EXPLORATION

Tectonic setting of the world’s giant oil fields

Part 3 – A new classification scheme of the world’s giant fields reveals the regional geology where explorationists may be most likely to find future giants. In this, the last of a three-part series, six areas where giant fields are clustered are examined

Paul Mann and Lisa Gahagan, Institute for Geophysics, University of Texas at Austin; and Mark B. Gordon, GX Technology

lthough there are large variations in reserve estimates, giant fields contain at least 65% of the world’s proven reserves.¹ Therefore, knowledge of their tectonic setting, geologic history and conditions for hydrocarbon formation will contribute greatly to understanding the origin and future supply of the world’s hydrocarbons. Six areas ranging from Siberia to the Asia-Pacific where giant fields are densely clustered are presented. The authors discuss how the tectonics of these particular areas produced these clusters and their rationale for classification.

Introduction

A "giant" oil field is defined as one containing proved reserves exceeding 500 million bbl; a giant gas field contains proved reserves of greater than 3 Tcf.² "Reserves" refer to the ultimate recoverable amount and include the amount produced to date. Some fields are giants only when viewed on a boe basis.³

North American giant fields (without field outlines) were digitized from Carmalt and St. John’s published compilation.³ Field locations and outlines are derived from the Petroconsultants SA, digital database for non-North American giant fields (used with permission). The 592 giant fields of the world cluster in 20 regions (see Part 1). Detailed geologic maps of these regions are modified from the Exxon Tectonic Map of the World.⁴

Tectonic Settings

The following descriptions summarize the authors’ rationale for their classification of six areas where giants are concentrated.

Sunda. The 26 giant fields in Sunda occur in three main areas, Fig. 1. These are: 1) central and northern Sumatra, 2) Brunei on the western margin of Borneo, and 3) the Pattani trough, offshore Thailand. In Sumatra, inverted Late Neogene rift structures on continental collision are present in a backarc setting and were classified as a continental collision zone. The Borneo area was classified as an arc-continent collision zone, and the Pattani trough as a pull-apart basin on a strike-slip fault.

Fig. 1. Sunda. Click for enlarged view

West Siberian basin. The area’s 66 giants (Fig. 2) formed in a northward-plunging Jurassic-Quaternary sag basin overlying a Permo-Triassic rift system. Such rifts are aborted to form isolated intracontinental rifts that are surrounded by continental areas. Reservoirs lie in Cretaceous sag fill. Source rocks include both Cretaceous and Upper Jurassic shales.⁵

Fig. 2. West Siberian basin. Click for enlarged view

Siberia. Seven giant fields have been discovered in Precambrian-Cambrian sedimentary rocks in this underexplored region of the Siberian platform, Fig. 3.⁵ Reservoirs and sources comprise late Precambian-Cambrian clastic rocks and carbonate rocks, with seals provided by Cambrian evaporites. A rift setting for these fields was inferred.

Fig. 3. Siberia.

Northwest Australia. This margin rifted during the Middle Jurassic. The region’s five giant fields are mainly found in the overlying Upper Jurassic-to-recent passive margin section, which drapes earlier rift structures, Fig. 4. For this reason, the tectonic setting was classified as a passive margin fronting a major ocean basin.

Fig. 4. Northwest Australia.

Bass Strait, Australia / Tasmania. The three giant fields in the Bass Strait area are underlain by the Gippsland rift basin (Fig. 5), which is the most prolific oil and gas province in Australia. Geologic development of Gippsland basin is marked by a protracted and multistage Early to Late Cretaceous rift history, including separation of Australia and Antarctica and the opening of oceans east and west of Australia. Sources and reservoirs occur in the late to post-rift sequence.⁶ Despite location of the giants above a failed east-west rift that extends westward from the Tasman Sea, this setting was classified as a continental passive margin fronting a major ocean basin.

Fig. 5. Bass Strait, Australia / Tasmania.

China. The 10 giant fields in eastern China occur mainly in Bohai basin (Fig. 6), one of a family of early Cenozoic extensional basins that lie along the eastern margin of Asia from Russia to Vietnam.⁷ Paleocene to early Eocene rifting was diffuse, trans-tensional, and related to rollback of the subducted Pacific plate beneath the Asian continent, while Middle Eocene rifting appears to have been more organized in a large, right-stepping basin that formed as a very large pull-apart basin on right-lateral strike-slip faults. Narrower trans-tension zones are present north and south of the basin. Earthquakes indicate that right-lateral strike-slip faulting continues to present-day. For this reason, this tectonic setting was classified as strike-slip. Reservoirs include carbonate karst units that are sourced by Paleogene shales.

Fig. 6. China

Literature Cited

¹ Klett, T., and J. Schmoker, "Changes in observed field-size estimates of the world’s giant oil fields," Abstracts, p. A106, AAPG Annual Convention, Denver, Colorado, June 3 – 6, 2001.

² Halbouty, M., "Giant oil and gas fields of the decade 1990-2000," Online published version http://www.searchanddiscovery.com /documents. AAPG Annual Convention, Denver, Colorado, June 3 – 6, 2001.

³ Carmalt, S. W., and B. St. John, "Giant oil and gas fields," in M.T. Halbouty, ed., Future petroleum Provinces of the World, Memoir 40, AAPG, Tulsa, Oklahoma, 1986.

⁴ Exxon Tectonic Map of the World, World Mapping Project, Exxon Production Research Company, Houston, Texas, 1985.

⁵ Tull, S., "The diversity of hydrocarbon habitat in Russia," Petroleum Geoscience, Vol. 3, pp. 315 – 325, 1997.

⁶ Glenton, P., "Snapper field, Australia: Offshore Gippsland basin, southeast Australia," in Structural Traps V, compiled by N. Foster and E. Beaumont, AAPG Treatise of Petroleum Geology, Atlas of Oil and Gas Fields, AAPG, Tulsa, OK, pp. 227 – 250, 1991.

⁷ Allen, M., et al., "Early Cenozoic two-phase extension and Late Cenozoic thermal subsidence and inversion of the Bohai basin, northern China," Marine and Petroleum Geology, Vol. 14, pp. 951 – -972, 1997.

The authors
	Paul Mann has worked at the University of Texas Institute for Geophysics since 1983 and is currently a senior research scientist there. He earned a PhD in geological sciences from the State University of New York at Albany. He has held visiting professorships in New Zealand and France. Mann has done geological / geophysical fieldwork throughout the Caribbean region, as well as participated on several research cruises offshore New Zealand and in the Western Pacific. He invites comments at: paulm@ig.utexas.edu.
	Lisa Gahagan has been project manager for PLATES, a global plate tectonic research project at the Institute for Geophysics of the University of Texas at Austin, since its inception in 1991. She earned a master’s in geological sciences from the University of Texas at Austin.
	Mark B. Gordon has done geological research in Central America, Cuba, France and Saudi Arabia as a post-doctoral scientist at Rice University and the University of Paris. He has been working at GX Technology since 1997 doing pre-stack depth seismic processing on basins throughout North America. He earned a PhD in geological sciences from the University of Texas at Austin.

Part 1

Part 2