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 as part of the solution rather than as part of the problem.
“If we’re looking for efficient sources of energy with manageable environmental footprints, deepwater may be the place to look,” said Henry S. Pettingill, consultant and former geologist for Shell and Noble Energy. “While most of the media focus seems to be on the environmental strain related to consumption of energy, we should also consider the environmental cost of extracting and producing that energy.”
Pettingill is seeing 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. 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 (e.g. barrels of oil equivalent or “boe”), reserve density is also energy density, and 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 (Figure 1). These fields were chosen because they represent the diversity in areal footprint and net pay thickness. From this visual technique, we can appreciate fields with vast areas but relative low net pay – “pancake-shaped” – from those with smaller areas but relatively large net pay – “pipe-shaped”, with the latter having higher reserve density. The Mars-Ursa complex 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 field occupies an area smaller than most Houston neighborhoods, or about 70% of the area of Houston’s Bush Intercontinental Airport.
“Mars is a unique field,” said Dr Paul Weimer, who was Pettingill’s co-author in a presentation on the topic at the AAPG Global Super Basins Conference in February 2020. “It has a minimum of 14 reservoir levels in a very small area, and they are all stacked on top of each other.”
Figure 1: Areal footprints of select Giant Deepwater fields, along with their net pay thicknesses, all drawn at equal scale. Upper: Field with Deep marine sand reservoirs. Lower: Field with Carbonate reservoirs. Arrows denote the Reserve Density in barrels of oil equivalent per square meter (boe/m2).
Egypt’s Zohr gas field is close behind Mars, with more than 23 trillion cubic feet 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 trillion cubic feet. Scarborough would occupy about half of the entire Houston metropolitan area. It is notable that this appraised discovery has yet to come onstream 42 years after discovery.
Figure 2: Reserve Density in barrels of oil equivalent per square meter (boe/m2).Red = Gas Fields, Green = Oil Fields (most with associated gas).
Robert Bryce, in his 2010 book “Power Hungry”, defined power density as the amount of power that can be generated per square meter. Using the reserve densities of each field, Pettingill calculated their “power density”, in both watts and barrels of oil equivalent per day.
He then produced a power density chart comparing deepwater fields to a host of other power sources in a quest to learn which provided the most power and simultaneously took up the least amount of space (Figure 4).
Figure 3: Flow rates from Deepwater fields. Since barrels of oil equivalent is an energy equivalent and power is energy per time, this is a comparison of the power output of individual wells.
The Mars-Ursa field is the standout example, delivering more than 500 watts per square meter. Also, impressive, Israel’s Tamar gas field, because it has a high reserve density and flow rate, produces about 100 watts per square meter.
In contrast, a typical two-reactor nuclear plant from South Texas produced 56 watts per square meter, while in 2010 the average onshore U.S. gas well produced roughly the same amount.
Farther down the efficiency line are sustainable energy sources. The average solar plant delivers about 7 watts per square meter, whereas wind farms 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, an area 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 on the road to support drilling, hydraulic fracturing and production operations and in some cases oil evacuation.
Figure 4: Upper: Power Output of typical fuel sources used to generate power, including the Mars-Ursa Complex of the Gulf of Mexico deepwater and the Tamar gas field of the deepwater Levant basin. Lower: Power Density of typical fuel sources, shown in comparison to the deepwater fields Mars (U.S. Gulf of Mexico), Tamar (Israel Levant) and Scarborough (Northwest Shelf, Australia)
Because deepwater is 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 stated at a 2013 conference 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.”
He 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.
Pettingill 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 economic development with environmental needs.
“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.”
Pettingill teaches two courses with GeoLogica, one in the field and one in the classroom. His field course in the Pyrenees of Spain, Sand-rich Turbidite Systems: From Slope to Basin Plain (G016), examines the types of deposits that form the reservoirs in many of the fields discussed in this article. His classroom course is Creativity and Innovation Skills for E&P (G029). It is co-taught with Niven Shumaker and explores the types of “out of the box” thinking that Henry used to develop the energy density views presented above.