Changes

==Paleogeography and petroleum plays==

==Paleogeography and petroleum plays==

The paleogeographic and tectonic evolution of the southern Tethys area during the Phanerozoic plays an important role in determining the distribution of the source rocks and reservoirs as well as the origin of stratigraphic and tectonic traps, both strictly related to the geodynamic evolution of the area. This recognition of these controls on hydrocarbon resources and accumulation of oil and gas along the Tethyn margin has been the major thrust of the publications of many geologists, including Murris, 1980; Beydoun, 1986; Beydoun, 1988; Beydoun, 1991; May, 1991; and Sharland et al., 2001.

The paleogeographic and tectonic evolution of the southern Tethys area during the Phanerozoic plays an important role in determining the distribution of the source rocks and reservoirs as well as the origin of stratigraphic and tectonic traps, both strictly related to the geodynamic evolution of the area. This recognition of these controls on hydrocarbon resources and accumulation of oil and gas along the Tethyn margin has been the major thrust of the publications of many geologists, including Murris,<ref name=Murris_1980>Murris, R. J., 1980, Middle East: Stratigraphic evolution and oil habitat: AAPG Bulletin, v. 64, no. 5, p. 597–618.</ref> Beydoun,<ref name=Beydoun_1986>Beydoun, Z. R., 1986, The petroleum resources of the Middle East: A review: Journal of Petroleum Geology, v. 9, p. 5–29.</ref> <ref name=Beydoun_1988>Beydoun, Z. R., 1988, The Middle East: Regional geology and petroleum resources: Scientific Press, Beaconsfield, U.K., 292 p.</ref> <ref name=Beydoun_1991>Beydoun, Z. R., 1991, Arabian plate hydrocarbon geology and potential — a plate tectonic approach: AAPG v. 33, p. 77.</ref> May,<ref name=May_1991>May, P. R., 1991, The Eastern Mediterranean Mesozoic Basin: Evolution and oil habitat: AAPG Bulletin, v. 75, no. 7, p. 1215–1232.</ref> and Sharland et al.<ref name=Sharlandetal_2001>Sharland, P. R., Archer, R., Casey, D. M., Davies, R. B., Hall, S. H., Heward, A. P., Horbury, A. D., and Simmons, M. D., 2001, Arabian plate sequence stratigraphy: GeoArabia, Special Publication 2, 371 pp.</ref>

The giant fields of North Africa, Arabia, and the Middle East reflect episodes of enhanced primary productivity with high export production and storage of this organic matter in the sedimentary successions of different types of sedimentary basins.

The giant fields of North Africa, Arabia, and the Middle East reflect episodes of enhanced primary productivity with high export production and storage of this organic matter in the sedimentary successions of different types of sedimentary basins.

The factors that control primary productivity are light intensity, nutrient inputs (nitrate and phosphate), and climate (Begon et al., 1996). Today maximum productivity in the oceans is recorded on the inner shelf of the continental platforms and in ocean upwellings because of high nutrient concentration and relatively clear water (Haines, 1979; Barnes and Hughes, 1982). The paleogeographic configurations of the late Paleozoic-Mesozoic time interval is dominated by E-W oceans, particularly in North Africa, Arabia, and the Middle East, where they extended mainly from the equator to the tropics; indeed, for most of this interval the Tethyan Seaway was present at very low latitudes north of Africa and Arabia, indenting the Pangea supercontinent. Paleocurrent models for a general Pangea configuration (e.g., Kutzbach et al., 1990; Kiessling et al., 1999; Winguth et al., 2002, 2005) envisage a westward-flowing equatorial surface current which, upon reaching the continental shelves of the western Tethys Seaway, deflected southeastward and northeastward; in the meanwhile, a deep water circulation brought cold waters from high latitudes to the equator. Ocean upwellings of these cold and nutrient-rich bottom waters were created by monsoonal wind circulation (Crowley et al., 1989; Parrish, 1993; Peyser and Poulsen, 2008) along the Gondwanan margin and in the lee of continental blocks scattered in the Paleo- and Neo-Tethys, as well as at the equatorial divergence zone.

The factors that control primary productivity are light intensity, nutrient inputs (nitrate and phosphate), and climate.<ref name=Begonetal_1996>Begon, M., Harper, J. L., and Townsend, C. R., 1996, Ecology. Individuals, Populations and communities: Blackwell Scientific Publications, 1088 p.</ref> Today maximum productivity in the oceans is recorded on the inner shelf of the continental platforms and in ocean upwellings because of high nutrient concentration and relatively clear water.<ref name=Barnesandhughes_1982>Barnes, R. S. K., and Hughes, R. N., 1982, An introduction to marine ecology: Blackwell Scientific Publications.</ref> The paleogeographic configurations of the late Paleozoic-Mesozoic time interval is dominated by E-W oceans, particularly in North Africa, Arabia, and the Middle East, where they extended mainly from the equator to the tropics; indeed, for most of this interval the Tethyan Seaway was present at very low latitudes north of Africa and Arabia, indenting the Pangea supercontinent. Paleocurrent models for a general Pangea configuration (e.g., Kutzbach et al., 1990; Kiessling et al., 1999; Winguth et al., 2002, 2005) envisage a westward-flowing equatorial surface current which, upon reaching the continental shelves of the western Tethys Seaway, deflected southeastward and northeastward; in the meanwhile, a deep water circulation brought cold waters from high latitudes to the equator. Ocean upwellings of these cold and nutrient-rich bottom waters were created by monsoonal wind circulation<ref name=Crowleyetal_1989>Crowley T. J., Hyde, W. T., and Short, D. A., 1989, Seasonal cycle variation on the supercontinent of Pangaea: Geology, v. 17, p. 457–460.</ref> <ref name=Parish_1993>Parrish, J. T., 1993, Climate of the supercontinent Pangaea: Journal of Geology, v. 101, p. 215–233.</ref> <ref name=Peyserandpoulsen_2008>Peyser, C. E., and Poulsen, C. J. 2008, Controls on Permo-Carboniferous precipitation over tropical Pangaea: A GCM sensitivity study: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 268, p. 181–192.</ref> along the Gondwanan margin and in the lee of continental blocks scattered in the Paleo- and Neo-Tethys, as well as at the equatorial divergence zone.

The combination of extended continental platforms in the proximity of a nutrient-delivering supercontinent and developed ocean upwellings caused the increase of primary productivity during favorable climate conditions, particularly at low latitudes where light intensity was higher and rate of mineralization (hence greater nutrient supply) more rapid. Storage of increased production as organic matter in the sediments was in turn enhanced by high sedimentation rates and availability of accommodation space.

The combination of extended continental platforms in the proximity of a nutrient-delivering supercontinent and developed ocean upwellings caused the increase of primary productivity during favorable climate conditions, particularly at low latitudes where light intensity was higher and rate of mineralization (hence greater nutrient supply) more rapid. Storage of increased production as organic matter in the sediments was in turn enhanced by high sedimentation rates and availability of accommodation space.

The latitude and relative position of the Pangea and the Tethys therefore favored the deposition of source rocks, whereas the continuous and time-transgressive generation and evolution of different sedimentary basins controlled the creation of reservoirs and traps (such as those related to tectonic inversion of extensional structures), leading to the impressive concentration of oil and gas fields in this area.

The latitude and relative position of the Pangea and the Tethys therefore favored the deposition of source rocks, whereas the continuous and time-transgressive generation and evolution of different sedimentary basins controlled the creation of reservoirs and traps (such as those related to tectonic inversion of extensional structures), leading to the impressive concentration of oil and gas fields in this area.

The distribution of giant oil fields is related to the nature of the sedimentary basins. According to Mann et al. (2003), most of the giant oil and gas fields known until 2000 are related to continental passive margins facing the major ocean basins (34.66%), continental rifts and overlying sag basins (especially failed rifts at the edges or interiors of continents; 30.90%), and collisional margins produced by terminal collision between two continents (19.73%). These types of basins are common in the succession of North Africa and the Middle East. Due to the geodynamic evolution of this area, rift basins (mainly formed due to the opening of the Tethys oceans and to the extensional events affecting North Africa) rapidly evolved to passive margins (e.g., evolution of the peri-Gondwanan blocks) and then to active margins, with the development of collision-related basins (e.g., foredeep related to the accretion of the peri-Gondwanan blocks to the southern margin of Eurasia).

The distribution of giant oil fields is related to the nature of the sedimentary basins. According to Mann et al.,<ref name=Mannetal_2003>Mann, P., Gahagan, L, and Gordon, M. B., 2003, 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, 15-105.</ref> most of the giant oil and gas fields known until 2000 are related to continental passive margins facing the major ocean basins (34.66%), continental rifts and overlying sag basins (especially failed rifts at the edges or interiors of continents; 30.90%), and collisional margins produced by terminal collision between two continents (19.73%). These types of basins are common in the succession of North Africa and the Middle East. Due to the geodynamic evolution of this area, rift basins (mainly formed due to the opening of the Tethys oceans and to the extensional events affecting North Africa) rapidly evolved to passive margins (e.g., evolution of the peri-Gondwanan blocks) and then to active margins, with the development of collision-related basins (e.g., foredeep related to the accretion of the peri-Gondwanan blocks to the southern margin of Eurasia).

As a consequence, different types of sedimentary basins were continuously created by the movement of continental blocks, so that at any time different basin types can be recognized (e.g., divergence on the southern side of the Tethys and convergence on the Asian margin) in North Africa and the Middle East. Therefore, favorable conditions for the development of petroleum plays were almost continually present. Though there are some differences in tectonic evolution across the margin plate scale, correlations of stratigraphy and thus petroleum systems are possible (Murris, 1980; Beydoun, 1986, 1988, 1991; May, 1991; Sharland et al., 2001).

As a consequence, different types of sedimentary basins were continuously created by the movement of continental blocks, so that at any time different basin types can be recognized (e.g., divergence on the southern side of the Tethys and convergence on the Asian margin) in North Africa and the Middle East. Therefore, favorable conditions for the development of petroleum plays were almost continually present. Though there are some differences in tectonic evolution across the margin plate scale, correlations of stratigraphy and thus petroleum systems are possible.<ref name=Murris_1980 /> <ref name=Beydoun_1986 /> <ref name=Beydoun_1988 /> <ref name=Beydoun_1991 /> <ref name=May_1991 /> <ref name=Sharlandetal_2001 />

When considering the distribution of the giant oil and gas fields, two major groups of sedimentary basins can be identified: one in North Africa mainly dominated by rift, sag, and passive margins, and one in the Middle East, where oil fields are mainly preserved in sag and passive margin and collision-related basins (Al-Husseini, 2000; Ziegler, 2001). In the latter sector, oil fields are clustered in two major sets: Eastern Arabia-Persian Gulf-Zagros and Caspian Sea.

When considering the distribution of the giant oil and gas fields, two major groups of sedimentary basins can be identified: one in North Africa mainly dominated by rift, sag, and passive margins, and one in the Middle East, where oil fields are mainly preserved in sag and passive margin and collision-related basins.<ref name=Alhusseini_2000>Al-Husseini M.I. and Moujahed, I., 2000, Late Permian to Holocene paleofacies: Evolution of the Arabian Plate and its hydrocarbon occurrences: GeoArabia, v. 5, no. 4, p. 527–542.</ref> <ref name=Ziegler_2001>Ziegler, M. A., 2001, Late Permian to Holocene paleofacies: Evolution of the Arabian plate and its hydrocarbon occurrences: GeoArabia, v. 6, no. 3, p. 445–504.</ref> In the latter sector, oil fields are clustered in two major sets: Eastern Arabia-Persian Gulf-Zagros and Caspian Sea.

North Africa experienced several stages of alternating passive margin (e.g., Devonian, Carboniferous) and rift settings, related to different geodynamic events (e.g., effects of the Tethys opening in late Paleozoic) which recurred in this area. Rift and passive margin stages are commonly separated by local or regional compressional events (e.g., Cretaceous) with reactivation/inversion of extensional structures (Boote et al., 1998). Favorable environmental conditions controlled the deposition of major source rocks, such as the Hot Shale during the Silurian. This unit reflects the environmental changes (warming and sea-level rise) that postdated the Ordovician glaciations (Figure 3). The history of repeated rifting, sag stage, and folding favored the creation of a large number of stratigraphic and tectonic traps that store, at different stratigraphic levels, several giant fields (Boote et al., 1998; Macgregor, 1998). The presence of a wide, shallow-water shelf in an arid environment (Triassic-Jurassic) led to the deposition of thick and widespread salt layers, which represent an effcient seal at the regional scale (Boote et al., 1998).

North Africa experienced several stages of alternating passive margin (e.g., Devonian, Carboniferous) and rift settings, related to different geodynamic events (e.g., effects of the Tethys opening in late Paleozoic) which recurred in this area. Rift and passive margin stages are commonly separated by local or regional compressional events (e.g., Cretaceous) with reactivation/inversion of extensional structures (Boote et al., 1998). Favorable environmental conditions controlled the deposition of major source rocks, such as the Hot Shale during the Silurian. This unit reflects the environmental changes (warming and sea-level rise) that postdated the Ordovician glaciations (Figure 3). The history of repeated rifting, sag stage, and folding favored the creation of a large number of stratigraphic and tectonic traps that store, at different stratigraphic levels, several giant fields.<ref name=Booteetal_1998>Boote, D., Clark-Lowes, D., and Traut M., 1998, Palaeozoic petroleum systems of North Africa, in D. Macgregor, R. Moody, and D. Clark-Lowes, eds., Petroleum geology of North Africa: GSL Special Publication 132, p. 7–68.</ref> <Macgregor_1998>Macgregor, D., 1998, Giant fields, petroleum systems, and exploration maturity of Algeria, in D. Macgregor, R. Moody, and D. Clark-Lowes, eds., Petroleum geology of North Africa: GSL Special Publication 132, p. 79–96.</ref> The presence of a wide, shallow-water shelf in an arid environment (Triassic-Jurassic) led to the deposition of thick and widespread salt layers, which represent an effcient seal at the regional scale.<ref name=Booteetal_1998 />

If in North Africa the basins are mainly related to rift basins followed by passive margin, the accretion of the peri-Gondwanan blocks to the southern margin of Eurasia led to the formation of a major concentration of giant fields along the northern passive margin of the peri-Gondwanan blocks and in the overlying peripheral basins related to their collision. In the southern Caspian Sea area, the giant fields are mainly stored in collision-related basins (Mann et al., 2003) whose origin was controlled by the docking of the peri-Gondwanan along the southern margin of Eurasia (e.g., Cimmerian orogeny). A similar origin is suggested for the Northern Caucasus Basins (Mann et al., 2003), whereas a complex history (from cratonic backarc extension and rifting followed by a sag basin stage) is recorded in the Pricaspian Basin (Weber et al., 2003).

If in North Africa the basins are mainly related to rift basins followed by passive margin, the accretion of the peri-Gondwanan blocks to the southern margin of Eurasia led to the formation of a major concentration of giant fields along the northern passive margin of the peri-Gondwanan blocks and in the overlying peripheral basins related to their collision. In the southern Caspian Sea area, the giant fields are mainly stored in collision-related basins<ref name=Mannetal_2003 /> whose origin was controlled by the docking of the peri-Gondwanan along the southern margin of Eurasia (e.g., Cimmerian orogeny). A similar origin is suggested for the Northern Caucasus Basins,<ref name=Mannetal_2003 /> whereas a complex history (from cratonic backarc extension and rifting followed by a sag basin stage) is recorded in the Pricaspian Basin.<ref name=Weberetal_2003>Weber, L. J., Francis, B. P., Harris, P. M., and Clark, M., 2003, Stratigraphy, facies, and reservoir distribution, Tengiz Field, Kazakhstan, in W. M. Ahr, P. M. Harris, W. A. Morgan, and I. D. Somerville, eds., Permo- Carboniferous carbonate platforms and reefs: SEPM Special Publication 78 and AAPG Memoir 83, p. 351–394.</ref>

In the Arabian peninsula, Mann et al. (2003) identified three basin types preserving giant fields: continent–continent collision for the elongate fields along the Zagros Mountain front; passive margin basins of the southern shore of the Neo-Tethys (central Arabian peninsula and Persian Gulf area); and continental rifts with overlying sag basins on the eastern Arabian Peninsula. Source rocks and reservoirs are present at different stratigraphic levels, reflecting a complex interaction of depositional and tectonics events (Fox and Ahlbrandt, 2002; Pollastro, 2003).

In the Arabian peninsula, Mann et al.<ref name=Mannetal_2003 /> identified three basin types preserving giant fields: continent–continent collision for the elongate fields along the Zagros Mountain front; passive margin basins of the southern shore of the Neo-Tethys (central Arabian peninsula and Persian Gulf area); and continental rifts with overlying sag basins on the eastern Arabian Peninsula. Source rocks and reservoirs are present at different stratigraphic levels, reflecting a complex interaction of depositional and tectonics events.<ref name=Foxandahlbrandt_2002>Fox, J. E., and Ahlbrandt, T. S., 2002, Petroleum geology and total petroleum systems of the Widyan Basin and Interior Platform of Saudi Arabia and Iraq: USGS Bulletin 2202-E, http://geology.cr.usgs.gov/pub/bulletins/b2202-e.</ref> <ref name=Pollastro_2003>Pollastro, R. M., 2003, Total petroleum systems of the Paleozoic and Jurassic, Greater Ghawar Uplift and adjoining provinces of Central Saudi Arabia and Northern Arabian-Persian Gulf: USGS Bulletin 2202-H, http://pubs.usgs.gov/bul/b2202-h.</ref>

==See also==

==See also==