Changes

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]]).

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.<ref>Kerans, C., F. J. Lucia, and R. K. Senger, 1994, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0078/0002/0150/0181.htm Integrated characterization of carbonate ramp reservoirs using Permian San Andres Formation outcrop analogs]: AAPG Bulletin, v. 78, no. 2, p. 181–216.</ref> Porosity variation in carbonate reservoirs occurs at the scale of high-frequency cycles.<ref>Ehrenberg, S. N., 2004, [http://archives.datapages.com/data/bulletns/2004/12dec/1653/1653.HTM Factors controlling porosity in Upper Carboniferous-Lower Permian carbonate strata of the Barents Sea]: AAPG Bulletin, v. 88, no. 12, p. 1653–1676.</ref> Larger scale trends in porosity variation can also occur at the systems tract or sequence level.<ref name=Ehrenberg2006>Ehrenberg, S. N., G. P. Eberli, M. Keramati, and S. A. Moallemi, 2006, [http://archives.datapages.com/data/bulletns/2006/01jan/0091/0091.HTM Porosity-permeability relationships in interlayered limestone-dolostone reservoirs]: AAPG Bulletin, v. 90, no. 1, p. 91–114.</ref>

The measurement of carbonate body dimensions is a topic that gets less attention than is the case for siliciclastic reservoirs. A recent exception is Qi et al. (2007), where geometric data for ooid shoal, tidal flat, and eolian carbonate macroforms were used for constructing a 3-D reservoir model for the Big Bow and Sand Arroyo Creek fields in Kansas. The model has four zones with the ooid grainstone lithofacies showing the highest porosities and permeabilities. These form large linear shoals associated with structural highs. The model can be used to make predictions as a result of the simple zonation, large macroforms, and a reasonable correspondence between facies and rock properties. Many carbonate reservoirs are more complex than this and rather more difficult to model.

The measurement of carbonate body dimensions is a topic that gets less attention than is the case for siliciclastic reservoirs. A recent exception is Qi et al. (2007), where geometric data for ooid shoal, tidal flat, and eolian carbonate macroforms were used for constructing a 3-D reservoir model for the Big Bow and Sand Arroyo Creek fields in Kansas. The model has four zones with the ooid grainstone lithofacies showing the highest porosities and permeabilities. These form large linear shoals associated with structural highs. The model can be used to make predictions as a result of the simple zonation, large macroforms, and a reasonable correspondence between facies and rock properties. Many carbonate reservoirs are more complex than this and rather more difficult to model.