Changes

==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> 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>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> Reprinted with permission from the AAPG.]]

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>

Carbonate sedimentation is very rapid and the build-up of carbonate sediment can exceed sea-level rise in a short period of time. For example, Neumann and Land<ref>Neumann, A. C., and L. S. Land, 1975, Lime mud deposition and calcareous algae in the Bight of Abaco, Bahamas: A budget: Journal of Sedimentary Petrology, v. 45, no. 4, p. 763–786.</ref> estimated that the carbonate sediment accumulation rate in the Bight of Abaco in the Bahamas is 120 mm (5 in.) per thousand years. This is about three times the estimated subsidence rate of 38 mm (1.4 in.) per thousand years. The phrase carbonate factory is commonly used to describe the manner in which large volumes of sediment are produced on tropical shelfs.

Carbonate sedimentation is very rapid and the build-up of carbonate sediment can exceed sea-level rise in a short period of time. For example, Neumann and Land<ref>Neumann, A. C., and L. S. Land, 1975, Lime mud deposition and calcareous algae in the Bight of Abaco, Bahamas: A budget: Journal of Sedimentary Petrology, v. 45, no. 4, p. 763–786.</ref> estimated that the carbonate sediment accumulation rate in the Bight of Abaco in the Bahamas is 120 mm (5 in.) per thousand years. This is about three times the estimated subsidence rate of 38 mm (1.4 in.) per thousand years. The phrase carbonate factory is commonly used to describe the manner in which large volumes of sediment are produced on tropical shelfs.

Vertically, carbonates can be characterized by high-frequency stacking, with shoaling-upward cycles a few meters thick. Westphal et al. (2004) described high-frequency depositional cycles from the Mississippian Madison Formation in the Wind River Basin of Wyoming. The cycles occur over a meter-scale thickness and consist of a lower transgressive and an upper regressive hemicycle. The transgressive hemicycle is dominated by tidal flat sediments (laminated mudstone and wackestone) and subtidal deposits (e.g., stromatilites). The regressive hemicycle comprises high-energy carbonate sand-shoal facies ([[:File:M91FG197.JPG|Figure 3]]).

Vertically, carbonates can be characterized by high-frequency stacking, with shoaling-upward cycles a few meters thick. Westphal et al.<ref name=Westphal /> described high-frequency depositional cycles from the Mississippian Madison Formation in the Wind River Basin of Wyoming. The cycles occur over a meter-scale thickness and consist of a lower transgressive and an upper regressive hemicycle. The transgressive hemicycle is dominated by tidal flat sediments (laminated mudstone and wackestone) and subtidal deposits (e.g., stromatilites). The regressive hemicycle comprises high-energy carbonate sand-shoal facies ([[:File:M91FG197.JPG|Figure 3]]).

High-frequency upward-shoaling cycles commonly comprise individual hydraulic or flow units within carbonate reservoirs (Kerans et al., 1994). Porosity variation in carbonate reservoirs occurs at the scale of high-frequency cycles (Ehrenberg, 2004). Larger scale trends in porosity variation can also occur at the systems tract or sequence level (Ehrenberg et al., 2006).

High-frequency upward-shoaling cycles commonly comprise individual hydraulic or flow units within carbonate reservoirs (Kerans et al., 1994). Porosity variation in carbonate reservoirs occurs at the scale of high-frequency cycles (Ehrenberg, 2004). Larger scale trends in porosity variation can also occur at the systems tract or sequence level (Ehrenberg et al., 2006).