'Systems' Study Gave Insights

Subtle Traps Search to Play Vital Role

Is the world running out of oil?

Is the sun setting on the era of fossil fuels?

Over the past few years the U.S. Geological Survey spent 100 man-years studying the topic of the world's future energy supplies, and then two years ago issued a new global assessment of the undiscovered conventional oil and gas resources and potential additions to reserves from field growth.

The numbers from the study are important — but the research also led to some surprising geologic insights that could help explorationists uncover new reserves around the globe.

"We studied all the significant petroleum systems of the world," said Thomas S. Ahlbrandt, world energy project chief for the USGS in Denver. "We looked at 128 assessed provinces and identified 149 total petroleum systems, which include undiscovered resources. Then we further subdivided those systems into 246 assessment units, or more homogeneous entities within the petroleum system."

The assessment process coupled geologic analysis with a probabilistic methodology to estimate remaining potential. For these provinces, the endowment of recoverable oil, which includes cumulative production, remaining reserves, reserve growth and undiscovered resources, is estimated at about three trillion barrels of oil. The natural gas endowment is estimated at 2.6 trillion barrels of oil equivalent.

Oil reserves are currently 1.1 trillion barrels of oil, with world consumption of about .028 trillion barrels per year. Natural gas reserves are about .8 trillion barrels of oil equivalent and world consumption is around .014 trillion barrels of oil equivalent.

So without any additional discoveries of oil, gas or natural gas liquids, the world has about two trillion barrels of oil equivalent of proved petroleum reserves, according to the study.

With additional reserve additions from undiscovered resources — or potential addition due to reserve growth — there is a total oil and gas endowment of about 5.6 trillion barrels of oil equivalent. The USGS estimates that the world has consumed about one trillion barrels of oil equivalent of this endowment, or 18 percent, leaving about 82 percent of the endowment to be used or found. Half of the world's undiscovered oil and gas potential is offshore, the study indicates — and Arctic basins, with about 25 percent of undiscovered petroleum resources, make up the next great frontier.

"From a geologic perspective, this study was a unique opportunity to look at petroleum systems throughout the world — and to our surprise we found that we have much to learn about these systems," Ahlbrandt said. "The resource volume data we compiled is important, but just as critical is the geologic knowledge we gleaned from the study."

(This geologic information, including maps, is reported in Chapter AR, "Analysis of Assessment Results," USGS World Petroleum Assessment DOS-60, a four CD-ROM set, or can be found at http://energy.cr.usgs.gov/oilgas/wep/wepindex.htm.)

Conventional wisdom was challenged by many aspects of the analysis.

Image Caption

Results of the USGS world petroleum assessment, covering conventional oil, gas and NGL. Means in billion barrels.
Graphic courtesy of U.S. Geological Survey

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Is the world running out of oil?

Is the sun setting on the era of fossil fuels?

Over the past few years the U.S. Geological Survey spent 100 man-years studying the topic of the world's future energy supplies, and then two years ago issued a new global assessment of the undiscovered conventional oil and gas resources and potential additions to reserves from field growth.

The numbers from the study are important — but the research also led to some surprising geologic insights that could help explorationists uncover new reserves around the globe.

"We studied all the significant petroleum systems of the world," said Thomas S. Ahlbrandt, world energy project chief for the USGS in Denver. "We looked at 128 assessed provinces and identified 149 total petroleum systems, which include undiscovered resources. Then we further subdivided those systems into 246 assessment units, or more homogeneous entities within the petroleum system."

The assessment process coupled geologic analysis with a probabilistic methodology to estimate remaining potential. For these provinces, the endowment of recoverable oil, which includes cumulative production, remaining reserves, reserve growth and undiscovered resources, is estimated at about three trillion barrels of oil. The natural gas endowment is estimated at 2.6 trillion barrels of oil equivalent.

Oil reserves are currently 1.1 trillion barrels of oil, with world consumption of about .028 trillion barrels per year. Natural gas reserves are about .8 trillion barrels of oil equivalent and world consumption is around .014 trillion barrels of oil equivalent.

So without any additional discoveries of oil, gas or natural gas liquids, the world has about two trillion barrels of oil equivalent of proved petroleum reserves, according to the study.

With additional reserve additions from undiscovered resources — or potential addition due to reserve growth — there is a total oil and gas endowment of about 5.6 trillion barrels of oil equivalent. The USGS estimates that the world has consumed about one trillion barrels of oil equivalent of this endowment, or 18 percent, leaving about 82 percent of the endowment to be used or found. Half of the world's undiscovered oil and gas potential is offshore, the study indicates — and Arctic basins, with about 25 percent of undiscovered petroleum resources, make up the next great frontier.

"From a geologic perspective, this study was a unique opportunity to look at petroleum systems throughout the world — and to our surprise we found that we have much to learn about these systems," Ahlbrandt said. "The resource volume data we compiled is important, but just as critical is the geologic knowledge we gleaned from the study."

(This geologic information, including maps, is reported in Chapter AR, "Analysis of Assessment Results," USGS World Petroleum Assessment DOS-60, a four CD-ROM set, or can be found at http://energy.cr.usgs.gov/oilgas/wep/wepindex.htm.)

Conventional wisdom was challenged by many aspects of the analysis.

"From that work we prioritized, 10 of the most important geologic insights related to the elements of the total petroleum systems," Ahlbrandt said.

"I used to think I knew a lot about petroleum systems," he continued. "I have worked 20 years in exploration and another 20 with the USGS, but I think I have a less clear understanding now than I did when I started.

"We have much left to learn about petroleum systems, he said, "and how they work."

Lessons Learned

Here's what the USGS scientists learned.

➤ While petroleum is trapped in many ways, less than half of the known petroleum in the world occurs in exclusively structural traps.

"When we looked at the mean resource by trap style we broke it down by stratigraphic, paleogeomorphic, combination structural and stratigraphic, compressional, extensional and non-tectonic structures," said Ahlbrandt, a 2002-2003 AAPG Distinguished Lecturer.

"Compressional structures were the largest category in terms of numbers — but we looked at the various trap styles in terms of resource volumes and came to the interesting conclusion that although many people look for structural traps, there are significant resources in other trap styles.

"For a recent conference in China, Michel T. Halbouty made the point that in China — and I think it's relevant for the world — that the search for the subtle trap will play an increasing role in world exploration," he said.

Some of the biggest fields in the world — like in the Sirte Basin in Libya — are combination stratigraphic and structural traps, and "those kinds of plays will be more important in the future," he said.

The advent of high-resolution geophysics expands the petroleum industry's capability to search for these non-structural traps, which has been significant in the explosion of this technology, he said.

➤ Type II source rocks are by far the dominant source rock type, and source rocks occur throughout the sedimentary rock record.

Ahlbrandt said that a considerable amount of literature indicates Type I and Type III source rocks are important contributors.

"However, when we looked at the petroleum systems around the world we soon realized that volumetrically for both discovered and undiscovered resources, Type II kerogens, or marine kerogens, are at least 10 times more significant than any other source rock," he said.

"We didn't fully appreciate that until we did this study," Ahlbrandt added. "This affects your thinking in terms of where you explore and what types of source rocks to look for.

"Lacustrine and coal source rocks are important where they are present," he said, "but the world 's petroleum source rocks are dominated by Type II source rocks."

➤ Source rocks are found throughout the sedimentary rock record since Pre-Cambrian time, but Mesozoic source rocks, particularly Jurassic to Cretaceous, are the most important volumetrically.

"When you look at petroleum volumes associated with Mesozoic source rocks, especially the Jurassic and Cretaceous, they are at least four times larger than Paleozoic source rocks and about five times larger than Cenozoic source rocks," Ahlbrandt said. "When we actually looked at volumes associated with these source rocks it was startling to us to see how dominant these Mesozoic source rocks really were."

➤ Young Cenozoic petroleum systems are volumetrically dominant, and much petroleum has clearly been lost from older petroleum systems.

Based on this research, the ideal petroleum system has Jurassic or Cretaceous source rocks that reach the window of maturity in the Cenozoic, Ahlbrandt said.

"These are relatively young rocks in the rock record, so with the dominance of these very young petroleum systems it becomes clear that a great deal of petroleum has been generated and lost into the atmosphere or to the surface through time," he said. "Obviously, there were petroleum systems earlier in the rock record, but they were destroyed.

"The fate of petroleum is to escape to the surface, and it's apparent that has happened over the ages."

Researchers could see there are petroleum systems that are currently active and others that are in various stages of destruction.

"In many areas of the world we could identify the fields, but the kitchen may have been destroyed," Ahlbrandt continued. "The critical element for the preservation of an old petroleum system is a quality evaporitic seal, like salt. You see this phenomenon in the Triassic salts of Algeria and the Cambrian salts in the Arabian Gulf where older petroleum systems are quite prolific."

➤ Critical elements of petroleum systems are cyclic and concentrated near era boundaries.

"This statement diminishes the significance of what it represents — the concentration of good seals, source rocks and reservoirs near era boundaries suggests there is a possible mega-cyclic control on petroleum systems," Ahlbrandt said.

The rapidly changing conditions at era boundaries or significant regional unconformities account for this concentration.

"For example, often there is secondary porosity development along unconformities," he continued. "In the Middle East there is excellent porosity development in the Arab Formation, sealing salts and anhydrites above the reservoirs and adjacent source rocks near major unconformities. Times of stress and tectonic change in the geologic record were times of high productivity of source rocks and elements crucial to the formation of hydrocarbon accumulations."

➤ Despite enormous recent success in deepwater reservoirs, volumetrically they are currently the least significant of those considered; continental reservoirs were dominant.

To learn this, the USGS team first divided reservoir rocks of the world into continental, continental paralic, paralic, paralic-shallow marine, shallow marine and deep marine.

"When you look at all of those depositional environments volumetrically, on average the deep marine is the least significant," Ahlbrandt said. "The most important were continental deposits like alluvial, aeolian and lacustrian depositional settings. Almost universally people have been surprised by that fact."

Of course, Ahlbrandt said, much exploration is currently dedicated to deepwater deposits, so their importance will grow significantly in the future. Deepwater marine deposits have an advantage in that Type II kerogens are available to source the reservoirs — they are just under-developed to date.

"While deepwater marine deposits are getting the lion's share of attention today, it should be remembered there are other types of reservoirs out there that are very prolific," he said. "Continental deposits particularly should not be overlooked, because they have certainly proven their worth many times over."

➤ Future discoveries will be dominantly from clastic reservoirs.

"For the undiscovered volumes by lithology, we had about 1.1 trillion barrels for clastics, and about 300 billion (barrels) for undiscovered carbonate reservoirs," Ahlbrandt said. "The discovered volumes are a little more equitable, with carbonates being a little more than half of the discovered clastics. However, for the future, clastic reservoirs are estimated to be roughly three to four times more volumetrically significant than carbonate reservoirs."

For example, recent giant discoveries all along the South Atlantic margin offshore West Africa and the eastern coast of South America are clastic reservoirs. The large new finds in the South Caspian Sea also were found in clastics. Even the Middle East region, sediments above the Jurassic have significant clastic sequences, such as Cretaceous deltaic reservoirs at Bergan (world's second largest field).

➤ Salt is a very effective long-term seal, and salt seals are a critical preservational component of older Paleozoic petroleum systems.

"If you look at the percentage of discovered and undiscovered petroleum volumes in the Cenozoic, for example, between 20 and 30 percent of the seals are evaporites," Ahlbrandt said. "If you go to Paleozoic or Paleozoic and Mesozoic combined, between 50 and 70 percent of the significant seals are evaporites. So, it's easy to see how concentrated the importance of salts are in older rocks to preserve hydrocarbons.

"The importance of salt as a seal has been recognized," he added, "but when you start looking at relative volumes and concentration it becomes evident that salt is critical to older petroleum systems."

➤ Most of the world's petroleum systems are dominated by vertical migration or limited lateral migration of less than 20 kilometers from the mature source rock area.

"I wouldn't have made that statement when we started this study — this was a very surprising finding for us," Ahlbrandt said. "When we went through our various petroleum system assessments we expected to see significant long-range lateral migration, but it wasn't as common as we thought it would be."

About 80 percent of the hydrocarbon resources are relatively local in proximity to the source rock, the study showed. As a result, identifying the source rock is a particularly important component of the exploration process.

➤ Many major conventional natural gas systems are closely linked to large unconventional or continuous resources.

U.S. companies and organizations have pioneered much work in unconventional resources like basin-centered gas, coalbed methane and gas hydrates, but the study team attempted to identify where unconventional or continuous resources occur in the world.

Ahlbrandt said that effort led to an interesting and almost predictive observation that wherever the scientists found large conventional gas accumulations they found indicators of much larger unconventional basin-centered gas type accumulations.

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