Deepwater provinces have been one
of the significant rank exploration plays in the past decade, and
for good reason — if the international petroleum industry is to
continue meeting global demand, these tremendous deepwater reserves
are critical.
But what can the past and present activities in these
provinces teach about the future potential of the deepwater margins
around the world?
A lot, according to two geologists who have studied
worldwide deepwater provinces with an eye toward future deepwater
exploration and production.
Henry S. Pettingill, with Noble Energy, and Paul
Weimer with the Energy and Mineral Applied Research Center at the
University of Colorado, and AAPG treasures, believe that the profession's
better understanding of deepwater provinces already has impacted
success rates.
To back their point, the two AAPG members have compiled
an overview that addresses the critical aspects of geological habitat,
productive trends and potential reserves of global deepwater activities.
Pettingill and Weimer will present their paper, "Worldwide
Deepwater Exploration and Production: Past, Present and Future,"
at this month's Gulf Coast Association of Geological Societies annual
Section meeting.
"Deepwater" is defined as 500 meters or greater water
depth, and "ultradeep" is 2,000 meters or more.
First, some facts:
- About 58 billion barrels of oil equivalent total resources
have been discovered in deep water from 18 basins on six continents,
they report, with the majority of the resources from the Gulf
of Mexico, Brazil and West Africa.
- Only 25 percent of the total resources are developed or under
development, and less than 5 percent have been produced, which
illustrates the immaturity of deepwater exploration and production.
- More than half of this total has been discovered since 1995.
- While OPEC accounts for almost 80 percent of the world's current
oil reserves, only 17 percent of the deepwater oil resources are
in OPEC countries — and all of that total is in Nigeria and Indonesia.
- The global deepwater exploration success rate was just 10 percent
until 1985, but since that time has averaged about 30 percent
— an increase driven by the Gulf of Mexico and West Africa.
- Success rates have been highest in West Africa and lowest in
Asia.
The Congo Basin has enjoyed the best track record
with a geological success rate that from 1996-2000 exceeded 80 percent.
Pettingill and Weimer noted that since deepwater
drilling began in the late 1970s, 29 giant discoveries with recoverable
reserves greater than 500 million barrels of oil equivalent have
been made. In fact, of the 58 giant fields discovered in the 1990s,
about one-third are in deep water.
"While the total number of giant fields discovered
worldwide in recent decades has leveled off, the discovery of deepwater
giants is rapidly increasing," the authors said.
Going to the Source
Pettingill and Weimer's report outlines deepwater
geology and what it means for future exploration.
"The present-day deepwater environment is a continuation
of ultra deepwater depths established during the Mesozoic in most
frontier regions of the world," they said. "This prolonged period
of deposition in bathyal water depths can produce excellent source
rocks, although their efficiency varies through time and space."
Along the West African margin, multiple marine source
rocks exist, being progressively younger moving into the deeper
water, they observed. Some of these, such as the Akata Shale in
Nigeria and the Iabe/Landana formation in the Lower Congo Basin,
may be considered world-class.
"Finally, in some equatorial regions, Tertiary land
plant material, which is traditionally gas prone, can contribute
to oil producing source rocks," they said. "This material is initially
deposited in coastal and shallow marine depocenters, however, during
Tertiary lowstands, some of this humic material is transported into
deeper water and concentrated in zones that ultimately form oil
source rocks."
This type of source rock is recognized today in Nigeria,
Brunei and Southeast Borneo.
Pettingill and Weimer said oil quality can vary in
the world's deepwater basins and quality can be a development concern,
particularly in ultra deepwaters, where there is less overburden
to mature source material. Specific problems include the presence
of high sulfur oils, waxy oils, asphaltenes, low API gravity and
gas hydrates.
Size — and Other Factors
Deepwater field sizes are highly variable from basin
to basin, they reported, a result of the differences in trap area
and net feet of pay, and to a lesser extent, recovery factor.
"In the confined basin setting, which is often composed
of small mini-basins, fields often have large net pay values but
limited trap areas, such as the supra-salt areas of the northern
Gulf of Mexico," they said. "In contrast, the Campos Basin in Brazil
has less confinement between salt bodies, and trap areas can be
quite large, leading to large mean field size."
Traps in deepwater Nigeria and Angola tend to be
rollover anticlines and/or diapir flanks having large area, often
accompanied by stacked sand sequences of high net pay, which leads
to large field size, they added.
In the northern Gulf of Mexico lower field size is
a consequence of smaller trap area. The mean of the largest 19 Gulf
finds is less than that of all 19 Campos Basin discoveries, and
the mean of the largest 30 Gulf discoveries is smaller than the
mean of all 37 West Africa discoveries that have published reserves,
according to the authors. But, favorable contract terms and advancing
infrastructure in the Gulf make these smaller fields economic, they
said.
Many of the approximately 130 Gulf deepwater discoveries
are or will be developed as subsea tiebacks to existing infrastructure.
Other important factors in deepwater exploration
success, according to the authors, include:
- Advancements in seismic technology.
- Identifying reservoirs having high flow rates and per-well
reserves.
- Production system advances.
"Advances in seismic reflection imaging have arguably
been the most important element in allowing companies to explore
deep water, since they often reduce geological risk to acceptable
levels," they said. "Seismic direct hydrocarbon indicators, including
amplitude vs. offset, have been critical to understanding reservoir
and charge risk. Because of the associated risk reduction, direct
hydrocarbon indicators were a major driving force behind the initiation
of significant deepwater drilling in the 1980s."
More recently, pre-stack depth migration has become
critical for imaging deepwater traps, particularly along steeply
dipping salt flanks and underneath salt, they added.
Future Themes
The authors see five themes for future deepwater
exploration. These include:
♦
A continuation of established trends.
"Most of the world's established deepwater play areas
are at a relatively immature state of exploration, so continued
success within those proven plays is almost a certainty," they said.
Going beyond the established geological formula,
exploration is extremely immature within basins lacking updip production,
within confined basins and compressive margins, and in plays with
pre-Cenozoic and non-turbidite targets.
♦
Emerging trends:
... basins lacking updip production, pre-Cenozoic
targets, unconfined basins and compressive margins, non-turbidite
reservoirs and non-direct hydrocarbon indicator supported targets.
"Non-direct hydrocarbon indicator exploration is
expected to increase, particularly in pre-Cenozoic and deeply buried
objectives," they said. "In several cases, petroleum systems have
been established updip of deepwater, but produce only marginal or
non-commercial accumulations. The recent success in the Rio Muni
Basin of Equatorial Guinea is an example, and it also includes pre-Cenozoic
targets."
Contractional settings to date have been very lightly
explored in deepwater, with drilling activity mostly limited to
the terminal foldbelts or "toe thrusts" that constitute contraction
downdip of the main extensional areas of passive margins.
♦
Increased deliberate gas exploration.
Deepwater gas exploration should increase as pipeline
networks and liquefaction technology advance, in conjunction with
increased worldwide consumption, they said. Many of the world's
deepwater basins are gas-prone, but many lack markets nearby. Other
basins that are oil-bearing have large amounts of associated gas.
LNG plants that can accept deepwater gas are currently
planned in at least four locations, and a floating LNG plant has
been proposed for the Northwest Shelf of Australia, they said.
♦
Going deeper — both ultra-deepwater and deeper into the subsurface.
Ultra-deepwater and deeper drilling depths will be
important — including subsalt, sub-detachment and sub-volcanic
targets.
"Whereas current Gulf of Mexico deepwater exploration
wells routinely have total depths exceeding 6,000 meters, relatively
few exploration wells in other deepwater frontiers have been drilled
beyond 4,000 meters total depth," they said. "Subsalt objectives
occur in several deepwater basins around the globe. However, only
the Gulf of Mexico has seen deepwater subsalt drilling and at least
one subsalt structural trend in the Gulf has been leased but not
drilled.
"Ultra-deepwater frontiers occur in several margins
of the world."
♦
Politically driven opportunities.
New opportunities may arise in areas previously not
open due to monopolies, moratoriums and boundary disputes.
In Brazil, for example, the removal of a monopoly
has resulted in offerings of prospective deepwater areas for licensing;
the eastern Gulf of Mexico has had a 10-year leasing moratorium,
despite the proliferation of deepwater discoveries along its border,
but has seen its first leasing round recently; and in the Gulf of
Mexico the ultra-deepwater "donut hole" area between Mexico and
the United States has been resolved and the U.S. portion is open
for leasing.