Changes

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 ([http://www.crienterprises.com/Edu_Evap_Coastal_Sabkha.html sabkhas]) are present from which [[gypsum]] and [[halite]] are deposited. [[Sand dune|Eolian sand dunes]] composed of siliciclastic or carbonate grains may form on the supratidal surface.

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 ([http://www.crienterprises.com/Edu_Evap_Coastal_Sabkha.html sabkhas]) are present from which [[gypsum]] and [[halite]] are deposited. [[Sand dune|Eolian sand dunes]] composed of siliciclastic or carbonate grains may form on the supratidal surface.

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. [[Storm deposits and currents|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 [[bar]]s and [[spit]]s oriented parallel to shoreline. Shorelines that face heavy wave action accumulate [[carbonate sand]] or gravel. [[Tidal current]]s are concentrated in channels between islands and produce [[tidal delta]]s 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. [[Storm deposits and currents|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 [http://geonames.usgs.gov/apex/f?p=136:8:0::::: bars] and [[spit]]s oriented parallel to shoreline. Shorelines that face heavy wave action accumulate [[carbonate sand]] or gravel. [[Tidal current]]s are concentrated in channels between islands and produce [[tidal delta]]s on the lee side of the island.

The shelf margin complex ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig2.png|Figure 2]]) is characterized by the presence of [[carbonate sand]]s and [[reef]]s. [[Reef]]s 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 sand]]s 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 current|Tidal]] and [[Storm deposits and currents|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 sand]]s and [[reef]]s. [[Reef]]s 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 sand]]s 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 current|Tidal]] and [[Storm deposits and currents|storm currents]] mold the sand belt into tidal channels and bars.

The upward-shoaling model is based on a depositional model of sediment [[Aggradation|aggrading]] to sea level. As the water shallows, the energy conditions increase, resulting in a vertically stacked sequence from [[mudstone]]s and [[wackestone]]s at the bottom to [[packstone]]s and [[grainstone]]s at the top ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig3.png|Figure 3]]).

The upward-shoaling model is based on a depositional model of sediment [[Aggradation|aggrading]] to sea level. As the water shallows, the energy conditions increase, resulting in a vertically stacked sequence from [[mudstone]]s and [[wackestone]]s at the bottom to [[packstone]]s and [[grainstone]]s at the top ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig3.png|Figure 3]]).

[[Postaccumulation cementation|Cementation]] and [[Reservoir quality#Compaction|compaction]] occur with burial, reducing the reservoir quality of the [[Mud-supported carbonate|mud-supported]] mudstones and wackestones more than the [[Grain-supported carbonate|grain-supported]] packestones and grainstones. The result is a vertical sequence of lower [[porosity]] and [[permeability]] mud-supported rocks at the base and higher porosity and permeability grain-supported rocks at the top. Consequently, the best quality [[Flow units for reservoir characterization|flow unit]] occurs at the top of the sequence. The permeable units are confined to the grainstone [[bar]]s.

[[Postaccumulation cementation|Cementation]] and [[Reservoir quality#Compaction|compaction]] occur with burial, reducing the reservoir quality of the [[Mud-supported carbonate|mud-supported]] mudstones and wackestones more than the [[Grain-supported carbonate|grain-supported]] packestones and grainstones. The result is a vertical sequence of lower [[porosity]] and [[permeability]] mud-supported rocks at the base and higher porosity and permeability grain-supported rocks at the top. Consequently, the best quality [[Flow units for reservoir characterization|flow unit]] occurs at the top of the sequence. The permeable units are confined to the grainstone bars.

If the grainstone bars are exposed to [[meteoric diagenesis]], significant separate vug porosity can develop, causing a loss of permeability while retaining porosity. In the dolomite environment, grainstone bars are commonly cemented with [[anhydrite]], and [[intercrystalline porosity]] in mud-supported sediments forms the permeable facies.

If the grainstone bars are exposed to [[meteoric diagenesis]], significant separate vug porosity can develop, causing a loss of permeability while retaining porosity. In the dolomite environment, grainstone bars are commonly cemented with [[anhydrite]], and [[intercrystalline porosity]] in mud-supported sediments forms the permeable facies.