assuncão, 1971.
“Carta
Geológica de Portugal, Noticia Explicativa da Folha, Ilha
Terceira, Escala 1:50,000.”
Serviços Geológicos de
Portugal, 43 p.
selecting the methods that contribute directly to
the program objectives, rejecting or minimizing
activities that contribute only marginally to risk
reduction by providing extraneous or ambiguous
data;
349