EXPLORATION

Tectonic setting of the world’s giant oil fields

Part 2 – 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 second of a three-part series, seven more areas where giant fields are densely clustered are presented

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. The authors discuss how tectonics produced regional clusters of giant fields 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 Seven of these regions appear in this article; the remaining six will be discussed in Part 3.

Tectonic Settings

The following descriptions summarize the authors’ rationale for their classification of seven of the 20 areas where giants are concentrated.

North Sea. The Central Graben is host to 30 giants, Fig. 1. It is a failed rift extending into the Eurasian continental crust. Giants are localized along the complex normal and strike-slip faults running down the graben axis. The earliest rift phase occurred during the Carboniferous and Jurassic, with the rift-system trend closely controlled by pre-existing basement trends. Source rocks were deposited in the initial rift during the Late Paleozoic and Kimmeridgian, with reservoirs at several levels. An overlying steer’s head sag basin was deposited over the Central Graben in Late Cretaceous time. Structures were mainly formed during Jurassic rifting, Tertiary magmatism and fault inversion related to the Alpine collision.

Fig. 1. North Sea region. Click for enlarged view

North Africa. These 26 giants can be subdivided into two regions (Fig. 2): 1) to the west, the Illizi province of Algeria; and 2) to the east, the giants of Libya’s Sirte rift. Tectonic history of North Africa is marked by convergence during the Paleozoic Hercynian orogeny, which left a major unconformity separating folded Cambro-Ordovician rocks from unfolded Permian-Triassic clastic sedimentary and volcanic rocks. During the Pangean breakup, rifts formed across northern Africa, including the Atlas rift system. Following rifting, the area subsided and received a thick section of evaporitic and clastic sediments.

During the Late Cretaceous, convergence began between Africa and Eurasia and resulted in the Alpine mountain chains in northern Africa, including inversion of the Atlas rift to form the Atlas mountain belt. Giant fields in structural traps occur beneath the Hercynian unconformity and are sourced by Ordovician and Silurian black shales.

The Sirte basin is a rift basin with a complex extensional history that began in the late Cretaceous and extended into the Tertiary.⁵ Source rocks are Late Cretaceous shales that thicken into the rift basins. Reservoirs comprise reef buildups on structural highs. Traps are combinations of structural and stratigraphic traps. These giants were classified as continental rifts with overlying steer’s head basins.

Fig. 2. North Africa region.

Caspian Sea. In this region (Fig. 3), formation of giants was initially controlled by a Jurassic rifting event at the northern Tethys margin. Later source rocks deposited in this rift framework are Paleocene-Eocene. The area’s 26 giant reservoirs are at various levels within the Cenozoic section, and structural traps formed from the Cenozoic to present-day.⁸

Fig. 3. Caspian Sea region.

Arabian Peninsula / Persian Gulf. There are 151 giants in this region, Fig. 4. They are concentrated in a large foreland basin formed during the Late Cenozoic collision of the Arabian Peninsula with Eurasia. Downward flexure of the Arabian Peninsula beneath the Zagros Mountains of Iran / Iraq was caused by the northeastward consumption of the Tethys Ocean at the Zagros suture zone. Additional causes of this flexure were the eventual Cretaceous-recent convergence and collision of the Arabian plate against the Eurasian plate. This protracted convergent event has created the Persian Gulf and Mesopotanian lowlands as a sag in the foreland basin, as well as formation of the Zagros Mountains, with a culmination of fold-thrust deformation in Miocene and Pliocene time.

However, other than minor tilting, large areas of the foreland appear completely undisturbed by Zagros-related convergent deformation, as manifested in the variety of giant-field shapes. For that reason, formation of elongate giants parallel to folds and thrusts in the Zagros Mountain and foreland basin was classified as a continental collision margin, while those giants to the southwest were counted as continental rifts and overlying steer’s head sag basins.

The basal stratigraphic section underlying the present-day foreland basin was deposited along a Cambrian-Permian passive-margin setting along the southern Tethys margin. Deeply buried salt, possibly deposited in Cambrian rifts, was activated by small-displacement basement faults during Permian to Jurassic time. These gave rise to salt ridges and diapirs, forcing folds in the overlying sedimentary section, which include some of the largest giant fields, such as Ghawar, Saudi Arabia. These folds are at a high angle to later folds and thrusts related to the Zagros convergence. Source rocks in this basin phase include Cambrian-to-Permian units, with the main reservoir in the Permian.

A second hydrocarbon-formation period occurred from the Triassic through Tertiary, with Middle Jurassic source rocks and Upper Jurassic reservoirs. Migration is primarily upward from underlying source rocks in giant fields that are removed from the Zagros deformation.⁷ Structural traps formed in the area adjacent to the Zagros foldbelt and relate to early collisional effects in Eocene and younger time.

Fig. 4. Arabian Peninsula and Persian Gulf region. Click for enlarged view

West Africa. These 20 giants (Fig. 5) occur along the rifted margin formed by the opening of the South Atlantic Ocean. Rift history comprised a Neocomian-to-Aptian period of continental rifting, with lacustrine or brackish sediments infilling half-grabens. This was followed by an Albian-to-recent passive-margin phase, which was dominated by landward-derived, prograded clastic-carbonate platforms, locally deformed by underlying salt deposits.⁶

The two stages are separated by formation of a large, Aptian salt deposit along most of the West African margin. The salt deposit derived from evaporation of a standing body of water formed by separation of the continents. This immense salt deposit formed an important seal for hydrocarbons derived from the pre-rift section, as well as having created structural traps in the overlying passive-margin section. Classification of these giants is difficult, since source rocks mainly occur in pre-salt lacustrine units, while reservoirs occur mainly in carbonate rocks of the post-salt, marine passive-margin section. The authors classified these giants as forming on a continental passive margin fronting a major ocean basin.

Fig. 5. West Africa region. Click for enlarged view

Black Sea. There are 10 giant fields in this region, Fig 6. The Black Sea is a composite basin formed by rifting in the Aptian (western basin) and Paleocene-Eocene (eastern basin) along the northern edge of Tethys. Source rocks range in age from Paleozoic through Cenozoic, with dominantly Eocene reservoirs. Structures formed during the closure of Tethys and include the inverted Dneiper-Donetsk rift to the north of the Black Sea.

Fig. 6. Black Sea region.

Ural Mountains. This region (Fig. 7.) formed the rifted eastern margin of Baltica during Ordovician-to-Permian time, with grabens forming major Paleozoic depocenters in the basin. A foreland basin was superimposed on this margin during collision of the Ural arc during the Late Permian to Early Jurassic. Structural trap formation occurred during this orogeny and created the area’s 23 giants, which resulted in folds forming as far as 300 mi west of the Uralian deformation front. Source rocks are traceable Devonian shales deposited during the early graben phase prior to collision.⁹

Fig. 7. Ural Mountains region. Click for enlarged view

Next Month

In Part 3, the six remaining areas where giants are clustered will be discussed. These are: West Siberia; Siberia; China; Sundra; Western Australia; and Bass Strait / Australia / Tasmania.

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 Lewis, C., "Sarir field [Sirte basin, Libya]," in Structural Traps II, compiled by N. Foster and E. Beaumont, AAPG Treatise of Petroleum Geology, Atlas of Oil and Gas Fields, AAPG, Tulsa, Oklahoma, pp. 253 – 267, 1990.

⁶ Bandouy, S. and C. Legorjus, "Sendji field – People’s Republic of Congo, Congo basin," in Structural Traps V, compiled by N. Foster and E. Beaumont, AAPG Treatise of Petroleum Geology, Atlas of Oil and Gas Fields, AAPG, Tulsa, Oklahoma, pp. 121-149, 1991.

⁷ Pierce, W., "Southern Arabian basin oil habitat: Seals and gathering areas," Society of Petroleum Engineers, SPE25606, Bahrain Oil Show, pp. 103 – 111, 1993.

⁸ Hall, S. and V. Sturrock, "Tectonic control on the creation of supergiant fields in the central and south Caspian area," Houston Geological Society Bulletin, Vol. 43, pp. 13 – 17, 2001.

⁹ Tull, S., "The diversity of hydrocarbon habitat in Russia," Petroleum Geoscience, Vol. 3, pp. 315 – 325, 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 holds 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 holds 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 holds a PhD in geological sciences from the University of Texas at Austin.

Part 1

Part 3