What is a giant oil or gas field? What is its importance?
We do not find total agreement on size definitions. Even the American Association of Petroleum Geologists and Oil & Gas Journal have not always used the same definition. This column summarizes giant field status and lore.
AAPG and giant fields
AAPG Memoir 40, published in 1986 and edited by long-time geologist Michel T. Halbouty, defined a giant field as "having 500 million bbl of recoverable oil or equivalent gas," with gas converted at a ratio of 6,000 cu ft/bbl.
Halbouty's studies of giant fields date to Memoir 14 in 1968.
AAPG still uses the above cutoffs, expressed as 500 million bbl of oil and 3 tcf of gas.
Memoir 40 listed 509 fields. It contained a table that showed each field's reserves in oil, gas, and barrels of oil equivalent. It omitted any field deemed uneconomic.
Among the 509 fields were many in the then-USSR and its satellites and some in China.
Memoir 40 noted that the number of giant fields discovered per year had declined, mainly because of economic factors; however, we now know of 53% more giant fields than in 1968.
Through 2000, AAPG tallied 583 giant fields, or perhaps 0.1% of all the world's fields. The 583 included 37 oil and 40 gas giants discovered during the 1990s.
Seven of the eight giant field discoveries in 2000 were gas finds. This continued a trend toward more giant gas than oil discoveries in recent years.
Other recent trends: More giant discoveries are occurring in deep water and in stratigraphically controlled traps as a percentage of all traps.
AAPG plans to publish Memoir 74, a more recent compendium of giant field information. It could be available as early as January 2002, and work is already starting on the next project.
An OGJ contribution
OGJ's Jan. 13, 1969, issue contained the article, "The world's monster oil fields, and how they rank," by Robert J. Burke and Frank J. Gardner.
The article contained a table of 71 fields worldwide. It did not attempt to define a giant field, but the authors used the word supergiant to refer to fields of more than 10 billion bbl of estimated ultimate recovery.
The article barely mentioned gas, but it pointed out that a few gas fields are so large they will ultimately produce 1 billion bbl or more of condensate.
It also quoted an unspecified consultant: "The best reserve estimate is that one which makes buyer and seller equally angry."
The Burke and Gardner article also stated that 15% of the free world's crude (as of yearend 1968) was in two fields, Greater Burgan in Kuwait and Ghawar in Saudi Arabia, and that 47.5% of recoverable free world oil was in six fields, five in the Middle East and one in Venezuela.
Halbouty said 15 oil fields could produce 1 million b/d each 15 years ago. Today the only four are Ghawar, Kirkuk, Burgan, and Cantarell.
Other sources
Richard Nehring's June 1978 Rand Corp. report to the US Central Intelligence Agency used 500 million bbl as the minimum cutoff for a giant field and 5 billion bbl for supergiant (recoverable volumes). It did not address gas.
PennWell Corp. published the Dictionary of Petroleum Exploration, Drilling & Production, by University of Tulsa Continuing Education Prof. Norman Hyne, in 1991. Hyne's book cited 100 million bbl for a giant oil field and two cutoffs for a giant gas field: either 600 bcf or 1 tcf. It also gave the following definition of a giant field: "A field that has more than 500 million bbl of recoverable oil or 3 tcf of natural gas in the Middle East, North Africa, and Asiatic Russia." Hyne's otherwise helpful dictionary contains no definition of supergiant.
"Giant" and "supergiant" are indefinite terms, and it wouldn't be right for an OGJ editor or other author to try to dictate definitions. It would be reasonable for authors to define giant and supergiant as they use the terms.
As Ray Thomasson and Fred Meissner pointed out in the Dec. 3 and Dec. 10, 2001, issues of OGJ, some giant fields are discovered, then rediscovered. Some are rediscovered more than once as economics and technologies evolve.
