Changes

The reason for the very large size of some carbonate reservoirs is not surprising when one considers the sheer scale of even modern-day carbonate settings. The shallow submerged platform area of the Bahamas extends more than 400 km (248 mi) north–south and covers an area of about 125,000 km2 (48,263 mi2). The size of individual sediment bodies on the Bahama Banks can be impressive too ([[:File:M91FG196.JPG|Figure 1]]). The Joulters Cay ooid shoal is a single carbonate sand body with a mobile border 25 km (15 mi) long and between 0.5 and 2 km (0.3 and 1.2 mi) wide.<ref>Major, R. P., D. G. Bebout, and P. M. Harris, 1996, Facies heterogeneity in a modern ooid sand shoal—An analog for hydrocarbon reservoirs: Bureau of Economic Geology, Geological Circular 96-1, University of Texas at Austin, 30 p.</ref>

The reason for the very large size of some carbonate reservoirs is not surprising when one considers the sheer scale of even modern-day carbonate settings. The shallow submerged platform area of the Bahamas extends more than 400 km (248 mi) north–south and covers an area of about 125,000 km2 (48,263 mi2). The size of individual sediment bodies on the Bahama Banks can be impressive too ([[:File:M91FG196.JPG|Figure 1]]). The Joulters Cay ooid shoal is a single carbonate sand body with a mobile border 25 km (15 mi) long and between 0.5 and 2 km (0.3 and 1.2 mi) wide.<ref>Major, R. P., D. G. Bebout, and P. M. Harris, 1996, Facies heterogeneity in a modern ooid sand shoal—An analog for hydrocarbon reservoirs: Bureau of Economic Geology, Geological Circular 96-1, University of Texas at Austin, 30 p.</ref>

[[File:M91FG196.JPG|thumb|300px|{{figure number|1}}Ooid shoal, Bahamas; the bottom edge of the photograph represents a 4.5-km (2.7 mi)-wide transect. The lower inset is an illustration of a cliff exposure of laterally accreting (shingled) oolites from the Lower Cretaceous of Northern Mexico (from Osleger).<ref name=Osleger>Osleger, D. A., R. Barnaby, and C. Kerans, 2004, [http://archives.datapages.com/data/specpubs/memoir80/CHAPTER5/CHAPTER5.HTM A laterally accreting grainstone margin from the Albian of northern Mexico: Outcrop model for Cretaceous carbonate reservoirs], in G. M. Grammer, P. M. Harris, and G. P. Eberli, eds., Integration of outcrop and modern analogs in reservoir modeling: [http://store.aapg.org/detail.aspx?id=658 AAPG Memoir 80], p. 93–107.</ref> Reprinted with permission from the AAPG.]]

[[File:M91FG196.JPG|thumb|300px|{{figure number|1}}Ooid shoal, Bahamas; the bottom edge of the photograph represents a 4.5-km (2.7 mi)-wide transect. The lower inset is an illustration of a cliff exposure of laterally accreting (shingled) oolites from the Lower Cretaceous of Northern Mexico (from Osleger).<ref name=Osleger>Osleger, D. A., R. Barnaby, and C. Kerans, 2004, [http://archives.datapages.com/data/specpubs/memoir80/CHAPTER5/CHAPTER5.HTM A laterally accreting grainstone margin from the Albian of northern Mexico: Outcrop model for Cretaceous carbonate reservoirs], in G. M. Grammer, P. M. Harris, and G. P. Eberli, eds., Integration of outcrop and modern analogs in reservoir modeling: [http://store.aapg.org/detail.aspx?id=658 AAPG Memoir 80], p. 93–107.</ref>]]

==Carbonates are different from sandstones==

==Carbonates are different from sandstones==

==Geometry==

==Geometry==

[[File:M91FG67.JPG|thumb|300px|{{figure number|2}}Shingled geometries are common in certain depositional environments and can result in a number of isolated reservoir segments. However, this type of geometry is easy to overlook, and a layer-cake geometry is often erroneously imposed (from Sneider and Sneider).<ref name=Sneider2>Sneider, R. M., and J. S. Sneider, 2001, [http://archives.datapages.com/data/specpubs/memoir74/m74ch06/m74ch06.htm New oil in old places: The value of mature-field development], in M. W. Downey, J. C. Threet, and W. A. Morgan, eds., Petroleum provinces of the twenty-first century: [http://store.aapg.org/detail.aspx?id=77 AAPG Memoir 74], p. 63–84.</ref> Reprinted with permission from the AAPG.]]

[[File:M91FG67.JPG|thumb|300px|{{figure number|2}}Shingled geometries are common in certain depositional environments and can result in a number of isolated reservoir segments. However, this type of geometry is easy to overlook, and a layer-cake geometry is often erroneously imposed (from Sneider and Sneider).<ref name=Sneider2>Sneider, R. M., and J. S. Sneider, 2001, [http://archives.datapages.com/data/specpubs/memoir74/m74ch06/m74ch06.htm New oil in old places: The value of mature-field development], in M. W. Downey, J. C. Threet, and W. A. Morgan, eds., Petroleum provinces of the twenty-first century: [http://store.aapg.org/detail.aspx?id=77 AAPG Memoir 74], p. 63–84.</ref>]]

[[File:M91FG197.JPG|thumb|300px|{{figure number|3}}High-frequency carbonate cycle on a meter scale from the Mississippian Madison Formation in the Wind River Basin of Wyoming (after Westphal et al.).<ref name=Westphal>Westphal, H., G. P. Eberli, L. B. Smith, G. M. Grammer, and J. Kislak, 2004, [http://archives.datapages.com/data/bulletns/2004/04apr/0405/0405.HTM Reservoir characterization of the Mississippian Madison Formation, Wind River basin, Wyoming]: AAPG Bulletin, v. 88, no. 4, p. 405–432</ref> Reprinted with permission from the AAPG.]]

[[File:M91FG197.JPG|thumb|300px|{{figure number|3}}High-frequency carbonate cycle on a meter scale from the Mississippian Madison Formation in the Wind River Basin of Wyoming (after Westphal et al.).<ref name=Westphal>Westphal, H., G. P. Eberli, L. B. Smith, G. M. Grammer, and J. Kislak, 2004, [http://archives.datapages.com/data/bulletns/2004/04apr/0405/0405.HTM Reservoir characterization of the Mississippian Madison Formation, Wind River basin, Wyoming]: AAPG Bulletin, v. 88, no. 4, p. 405–432</ref>]]

Carbonate sediments tend to show a ribbon-like geometry and are less commonly developed as widespread sheets. Examples of both geometries are shown by two of the major carbonate reservoir intervals in the Middle East.<ref>Ehrenberg, S. N., P. H. Nadeau, and A. A. M. Aqrawi, 2007, [http://archives.datapages.com/data/bulletns/2007/03mar/BLTN06054/BLTN06054.HTM A comparison of Khuff and Arab reservoir potential throughout the Middle East]: AAPG Bulletin, v. 91, no. 3, p. 275–286</ref> Sediments of the Permian–Triassic Khuff Formation were deposited on a very low relief shelf, sheltered from the open ocean by a barrier reef. These show a layer-cake geometry consisting of interbedded mudstones and fine-grained grainstones.<ref>Alsharhan, A. S., 2006, Sedimentological character and hydrocarbon parameters of the middle Permian to Early Triassic Khuff Formation, United Arab Emirates: GeoArabia, v. 11, p. 121–158.</ref> By contrast, sedimentation in the Jurassic Arab Formation occurred on a shelf differentiated into shallow shoals and intrashelf basins. These exhibit a progradational geometry.<ref>Meyer, F. O., and R. C. Price, 1992, A new Arab-D depositional model, Ghawar field, Saudi Arabia: Presented at the Society of Petroleum Engineers 8th Middle East Oil Show, SPE Paper 25576, 10 p.</ref>

Carbonate sediments tend to show a ribbon-like geometry and are less commonly developed as widespread sheets. Examples of both geometries are shown by two of the major carbonate reservoir intervals in the Middle East.<ref>Ehrenberg, S. N., P. H. Nadeau, and A. A. M. Aqrawi, 2007, [http://archives.datapages.com/data/bulletns/2007/03mar/BLTN06054/BLTN06054.HTM A comparison of Khuff and Arab reservoir potential throughout the Middle East]: AAPG Bulletin, v. 91, no. 3, p. 275–286</ref> Sediments of the Permian–Triassic Khuff Formation were deposited on a very low relief shelf, sheltered from the open ocean by a barrier reef. These show a layer-cake geometry consisting of interbedded mudstones and fine-grained grainstones.<ref>Alsharhan, A. S., 2006, Sedimentological character and hydrocarbon parameters of the middle Permian to Early Triassic Khuff Formation, United Arab Emirates: GeoArabia, v. 11, p. 121–158.</ref> By contrast, sedimentation in the Jurassic Arab Formation occurred on a shelf differentiated into shallow shoals and intrashelf basins. These exhibit a progradational geometry.<ref>Meyer, F. O., and R. C. Price, 1992, A new Arab-D depositional model, Ghawar field, Saudi Arabia: Presented at the Society of Petroleum Engineers 8th Middle East Oil Show, SPE Paper 25576, 10 p.</ref>

Horizontal wells are used to develop chalk fields.<ref>Megson, J., and R. Hardman, 2001, Exploration for and development of hydrocarbons in the Chalk of the North Sea: A low permeability system: Petroleum Geoscience, v. 7, no. 1, p. 3–12.</ref> Permeabilities are too low for conventional wells to be effective. Long horizontal wells, commonly 2 km or more in length, maximize the permeability-thickness and productivity of chalk fields. Fracture stimulation is used to enhance productivity (e.g., Cook and Brekke).<ref>Cook, C. C., and K. Brekke, 2004, Productivity preservation through hydraulic propped fractures in the Eldfisk North Sea Chalk field: SPE Reservoir Evaluation and Engineering, v. 7, no. 2, SPE Paper 88031-PA, p. 105–114.</ref> Waterfloods can be highly effective in chalk because the fine capillary structure will draw in water very efficiently, displacing much of the oil.<ref name=Surlyk /> The injection wells should be drilled to avoid any open fractures that are likely to connect up with production wells, as rapid water breakthrough will ensue.

Horizontal wells are used to develop chalk fields.<ref>Megson, J., and R. Hardman, 2001, Exploration for and development of hydrocarbons in the Chalk of the North Sea: A low permeability system: Petroleum Geoscience, v. 7, no. 1, p. 3–12.</ref> Permeabilities are too low for conventional wells to be effective. Long horizontal wells, commonly 2 km or more in length, maximize the permeability-thickness and productivity of chalk fields. Fracture stimulation is used to enhance productivity (e.g., Cook and Brekke).<ref>Cook, C. C., and K. Brekke, 2004, Productivity preservation through hydraulic propped fractures in the Eldfisk North Sea Chalk field: SPE Reservoir Evaluation and Engineering, v. 7, no. 2, SPE Paper 88031-PA, p. 105–114.</ref> Waterfloods can be highly effective in chalk because the fine capillary structure will draw in water very efficiently, displacing much of the oil.<ref name=Surlyk /> The injection wells should be drilled to avoid any open fractures that are likely to connect up with production wells, as rapid water breakthrough will ensue.

==References==

==References==

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