“The
ancient seas gave us the oil that is now being drawn out of the earth. Can the
ocean today be induced to give up some of the oil that must be trapped in sedimentary
rocks under its floor, covered by water scores or hundreds of fathoms deep?”
The answer to that elegant question put by Rachel Carson in 1951, in her book
The Sea Around Us, finally is a resounding “yes”!
A floating petroleum production system
owned by Petrobras (Brazil’s national oil company) produces from the Marlim
oil field, Campos basin, offshore Brazil. Petrobras is a leading petroleum company
in deepwater exploration and production with three decades of experience in
deepwater basins offshore Brazil. Photo courtesy of Petrobras.
Exploration for and discovery of petroleum first began on land. Next, exploration
and production moved to offshore waters — what’s been called the “second
wave” of petroleum discovery. Now, a new wave of discovery is in full swing,
with petroleum exploration reaching offshore environments deeper than ever before.
Deepwater environments usually begin at water depths of 600 feet, where the
continental shelf terminates and the continental slope starts. The slopes, diving
to 12,000 feet in depth, are depositional basins for sand, silt and mud that
have been transported from the continent and accumulated over millions of years.
Beyond the slope, in the abyssal plain, currents are weak and sediments are
thin. The authors of a recent book, Deepwater Petroleum Exploration and Production
(2003), have called the deepwater oil bonanza “the third wave.”
In 2002, the world’s oil reserves in deepwater basins deeper than 1,500
feet amounted to about 60 billion barrels of oil, which was almost 6 percent
of the world’s proven oil reserves. In the same year, global deepwater
oil production was about 2.4 million barrels a day, about 3.2 percent of global
production. Petroleum discovery and production in deepwater environments continue
to expand. This year, production from the deepest water depth yet began in Shell’s
Coulomb field, in 7,600 feet of water in the Gulf of Mexico.
According to Douglas Westwood’s World Offshore Drilling Report 2003-2007,
petroleum companies spent $19.2 billion on deepwater drilling during 1988 to
2002, and that figure is set to double to $40 billion between 2003 and 2007.
A report called The Future of Deepwater by two U.K. consultant companies,
Wood Mackenzie and Robertson, also released this year, estimates that 114 billion
barrels of oil and 68 billion barrels of oil equivalent from natural gas in
deepwater await discovery.
We understand far less about petroleum-forming factors in deepwater than in
shallow water or on land. However, the recent discoveries of large oil fields
in deepwater basins encourage us to intensify our studies of deepwater depositional
processes and geologic structures, and to strengthen further discoveries with
geoscience-based knowledge and methods.
Finding oil and gas, whether on continents or in deepwater, requires identification
of three important factors responsible for the generation and accumulation of
petroleum: a source rock, rich in organic material and heated mildly; a reservoir
rock that can store and transmit petroleum; and a trap that creates 3-D impermeable
seal rocks.
Deepwater petroleum fields are usually found along subaqueous delta fronts in
tectonically quiescent areas. The South Atlantic margins (offshore western Africa
and offshore Brazil), the Gulf of Mexico and the South China Sea margins are
remarkable for deepwater discoveries. These continental margins are covered
by wide and thick piles of sediments, deposited by the large river systems of
the world, such as the Mackenzie, Mississippi, Amazon, Congo, Niger and Mahakam,
that form vast deltas before entering the ocean. Ocean currents and gravity-driven
slope flow carry the sediments even farther into deepwater environments.
Petroleum traps in deepwater basins result from a variety of faults and folds
of sedimentary layers. These deepwater reservoirs are sand-rich sediments transported
by turbidity currents on ocean slopes. Thus, a quantitative understanding of
the geometry and development of fault and fold structures in deep seas, as well
as the distribution and properties of sediments, is crucial for successful deepwater
petroleum discoveries.
Drilling and production in deepwater environments pose particular economic and
technological challenges. A deepwater well in the Gulf of Mexico costs $30 million
to $100 million, while the costs of drilling in shallow water and onshore are
usually less than $5 million and $500,000 per well, respectively. Therefore
initially, only major oil companies have been able to afford deepwater ventures.
But in recent years, several medium-sized companies have also joined the deepwater
club.
The technology to drill and produce oil and gas in deepwater is divided into
three broad categories: fixed platforms, floating structures and subsea systems,
which are wellheads on the seafloor connected to fixed platforms or floating
structures. Fixed platforms are held in place either by the weight of the structure
(usually concrete) or by steel platforms driven into the seafloor, while floating
structures and subsea systems are more mobile and thus able to go to greater
depths.
Fixed platforms are feasible for installation in water depths up to 1,500 feet.
A type of fixed platform called a compliant tower can withstand turbulent lateral
forces and go a bit deeper, up to 3,000 feet. It consists of a vertical section
built from steel segments and fixed to the seabed, as well as a deck providing
space for the crew, drilling rig and production facilities.
The technology of fixed platforms had its humble beginnings in 1911, when the
Gulf Oil Corporation drilled many successful wells from wooden platforms in
Lake Caddo in East Texas. In 1947, Kerr-McGee Corporation made a breakthrough
— installing a huge steel platform combined with a drilling tender (converted
from a landing ship tank) 10 miles off the Louisiana coast. This produced the
first oil from a well that was out of sight from land, and marks a milestone
in the offshore petroleum industry. In 1988, Shell installed the Bullwinkle
in 1,354 feet of water in the Gulf of Mexico, marking another high point in
fixed platform technology.
To get deeper, companies can use floating systems, including tension lag platforms,
spar platforms and “floating platform, storage and offloading” (FPSOs).
A type of drillship, FPSOs are not limited by water depth, and they are free
to move laterally and vertically. They consist of a large tanker-shaped vessel,
moored in place by wires, which can produce petroleum from seafloor wells and
store it for transportation by shuttle tankers.
Credit for designing such oil well drillships goes to the CUSS group (named
after Continental, Union, Shell and Superior Oil companies), which drilled in
water depths reaching 400 feet in the 1950s offshore California. In recent years,
the drilling company Transocean has introduced “dual activity” drillships
that conduct drilling operations simultaneously, and thus save time and money.
In 2001, a Transocean drillship working for Unocal drilled in 9,727 feet in
the Gulf of Mexico. Last November, another Transocean drillship, Discover
Deep Seas, working for ChevronTexaco, set a new record by drilling at a
depth of 10,011 feet in the same basin. There are 97 operating FPSOs in the
world now, and another 19 are under construction.
Despite the huge potential of large deepwater petroleum fields, operations in
these environments are risky, and the consequences of a mistake in estimating
reserves, finding accumulation sites, drilling, well productivity and flow assurance,
or deepwater environmental preservation can be serious and costly. Geologic
research and well-trained geoscientists can help mitigate these risks and thus
accelerate the rates of discovery and production. Given the worldwide rise in
demand for oil, deepwater exploration and production will continue to be a venture
of paramount importance in the coming years.
 |
Geotimes Home | AGI Home | Information Services | Geoscience Education | Public Policy | Programs | Publications | Careers  |