July 23rd 2008
Three super-giant fields
discovered offshore Brazil
Petrobras
recently announced the discovery of
three super-giant oil fields in Brazil’s
offshore Santos Basin. The company
confirmed its Tupi discovery on Block
BM-S-11 (Fig. 1) on November 8, 2007 and
stated that the field has probable
recoverable reserves between 5 and 8
billion bbl of light oil. It now appears
that another, possibly much larger, oil
field called Carioca-Sugar Loaf has been
discovered about 50 mi west of Tupi. The
Carioca discovery well (1-SPS-050) was
spudded in April 2007 on Block BM-S-9.
The well reached a 21,876-ft TD in the
pre-salt Aptian Guaratiba Formation in
July 2007. It tested 2,900 bpd of 27°API
oil and 2 MMcfgd. A third discovery, the
Jupter well, was announced on January
21.
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Fig. 1
. Map of Santos Basin
subsalt plays and
blocks, and post-salt
fields. Base map from
ANP Round 8, http://anp.gov.br/brasil-rounds/round8/round8/mapas/Mapa_Round8_23outubro2006.pdf
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Two other
subsalt tests were drilled to the west
of Carioca during the fourth quarter of
2007. The first of these is Well
1-SPS-51 on Block BM-S-21, about 50 mi
southwest of the Carioca location.
Petrobras (operator) and partners Galp
Energia and BG Group Plc said that the
well proved a light oil field in a
pre-salt layer in the basin. The well is
located 174 mi off the coast of the
State of São Paolo in 7,329 ft of water,
and was drilled to a 17,552-ft TD. The
second subsalt test is the Sugar Loaf
well on Block BM-S-8, about 30 mi
northeast of 1-SPS-51 and 30 mi west of
Carioca.
Unofficial, but credible, reports have
suggested that these new wells may be
testing the same large structure
discovered at Carioca. If true, the
feature may cover almost 1,000 sq mi. On
December 11, UBS Pactual in Rio
de Janeiro reported that the
Carioca-Sugar Loaf feature “touches four
blocks: BM-S-22 and BM-S-9 have ‘large
stakes’ in Sugar Loaf; BM-S-21 has a
medium stake; and BM-S-8 a small stake.”
A week later, Next Energy News
wrote that “according to officials at
Petrobras, Sugar Loaf field may produce
up to 40 billion bbl of
oil...Preliminary exploration of the
area has determined that Sugar Loaf
field is five times the size of Tupi
field.”
Just
before the November 27 opening of
Brazil’s Bid Round 9, the national
petroleum agency, ANP, withdrew 40
blocks with significant subsalt
potential from the auction based on the
Tupi confirmation and on new information
from drilling. On December 20, Petrobras
posted an official statement on its
website confirming that the 1-SPS-51
well was an indicated discovery based on
interpretation of drilling results and
logs, but that no tests had yet been
conducted.
On
January 21, a third discovery, Jupiter,
was announced on the BM-S-24 Block.
Petrobras said that the 1-RJS-652
Jupiter well, located 24 mi to the east
of Tupi, is an important gas condensate
discovery. The company also said that
this structure was similar in size to
Tupi. The well had 394 ft of pay and a
17,231-ft TD. Jupiter apparently adds
another 5-8 billion bbl to the
recoverable reserves already announced
for Tupi.
Exploration results in the Santos Basin
prior to Tupi had been disappointing.
Only about 115 exploratory wells have
been drilled in the basin, compared to
more than 1,100 wells in the Campos
Basin to the north. Cumulative
production is about 260 MMcfg from
Merluza Field and 42 million bbl of oil
from the Coral-Tubarao-Estela do
Mar-Caravela Field complex, Fig. 1.
Estimated ultimate recovery from Merluza
is 305 MMcfg, and 228 million bbl of oil
and 245 Bcfg from Coral-Tubarao-Estela
do Mar-Caravela (IHS).
Exploration has focused on relatively
shallow post-salt clastic objectives,
which are poorer reservoirs than
correlative intervals in the Campos
Basin, Fig. 2. Principal hydrocarbon
reservoirs include Lower-Middle Albian
carbonates of the Guaruja Formation and
Turonian turbidite sandstones of the
Ilhabela Member of the Itajai-Acu
Formation. Source rocks for the
petroleum system are Lower Cretaceous
lacustrine shales of the sub-salt
Guaratiba Formation.
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Fig. 2
. Depth seismic line in
the Santos Basin subsalt
play, wells, blocks and
post-salt fields.
Modified from Formigli,
Jan. 2007: Pre-Salt
Reservoirs Offshore
: Perspectives
and Challenges,
Petrobras Web Site,
www2.petrobras.com.br/ri/pdf/2007_Formigli_
Miami_pre-sal.pdf
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The Tupi
discovery lies beneath a layer of Lower
Cretaceous (Aptian) salt that is more
than 6,500 ft thick. Source and
reservoir rocks are contained within the
syn-rift Lower Cretaceous (Barremian to
Aptian) Guaratiba Formation. Reservoirs
consist chiefly of lacustrine carbonates
and associated coastal plain, fluvial
and shallow marine clastic and carbonate
facies. Traps are formed in pre-rift and
syn-rift reservoirs by tilted fault
blocks, horsts and grabens that are
overlain and sealed by salt. Guaratiba
source rocks are thought to be
equivalent to the Lagoa Feia Shale
source rocks of the adjacent Campos
Basin.
Oil from
the Tupi area is 28-30°API with a
gas/oil ratio of about 15-20%. Oil
viscosity averages 1 cp, and the oil
contains significant paraffin.
Associated gas contains 8-18% CO2.
Tupi is located 186 mi offshore in water
7,220 ft deep. The reservoir is a 250-ft
thick, oil-saturated, porous carbonate
unit previously unknown in Brazil. It is
divided into three sequences, with the
uppermost interval containing more than
half of the reserves. Initial reservoir
pressure is 8,232 psi. Testing indicates
potential flowrates of 15,000-20,000
bopd per well. However, heterogenous,
laminated carbonate lithologies may
result in variable reservoir quality.
Horizontal drilling and hydraulic
fracturing are being considered by
Petrobras. Tupi reservoir rocks may be
similar to the Toca carbonates of the
Lower Cretaceous (Barremian-Aptian)
Bucomazi Formation in West Africa. At
the Kambala Field in Cabinda, Angola,
Toca reservoirs are 75 to 300 ft thick
and consist of partially to fully
dolomitized carbonates that have matrix
porosities of 2-10% and very low
permeability. At Kambala, production is
controlled by faulting and fracturing
and, while the field contains more than
1 billion bbl of oil in place,
cumulative production after 30 years is
less than 50 million bbl.
If
reports about the potential
size of the Carioca-Sugar Loaf structure
prove correct at an estimated 33 billion
bbl of oil (the mean of 25 and 40
billion bbl), and Tupi and Jupiter
reserve estimates are accurate at 6.5
billion barrels each (the mean of 5 and
8 billion bbl), Carioca-Sugar Loaf would
be the third-largest oil field in the
world. It would also be the largest
field discovered in the last 30 years,
with Tupi-Jupiter being the 6th largest
discovered in that timeframe, Fig. 3.
For further comparison, Prudhoe Bay is
the largest field in the US, with 15.2
billion barrels of recoverable oil, and
East Texas Field is the second largest
with 5.4 billion bbl. It is unlikely
that the Carioca-Sugar Loaf structure
will be a single field; it will probably
be a complex of somewhat smaller fields,
similar to the emerging deep GOM Wilcox
play, with each perhaps averaging the
size of Tupi.
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Fig. 3
. World oil & condensate
field ranking. Data from
Giant Oil and Gas
Fields, Jan. 2007
Edition,
sourcetoreservoir.com.
Click Image to
Enlarge.
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Reservoir
quality and heterogeneity are the
greatest risk factors from a geoscience
perspective. Well engineering challenges
include drilling and maintaining
production casing through the salt, and
possible fracture stimulation and
secondary recovery in reservoirs that
are in 7,000 ft of water and at drilling
depths greater than 22,000 ft.
Withdrawal of key blocks from Brazil’s
ANP Bid Round 9 is disquieting,
depending on the reason for withdrawal.
Perhaps it is an early example of the
sort of resource nationalism that some
have predicted as global oil supplies
decline. It is more likely, however,
that this action reflects an effort by
the ANP to ensure maximum bid levels
once more is known about the size of the
subsalt play. Despite risks and
challenges, these discoveries represent
the most important oilfield discoveries
in decades. If Tupi-Jupiter and
Carioca-Sugar Loaf are included, half of
the 50 largest oil fields in the world
were found before 1960, and 87% before
1980. The real message of these
discoveries is that we must not lose
sight of the as-yet unknown, but
possibly great, potential of basins that
are often the exclusive domain of
national oil companies.
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