Changes

==Carbonate sediments and environments==

==Carbonate sediments and environments==

The majority of carbonate sediments are produced in shallow, warm ocean waters by extraction of calcium carbonate from seawater by organisms to form their shells or skeletal material. The sediments are composed of a spectrum of sizes and pore geometries. The Dunham classification (Figure 1) describes depositional textures in a manner that can be related to pore geometries. The grain-supported textures tend to have larger pore sizes than do mud-supported textures. The textures have different geometries in different depositional environments.

[[file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig1.png|thumb|left|{{figure number|1}}Dunham's<ref name=pt06r29>Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture, in, Classifications of Carbonate Rocks—A Symposium: AAPG Memoir 1, p. 108–121.</ref> classification of carbonate rocks according to depositional texture.<ref name=pt06r138>Swanson, R. G., 1981, Sample examination manual: AAPG Methods in Exploration Series, 65 p.</ref>]]

[[file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig1.png|thumb|{{figure number|1}}Dunham<ref name=pt06r29>Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture, in, Classifications of Carbonate Rocks—A Symposium: AAPG Memoir 1, p. 108–121.</ref> classification of carbonate rocks according to depositional texture. (From <ref name=pt06r138>Swanson, R. G., 1981, Sample examination manual: AAPG Methods in Exploration Series, 65 p.</ref>.)]]

The majority of carbonate sediments are produced in shallow, warm ocean waters by extraction of calcium carbonate from seawater by organisms to form their shells or skeletal material. The sediments are composed of a spectrum of sizes and pore geometries. The Dunham classification ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig1.png|Figure 1]]) describes depositional textures in a manner that can be related to pore geometries. The grain-supported textures tend to have larger pore sizes than do mud-supported textures. The textures have different geometries in different depositional environments.

There are five basic carbonate depositional environments. From shore to basin, they are ''peritidal'' (tidal flat), ''shallow shelf interior, shelf margin complex, slope'', and ''basin'' (Figure 2).

[[file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig2.png|thumb|{{figure number|2}}Carbonate depositional environments. (Diagram by R. G. Loucks and C. R. Handford, unpublished.)]]

The peritidal depositional environment is complex (Figure 2). Sediments deposited between mean high and mean low tide are called ''intertidal sediments'', sediments deposited above mean high tide are called ''supratidal sediments'', and sediments deposited below mean low tide are called ''subtidal sediments''. In arid and semi-arid climates, evaporite flats (sabkhas) are present from which gypsum and halite are deposited. Eolian sand dunes composed of siliciclastic or carbonate grains may form on the supratidal surface.

There are five basic carbonate depositional environments. From shore to basin, they are ''peritidal'' (tidal flat), ''shallow shelf interior, shelf margin complex, slope'', and ''basin'' ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig2.png|Figure 2]]).

[[file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig2.png|thumb|{{figure number|2}}Carbonate depositional environments. (Diagram by R. G. Loucks and C. R. Handford, unpublished.)]]

The peritidal depositional environment is complex ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig2.png|Figure 2]]). Sediments deposited between mean high and mean low tide are called ''intertidal sediments'', sediments deposited above mean high tide are called ''supratidal sediments'', and sediments deposited below mean low tide are called ''subtidal sediments''. In arid and semi-arid climates, evaporite flats (sabkhas) are present from which gypsum and halite are deposited. Eolian sand dunes composed of siliciclastic or carbonate grains may form on the supratidal surface.

The shallow shelf interior environment (Figure 2) is dominated by low energy waters that allow lime mud to accumulate. Storms, however, churn the sediment into suspension, winnowing out the fine-sized material and concentrating the coarse material. Near shorelines, the shelf environment may be composed of offshore bars and spits oriented parallel to shoreline. Shorelines that face heavy wave action accumulate carbonate sand or gravel. Tidal currents are concentrated in channels between islands and produce tidal deltas on the lee side of the island.

The shallow shelf interior environment ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig2.png|Figure 2]]) is dominated by low energy waters that allow lime mud to accumulate. Storms, however, churn the sediment into suspension, winnowing out the fine-sized material and concentrating the coarse material. Near shorelines, the shelf environment may be composed of offshore bars and spits oriented parallel to shoreline. Shorelines that face heavy wave action accumulate carbonate sand or gravel. Tidal currents are concentrated in channels between islands and produce tidal deltas on the lee side of the island.

The shelf margin complex (Figure 2) is characterized by the presence of carbonate sands and reefs. Reefs are commonly found at the shelf edge where their rigid framework can withstand strong wave action and they can take advantage of the nutrients upwelling from the deeper waters. Carbonate sands derived from a reef or from plants and animals inhabiting the shelf edge accumulate along a wide belt that follows the break between the shelf edge and the slope. Tidal and storm currents mold the sand belt into tidal channels and bars.

The shelf margin complex ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig2.png|Figure 2]]) is characterized by the presence of carbonate sands and reefs. Reefs are commonly found at the shelf edge where their rigid framework can withstand strong wave action and they can take advantage of the nutrients upwelling from the deeper waters. Carbonate sands derived from a reef or from plants and animals inhabiting the shelf edge accumulate along a wide belt that follows the break between the shelf edge and the slope. Tidal and storm currents mold the sand belt into tidal channels and bars.

The slope (Figure 2) is dominated by sediment transport seaward from the shelf margin. Fine-grained sediment settles to the bottom forming thin-bedded mudstones, while slumps, debris flows, and turbidity currents form coarsegrained bodies of breccia, conglomerate, and carbonate sand. The resulting facies patterns depend upon the relief of the shelf margin and the nature of the shallow water portion of the margin<ref name=pt06r87>Mcllreath, I. A., James, N. P., 1984, Carbonate slopes, in Walker, R. G., ed., Facies Models: Geoscience Canada Reprint Series 1, p. 245–258.</ref>.

The slope ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig2.png|Figure 2]]) is dominated by sediment transport seaward from the shelf margin. Fine-grained sediment settles to the bottom forming thin-bedded mudstones, while slumps, debris flows, and turbidity currents form coarsegrained bodies of breccia, conglomerate, and carbonate sand. The resulting facies patterns depend upon the relief of the shelf margin and the nature of the shallow water portion of the margin.<ref name=pt06r87>Mcllreath, I. A., James, N. P., 1984, Carbonate slopes, in Walker, R. G., ed., Facies Models: Geoscience Canada Reprint Series 1, p. 245–258.</ref>

Sediments of the basin environment (Figure 2) are dominated by very fine grained skeletons of planktonic microorganisms, which when lithified, become chalks. The are found in deep basins as well as on drowned shelves.

Sediments of the basin environment ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig2.png|Figure 2]]) are dominated by very fine grained skeletons of planktonic microorganisms, which when lithified, become chalks. The are found in deep basins as well as on drowned shelves.

==Diagenesis==

==Diagenesis==