West
Africa may be grabbing the lion's share of international exploration
interest, but here's a message for the industry:
The other
side of the Atlantic Ocean looks pretty good, too.
In fact,
Central America, South America and the Caribbean are poised to be
hydrocarbon success stories. According to the U.S. Geological Survey's
2000 World Petroleum Assessment, the region ranks third in the world
for undiscovered oil and gas resources, behind the Middle East and
the former Soviet Union.
Specifically,
the USGS estimates mean totals of 105 billion barrels of oil and
487 trillion cubic feet of gas of undiscovered resource in the region.
Not surprisingly,
the greatest potential for giant oil and gas fields lies in the
offshore basins along the Atlantic margin of eastern South America,
from the Santos Basin in the south to the Guyana-Suriname Basin
in the north.
The potential
for giant fields is mainly offshore in water depths to 3,600 meters.
In addition,
the basins of northern South America around Trinidad and Barbados
as well as Venezuela are estimated to contain significant undiscovered
natural gas resources, as are some of the Andean-related basins.
That all
is according to Christopher Schenk, a USGS geologist who reported
on the agency's findings at last fall’s AAPG International
Conference and Exhibition in Barcelona. His paper was titled "Assessment
of Undiscovered Oil and Gas Resources in Central America, South
America and the Caribbean."
The USGS
studied 23 basins in Central and South America and the Caribbean
area. In each basin scientists geologically:
- Defined total petroleum
systems.
- Defined assessment
units within total petroleum systems.
- Assessed the volumes
of undiscovered conventional oil and gas in each assessment unit.
The USGS
defined 28 total petroleum systems and 55 assessment units in the
basins.
Schenk said
that 83 percent, or 87 billion barrels, of undiscovered oil is estimated
to be in the total petroleum systems in the Guyana-Suriname, Campos,
Santos, Falklands Plateau, East Venezuela, Maracaibo, Llanos and
the Putumayo-Oriente-Maranon basins, with two thirds of that total
estimated to be offshore.
For undiscovered
non-associated gas, an estimated 61 percent of the resource is in
the total petroleum systems of seven assessed basins — six of which
are predominately offshore: Foz do Amazonas, Espirito Santo, Santos,
Pelotas, Santa Cruz-Tarija, the Tobago Trough and East Venezuela,
which includes the Columbus Basin offshore Trinidad.
"The
exploitation of these oil and gas resources in South America," Schenk
said, "means that the emphasis on future exploration will move offshore
to water depths approaching 4,000 meters."
Geologic
Factors — and Results
Three major
geologic events shaped the development of total petroleum systems
and composite total petroleum systems in Central America, South
America and the Caribbean. The resource assessment grouped total
petroleum systems into these three categories.
These are:
The development of the Andean mountain chain.
The tectonic
development of the Andean chain in the Tertiary led to the formation
of a fold and thrust belt and a segmented foreland basin that extends
the length of the continent.
Pre-Andean,
passive-margin marine source rocks in many of the assessed provinces
reached generative maturity for oil or gas because of tectonic loading
or burial by thick Tertiary synorogenic clastic sequences.
"We
assessed several provinces with total petroleum systems that evolved
largely because of Andean tectonism, including systems in the Magallanes,
San Jorge, Neuquen, Santa Cruz-Tarija, Putumayo-Oriente-Maranon,
Talara, Progreso, Middle Magdalena and Llanos basins," Schenk
said.
Many of
the Andean basins have been heavily explored, and the potential
for large oil discoveries is minor.
"Basins
such as the Putamayo-Oriente-Maranon have been producing for decades,
and we did not give those a great deal of additional undiscovered
oil resources," Schenk said. "However, there were some Andean basins
with significant remaining natural gas potential."
An example:
In the Santa Cruz Basin along big, re-activated Paleozoic structures.
"The Bolivian
government has recognized this resource and is starting to exploit
it," Schenk said. "The pipeline into Brazil is from this region."
Colombia's
Llanos Basin is still prospective, according to the USGS.
"The
basin has been heavily explored, but it is structurally very complicated,
so there likely are remaining undiscovered oil resources," he said.
"Of course, the future won’t be as great as the past, but there
is still potential.
"The figures
for our study do not include potential for additional reserves in
existing fields, and this could be considerable for the Llanos Basin,"
he added. "It will likely be much like Prudhoe Bay, where the first
reserve estimates were nine billion barrels of oil and today the
total is up to 13 billion barrels."
To the north, the
evolution of the Caribbean plate.
The Caribbean
Plate's oblique collision with South America's passive northern
margin in the Tertiary formed a series of foreland basins across
Venezuela in which the deposition of thick synorogenic clastics
led to the maturation of the Upper Cretaceous La Luna Formation
and its stratigraphic equivalents, according to the USGS.
While the
Maracaibo and East Venezuela basins are extensively explored and
the La Luna total petroleum system is known in some detail, the
total petroleum system in other assessed Caribbean basins — the
Carupano Basin and the Tobago Trough, for example — are far less
understood, according to Schenk.
The Maracaibo
has been producing for 100 years from coastal fields, but he said
there may be some potential down deep in the deeper parts of the
basins.
"Again,
field growth will be a big factor in these basins, and that figure
is not included in our estimates," Schenk said. "We defined the
East Venezuela Basin to include the big gas basins off Trinidad,
so the bulk of the mean 93 trillion cubic feet of estimated undiscovered
gas resource is from that emerging region."
Several
significant gas discoveries have been made in the Carupano, a small
basin northwest of Tobago in the Tobago Trough. The basin is not
well known, and Schenk said there is about as much remaining undiscovered
potential as the total found to date.
"That
is a nice little gas basin that surrounds the countries of Tobago
and Grenada in international waters," he said. "The major problem
in this part of the world is ownership issues. The Trinidad-Venezuela
border, the Trinidad-Barbados border and the Guyana-Suriname border
are all under dispute. That will be a real hurdle for companies
interested in the region."
Still, there
are resources worth searching for in the area, such as the Lesser
Antilles Deformed Belt offshore Barbados.
"I
felt there was potential to the west offshore Barbados, where there
are some big structures and known source rocks," he said. "Conoco
recently drilled a well offshore southwest Barbados in about 5,000
feet of water, so it will be interesting to see the results of that
effort when they are released."
The opening of the Atlantic
— especially
the south Atlantic region, where the lion's share of undiscovered
South America potential is buried.
The Atlantic
opening and subsequent rifting in the Jurassic and Lower Cretaceous
led to the formation of a series of rift-drift basins along the
length of the continent's eastern margin.
Many of
the source rocks in these basins are genetically related to synrift
and transitional-marine sediments, and clastic wedges of the drift
phase are interpreted to have sufficient overburden to mature several
of the source rocks, the USGS report indicated.
The Guyana-Suriname
Basin, the region's northernmost province, is virtually untouched.
"I
spent a week (there) with the state oil company looking at all their
data," he said, "and the basin looks very prospective."
There is:
- Production onshore
that proves there is a working oil system.
- Likely a Cretaceous
source rock offshore, which coupled with the turbidite reservoirs
make a good package.
- Good evidence that
the onshore oil fields are sourced from down dip in the offshore
area.
The USGS
estimates a mean 15.2 billion barrels of oil undiscovered resource
for the basin.
Other Potential
Unlike the Guyana-Suriname, the Foz do
Amazonas Basin has very little potential for oil, according to
the USGS — but mean undiscovered natural gas estimates totaled
a whopping 30 trillion cubic feet of gas.
The basin
includes the total petroleum system associated with 10 kilometers
of deltaic and slope sediments deposited off the Amazon shelf since
the Miocene. Schenk said several deltas around the world have produced
considerable gas from very similar settings.
There is
one gas field in the basin. Plus, bottom simulating reflectors indicate
thick gas hydrates, which suggest there may be thermogenic gas generation
at depth in the total petroleum system.
The Sergipe-Alagoas and Espirito
Santo are small Brazilians basins with both onshore and offshore
components. However, a great deal of oil has already been produced
from these basins, so the remaining undiscovered resource is not
large compared to other basins in Brazil, Schenk said.
The Campos Basin, one of the world's most
studied petroleum systems, has three assessment units: the Late
Cretaceous-Tertiary turbidities that account for giant fields like
Roncador, Marlim and Albacora; the Cretaceous Carbonates that contains
the first offshore field discovered in the basin; and the salt dome
province Tertiary sandstones, which is hypothetical and defined
to include petroleum trapped in salt structures in ultra-deep waters
of the Campos Basin.
The USGS
estimated a mean average of more than 16 billion barrels of oil
for the basin and over 19 trillion cubic feet of primarily associated
gas.
The Santos Basin is deeper and hotter than the Campos and
perhaps the most prospective under explored basin in South America.
While there
is a good deal of uncertainty in the USGS numbers, mean undiscovered
oil resources for the Santos are about 23 billion barrels of oil
and the mean natural gas estimate is over 80 trillion cubic feet.
Petrobras
is now zeroing in on this basin and beginning to exploit the natural
gas potential, Schenk said.
The Pelotas Basin is related to the thick wedge of clastic
sediments deposited in the Rio Grande cone and other centers of
clastic deposition along the Pelotas shelf edge.
The composite
total petroleum system in the basin is defined based on the presence
of thick gas hydrates in the upper part of the sedimentary section
— a possible indication that thermogenic gas is being generated
at depth in the clastic wedge, according to the USGS.
The Falklands Plateau and the Malvinas
Basin have both been studied, and large structures have been
mapped in the Falklands Plateau, but there is uncertainty over the
presence of an adequate source rock.
"There
is a lot of questions about how the fluid system would work here
— whether enough source rock has been heated up and whether it
would have generated enough fluid to fill the prospects," Schenk
said. "There have been several wells drilled and they were disappointing.
Today it looks like there are good structures with nothing in them.
"There may
still be some potential," he added, "but somebody is going to have
to demonstrate that there is an adequate source rock."