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 (Brazils 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 whats 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 worlds 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 worlds 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 Shells Coulomb field, in 7,600 feet of water in the Gulf of Mexico.
According to Douglas Westwoods 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.