www.aapg.org/explorer/2007/03mar/giant_fields.cfm -
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Published on: 1/1/2007
Last Visited: 3/6/2007
Maps courtesy of Paul Mann
Paul Mann will present the paper "Emerging Trends form 69 Giant Oil and Gas Fields Discovered from 2000-2006" at 1:20 p.m. Monday, April 2, at the AAPG Annual Convention in Long Beach, Calif.
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"It's an interesting (giant fields) cluster area because this is the only cluster on earth that is derived from upper Precambrian source rocks," said Paul Mann of the Institute of Geophysics at the University of Texas-Austin.
"This is an old rift-passive margin setting now in an interior cratonic area -- most don't make it this far given the deformation events that get superimposed on top of the margin throughout its history and its amalgamation into the craton," he noted.
Mann and his colleagues who map giant fields expect about 33 giants to be discovered worldwide in the period 2007-09.
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Map courtesy of Paul Mann
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Paul Mann will present a project update at the AAPG Annual Convention in Long Beach, Calif., in the session "Emerging Trends from 69 Giant Oil and Gas Fields Discovered from 2000-2006."
Mann is senior research scientist for the University of Texas Institute of Geophysics (UTIG) in Austin.
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After analyzing giant fields discovered up to 2000, Mann and his colleagues predicted that new giant discoveries for 2000-09 would occur primarily in passive margin and rift environments, especially in deepwater basinal settings.
Take a closer look Asia Oceania.
Map courtesy of Paul Mann
They also projected the addition of giant fields in known areas, including hydrocarbon provinces of the Persian Gulf, West Siberia and Southeast Asia.
So far, those predictions have been spot on.
Mann said the giant fields project dates back to a series of World Oil articles published in 2001.Another descriptive paper appeared in AAPG Memoir 78, Giant Oil and Gas Fields of the Decade 1990-1999.
That monograph was edited by the late Michel Halbouty, who had prepared previous volumes tracking decadal giant discoveries starting in 1960.
"What I found lacking in the previous giants literature was a global and tectonic scale of observation," Mann said.
"The term ,giant oilfield' gets bandied around a lot.My interest was to remove their mystique by taking the locations of all giant oil and gas fields and plotting them on geologic maps," he added.
That required (a) valid information on known giant fields, and (b) an adequate and reliable geologic world base map.
Information on existing giants came from Petroconsultants, later part of IHS.For the base map, Mann chose Exxon's world geologic map compiled in the 1980s, "since it's a global map with a standard color and symbol scheme," he noted.
"We are merging state-of-the-art production and discovery information with the latest thinking on basin-forming mechanisms," he said.
As he plotted the known giants, many showed up in proximity in "clusters" of large field groups.
"The advantage of this approach was that it took a global look at these 23 cluster areas," Mann said."We also classified the types of basins in which these fields occur."
That also proved challenging.Mann re-examined previous basin classifications devised since the 1960s and ,70s, and updated basin nomenclature to be more in line with current thinking on modern plate tectonics and subsurface geology.
As a result, patterns began to emerge.
"We found out that there were two basin types, which account for 60-plus percent of all the giants that exist," he said.
Generally speaking, those are basins with rift or passive margin settings.
"The reason is that these passive margin and rift environments are very stable.If you think of a collisional or a strike-slip environment, it's an unstable and unpredictable tectonic environment for a large oil or gas reservoir to survive," Mann said.
"The preservation potential of giants is greatly enhanced in passive margin and rift environments, especially when they are distant from active plate boundaries," he observed.
An example is the Gulf of Mexico, he said, "which was largely bypassed by the lithospheric-scale tectonic deformation associated with its unstable neighbor to the south, the Caribbean plate."
,An Exciting Database'
Mann's research identified 863 giant fields discovered through 1999.Since then, another 33 new oil giants and 36 new gas giants have been found.
"It's an exciting database that allows us to sort on various properties of the giants, including tectonic setting, reservoir type, source rock, discovery year and many other variables," Mann said.
"We can also place the giants onto plate tectonic reconstructions produced by UTIG's ,PLATES' project and track them through time and space -- for example, all giants that share the same late-Jurassic source rock," he noted.
Mann said plots of discovery year show a century-long march of exploration from North America and the Persian Gulf to Asia and the Southern Hemisphere.
"The oldest giant in our list is from 1868," he said.
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The sag-rift combination appears especially promising for giants, Mann noted.
"We classify them as rift basins because they look to be sag basins overlying older rift basins.It works well because the sag basin either acts as a trap or seal to keep oil in the rift basin, or it can be the reservoir," he said.
Another example of rift-like settings are failed rifts, where separation failed to occur -- in the North Sea or Australia's Bass Strait, for example.
"We also use the term second-chance giants.Faulting may breach a lower reservoir but the upwardly migrating oil is trapped in an overlying reservoir," Mann said."An example is migration out of the Eocene of Lake Maracaibo and into the Miocene in the Bolivar coastal fields.
"The more stacking and redundancy of reservoirs, the more opportunities for second-chance giants," he added.
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"Occasionally you also find what we call lock-box giants, like the Permian Basin, that got sealed over by evaporites and left alone in a cratonic area as plate boundaries moved away," Mann said.
Those exceptional giants are more likely to be encountered in cratonic areas like central Russia, he added.
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"The longer it sits around, the more that can happen to it, in a negative sense," Mann observed.
Conversely, some areas may become more promising than they first appear.
"There are examples where you had relatively poor onshore deposits and very rich offshore.I think the eastern margin of India is an example of that trend," Mann said.
"Unlike the Gulf of Mexico, where there has been a steady march from onland to shelf to slope to deepwater, eastern India has gone directly to deepwater," he observed."Now other oil-poor countries with deepwater areas are eager to prove their potential."
The rate of giant field discoveries peaked in the 1960s and ,70s, then began a fairly steady decline.Mann said the significant number of new giants found in the past five years is an exception.
"During this period we've had a reversal of the trend.It's exciting to see that the 2000-06 giant discoveries show a small upward spike in the overall discovery curve that has been in decline since the 1970s," he said.
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One positive sign comes from new giants found in eastern China, India and Myanmar, which could lead to more success, Mann said.
"Just in the past few years they've made some giant field discoveries, so these could be new clusters emerging," he explained.
Mann and his colleagues project the discovery of another 33 new giant fields during the rest of the decade.That would make 2000-09 the third-highest discovery decade in history.
But the overall decline in the discovery rate for giant fields continues, he noted.And few large frontier areas remain for giant-sized elephant hunting -- Antarctica and the Arctic, Mann suggested.
"Overall, the globe is shrinking under this latest wave of exploration, so we may be reaching the end of the rope as to where the new clusters may be," he said.
For the industry, elephant-hunting involves a choice between established cluster areas of large fields and promising new areas not fully developed.
"You don't want to give up finding new giants, but you also don't want to give up exploring these known clusters," Mann said.
"The real frontier seems to be the continental margins," he added.
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"The unfortunate truth is that oil and gas is unevenly distributed over the Earth's surface," Mann said.
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