In the oil and gas industry, the size of a discovery matters. And these days, so does the environmental footprint of extracting its resources. As the world continues to research sustainable energy sources, one geologist – in a rare twist – is looking to giant deepwater oil and gas fields for solutions.
“If we’re looking for efficient sources of energy with manageable environmental footprints, deepwater may be the place to look,” said AAPG Member Henry S. Pettingill, consultant and former geologist for Shell and Noble Energy.
While it often seems that environmental strain related to energy consumption is the focus, Pettingill suggests considering the environmental cost of extracting and producing that energy.
The Stockholm Environmental Institute once reported that the United States could reduce carbon emissions by 26 million metric tons in 2030 alone by not approving new Arctic and Gulf of Mexico projects and refusing to renew existing offshore leases.
Pettingill, however, sees deepwater oil and gas production in a more favorable light – both to the industry and to the environment.
“Most notably, about half the reserves of the deepwater giant fields are natural gas, which emits far lower emissions than coal or oil,” he said. “Since there is abundant supply and the economics can be favorable, many see it as the bridge fuel between now and the day that renewables and safe nuclear can provide a more substantial portion of our global energy mix.”
Pettingill has been studying deepwater giant oil and gas fields, comparing their reserves to their surface areas, ranking them according to their “reserve density,” or their volume in hydrocarbons per square meter.
“Because hydrocarbon volumes are expressed in energy equivalents, such as barrels of oil equivalent, reserve density is also energy density,” he explained. “This allows us to visualize how much energy is concentrated in one place, and generally speaking, points to the level of economic and environmental efficiencies associated with extraction of those reserves.”
His first step was to produce a chart of the giant deepwater fields to determine which fields have the largest and smallest reserves per areal footprint. He chose fields that represented diversity in areal footprint and net pay thickness.
The Mars-Ursa field in the northern Gulf of Mexico topped the list with an estimated 2.3 billion barrels oil equivalent contained within an area significantly less than 100 square kilometers, giving it a reserve density of about 40 barrels per square meter. The Mars-Ursa field occupies an area about 70 percent the size of George Bush Intercontinental Airport, said the Houston-based Pettingill.
“Mars is a unique field,” added Paul Weimer, 2020 AAPG Sidney Powers’ medalist and Pettingill’s co-author in a presentation on the topic at the AAPG Global Super Basins Leadership earlier this year. “It has a minimum of 14 reservoir levels in a very small area, and they are all stacked on top of each other.”
Egypt’s Zohr gas field is close behind Mars, with more than 23 TCF of recoverable gas distributed over an area of roughly 100 square kilometers, and a reserve density of about 38 barrels per square meter.
At the other end of Pettingill’s spectrum, the Scarborough gas discovery off the northwest coast of Australia has a reserve density of just 1.5 barrels of oil equivalent per square meter – its area spanning a vast 800 square kilometers, with recoverable volumes of 7.3 TCF. Scarborough would occupy about half of the Houston metropolitan area.
Using reserve densities of each field, Pettingill calculated their “power density,” or the amount of power that can be generated per square meter. He borrowed the “power density” concept from a book titled “Power Hungry,” by Robert Bryce. Using the reserve densities of each field, Pettingill calculated their power densities in both watts and barrels of oil equivalent per day.
In a quest to learn which provided the most power and simultaneously took up the least amount of space, he then produced a power density chart comparing deepwater fields to a host of other power sources.
The Mars-Ursa field is the standout example, delivering more than 500 watts per square meter. Also, impressive is Israel’s Tamar gas field, which has a high reserve density and flow rate and produces about 100 watts per square meter.
In contrast, a typical two-reactor nuclear plant in South Texas produces 56 watts per square meter, while the average onshore U.S. gas well produces roughly the same amount, Pettingill said.
Farther down the efficiency line are sustainable energy sources, including the average solar plant, which delivers about 7 watts per square meter, and wind farms that deliver about 1 watt per square meter. At the lowest end, cornfields used for ethanol deliver less than one-tenth of a watt per square meter.
“Wind farms, solar energy and unconventional hydrocarbons require very large amounts of area per megawatt generated,” Pettingill said. “A deepwater field with a small footprint is much more economically and environmentally efficient.”
For example, to replace the Mars-Ursa power output with corn ethanol, about one-half the state of Texas would have to be covered in cornfields, he said.
And, unlike many shale plays – which often require an extensive pipeline network connecting many wells over many miles – offshore fields use limited pipelines and do not rely on a steady stream of trucks to support drilling, hydraulic fracturing and production.
Because deepwater fields are known for very high flow rates, hydrocarbons can be quickly pumped straight to a processing facility.
“It gets to the user much faster, which in turn provides an economic advantage, with lower environmental burden from extraction than some other forms of energy,” Pettingill said.
Recalling his time at Noble Energy, he said, “The day we turned on the Tamar gas field, Israel was able to replace coal with natural gas as the primary feedstock to their power plants. Prior to that day, they never had a substantial reliable natural gas source, and now they are exporting gas.”
Noble once reported that the amount of coal removed from Israel’s energy supply is the equivalent to taking every car off the highway in Israel for 17 years.
Pettingill added that if Israel were to replace the power generation of the Tamar gas field with corn ethanol, then cornfields 11 times the area of Israel would be needed for the same amount of power.
However, he does acknowledge that offshore production can only be considered environmentally sound if strict measures are followed to prevent spills, leaks and damage to the seabed, and other forms of harm to wildlife.
In reflecting on the history of the industry, in which economics has always driven exploration and development, Pettingill suggests that reserve density and power density be factored into the equation, especially as the world gravitates toward projects that balance economy with ecology.
“Oil and gas are still good. What we do matters,” he said. “We are delivering something that cannot be replaced in an economically competitive way. But the message here is that deepwater production is economically friendly and environmentally manageable.”