Giant Oil and Gas Fields
The world's 910 giant oil and gas fields are those with 500 million bbl of ultimately recoverable oil or gas equivalent. Remarkably, almost all of these 910 giant fields - which by some estimates account for 50% of the world's petroleum reserves - cluster within 27 regions, or about 30%, of the Earth's land surface. In the study published by Mann et al. (2004) in AAPG Memoir 78, we have compiled maps showing the location of all 910 giants located on tectonic and sedimentary basin maps of these 27 key regions. Our giants database for pre-1990 giant fields outside of North America was kindly provided by Petroconsultants (now part of IHS Energy Group). M.K. Horn has updated the giant field database through 2004 and kindly made it available in the public domain:

Horn, M.K., 2003, Giant fields 1868-2003 (CD-ROM), in Halbouty, M.K., ed., Giant oil and gas fields of the decade 1990-1999: AAPG Memoir 78, 340p.

Horn, M.K., 2004, Giant fields 1868-2004 (CD-ROM): AAPG/Datapages Miscellaneous Data Series, version 1.2 (revision of Horn, 2003).

In the Mann et al. study, we classify the tectonic setting of the giants within these regions using six simplified classes of the tectonic setting for basins within these regions: 1) continental passive margins fronting major ocean basins (304 giants); 2) continental rifts and overlying sag or "steer's head" basins (271 giants); 3) collisional margins produced by terminal collision between two continents (173 giants); 4) collisional margins produced by continental collision related to terrane accretion, arc collision, and/or shallow subduction (71 giants); 5) strike-slip margins (50 giants); and 6) subduction margins not affected by major arc or continental collisions (8 giants). For giant fields with multi-phase histories, we attempt the difficult task of discriminating the single tectonic event/setting we consider to have the most profound effect on hydrocarbon formation, migration, and trapping. Our main classification criterion is the basin style dominating at the most typical stratigraphic and structural level of giant accumulations.

Continental passive margins fronting major ocean basins form the dominant tectonic setting that includes 35% of the world's giant fields. Continental rifts and overlying sag basins, especially failed rifts at the edges or interiors of continents, form the second most common tectonic setting that includes 31% of the world's giant fields. Terminal collision belts between two continents and associated foreland basins form the third setting with 20% of the the world's giant fields. Other setting classes including foreland basins at collision margins related to terrane accretion, arc collision, and/or shallow subduction; basins within strike-slip margins; and basins within subduction margins are relatively insignificant with 14% or less of the total basin population. Our tabulation indicates the importance of extensional settings formed during the early and late stages of oceanic opening for giant accumulations: the rift and passive categories combined account for almost two-thirds, or 66%, of all 910 giants. Our result differs significantly from previously published giant classifications where collisional settings form the dominant tectonic setting for oil giants.

We propose the following possibilities to explain the dominance of extensional rift and passive margin settings over all other tectonic settings: 1) localization of high quality source rocks in lacustrine and restricted marine settings during the early rift stage; 2) effectiveness of the sag or passive margin section above rifts to either act as reservoirs for hydrocarbons generated in the rift section and/or to seal hydrocarbons generated in the underlying rift section; 3) tectonic stability following early rifting that allows hydrocarbon sources and reservoirs to remain undisturbed by subsequent tectonic events acting on distant plate boundaries.

Trends in the discovery of giants in the period from 1990 to 2000 that we consider likely to continue into the 21st century include: 1) the discovery of fields in deepwater basinal settings along passive margins such as Brazil, West Africa, and the Gulf of Mexico associated with nodes of high quality source rocks areas and stratigraphic traps located using 3D seismic reflection data; 2) continued discoveries of giants in known areas including: expansion of the Persian Gulf hydrocarbon province to south into Yemen and the Arabian Peninsula and north into Iraq; expansion of the West Siberian basin in the Arctic offshore area; and radial expansion of the Illizi basin of Algeria; 3) continued discoveries in southeast Asia where Cenozoic rift, passive margin and strike-slip environments all coexist around the South China Sea or within the largely submerged Sunda continent; 4) along-strike expansion of elongate foreland trends in the Rocky Mountains, northern South America, southern Andes, Ural-Timan-Pechora and Barents Sea, and North Slope; and 5) expansion of discoveries in the Black Sea-Caspian region associated with closure and burial of northern Tethyal passive margin or arc-related basins.

Despite the association of giant fields with Cenozoic or Mesozoic plate edges (especially failed rifts trending at high angles to continental margins), the possibility always exists for further discovery of "lockbox-type" giants associated with now cratonic interior - but previous Paleozoic or Precambrian plate edges - as exemplified by known Paleozoic and Precambrian hydrocarbon giant clusters in the Permian basin in the USA, the Illizi basin of Algeria, and the Siberian Platform.

More recent work by M.K. Horn has utilized the giant fields database to compare selected features of giant fields including: 1) their discovery years; 2) remaining reserves, 3) reservoir lithologies; 4) trap type; 5) geologic age of reservoirs; and 6) depth of the primary producing reservoirs. For his online AAPG Search and Discovery article, see:

http://www.searchanddiscovery.com/documents/2004/horn/index.htm

Presentations related to this study:
Mann, P., Horn, M., and Cross, I., 2007, Emerging Trends from 69 Giant Oil and Gas Fields Discovered from 2000-2006. Presented at AAPG's 2007 Annual Meeting in Long Beach, CA, on Monday afternoon, 1:20 p.m. (Room Grand Ballroom - A). This presentation is posted on AAPG's Search and Discovery at
http://www.conferencearchives.com/aapg2007/sessions/player.html?sid=07041210.

Publications related to this study:
Mann, P., Gahagan, L., and Gordon, M., 2001, Classification of the tectonic setting of the world's giant oil fields, AAPG Annual meeting, June 3-6, Denver, Colorado.

Mann, P., Gahagan, L., and Gordon, M., 2001, Tectonic setting of the world's giant oil fields, World Oil, published in three installments: September issue (p. 42-50), October issue (p. 78-84); November issue (p. 56-60).
Installment one on World Oil website
Installment two
Installment three

Mann, P., Gahagan, L., and Gordon, M.B., 2004, Tectonic setting of the world’s giant oil and gas fields, in M. T. Halbouty, ed., Giant oil and gas fields of the decade 1990-1999, AAPG Memoir 78, p. 15-105.

Mann, P., Horn, M., and Cross, I., 2006, Tectonic Setting of 31 Giant Oil and Gas Fields Discovered from 2000-2005: Implications for Future Discovery Trends, AAPG annual meeting, Houston, Texas, April 9-12.