Changes

[[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>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 (Ehrenberg et al., 2007). 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 (Alsharhan, 2006). By contrast, sedimentation in the Jurassic Arab Formation occurred on a shelf differentiated into shallow shoals and intrashelf basins. These exhibit a progradational geometry (Meyer and Price, 1992).

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 (Alsharhan, 2006). By contrast, sedimentation in the Jurassic Arab Formation occurred on a shelf differentiated into shallow shoals and intrashelf basins. These exhibit a progradational geometry (Meyer and Price, 1992).

Carbonate sediments with ribbon geometries show a complex lateral facies progression in map view. A tendency for lateral accretion in successive cycles creates a subtle shingled geometry, which can make accurate correlation difficult (see Chapter 10, this publication, and [[:File:M91FG67.JPG|Figure 2]]). For example, laterally accreting grainstones show a shingled geometry on a kilometer scale in Albian carbonates in northern Mexico ([[:File:M91FG196.JPG|Figure 1]]).<ref name=Osleger /> It can be a mistake to fit a layer-cake geometry to these systems because this results in reservoir models where lateral connectivity is predicted to be more extensive than is the case (Tinker, 1996). Facies belts may be difficult to define as lithofacies variation in carbonates is frequently transitional rather than sharp.

Carbonate sediments with ribbon geometries show a complex lateral facies progression in map view. A tendency for lateral accretion in successive cycles creates a subtle shingled geometry, which can make accurate correlation difficult (see Chapter 10, this publication, and [[:File:M91FG67.JPG|Figure 2]]). For example, laterally accreting grainstones show a shingled geometry on a kilometer scale in Albian carbonates in northern Mexico ([[:File:M91FG196.JPG|Figure 1]]).<ref name=Osleger /> It can be a mistake to fit a layer-cake geometry to these systems because this results in reservoir models where lateral connectivity is predicted to be more extensive than is the case (Tinker, 1996). Facies belts may be difficult to define as lithofacies variation in carbonates is frequently transitional rather than sharp.