Changes

===Dolomitization===

===Dolomitization===

''Dolomitization'' is a diagenetic process that converts [[limestone]]s to [[dolostones]] through a microchemical process of calcium carbonate dissolution and dolomite precipitation. Dolomitization can change the rock fabric and the petrophysical properties significantly because the dolomite [[Crystallization|crystal]]s are commonly larger than the replaced limestone particles. Dolomite [[cement]] systematically grows on the dolomite crystal faces, reducing reservoir quality. Dolomitization requires the addition of large quantities of magnesium through fluid flow.

''Dolomitization'' is a diagenetic process that converts [[limestone]]s to [[dolostones]] through a microchemical process of calcium carbonate dissolution and dolomite precipitation. Dolomitization can change the rock fabric and the petrophysical properties significantly because the dolomite [[Crystallization|crystal]]s are commonly larger than the replaced limestone particles. Dolomite [[cement]] systematically grows on the dolomite crystal faces, reducing reservoir quality. Dolomitization requires the addition of large quantities of magnesium through [[Fluid flow fundamentals|fluid flow]].

===Evaporite mineralization===

===Evaporite mineralization===

The [[subtidal]]-[[supratidal]] model ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig3.png|Figure 3]]) is based on the transport of carbonate sediment onto the shore by [[Storm deposits and currents|storm]] and [[tidal current]]s resulting in the [[Depocenter#Sediment_supply_rate_and_facies_patterns|progradation]] of the tidal flat environment over the subtidal environment. Subtidal intervals are commonly composed of [[mudstone]]s, [[wackestone]]s, [[packstone]]s, and [[grainstone]]s in no predictable order. When present, [[Grain-supported carbonate|grain-supported]] sediments may be concentrated in the upper part of the subtidal section in the form of offshore bars and high-energy [[shoreface]] deposits. [[Intertidal]] and supratidal sediments are typically muddy except in association with high-energy subtidal sediments. A typical vertical sequence would show intercalated [[Mud-supported carbonate|mud]]- and grain-supported sediments in the subtidal interval overlain by [[algal mat]]s in the intertidal interval, and mud-cracked and desiccated wackestones and mudstones in the supratidal interval.

The [[subtidal]]-[[supratidal]] model ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig3.png|Figure 3]]) is based on the transport of carbonate sediment onto the shore by [[Storm deposits and currents|storm]] and [[tidal current]]s resulting in the [[Depocenter#Sediment_supply_rate_and_facies_patterns|progradation]] of the tidal flat environment over the subtidal environment. Subtidal intervals are commonly composed of [[mudstone]]s, [[wackestone]]s, [[packstone]]s, and [[grainstone]]s in no predictable order. When present, [[Grain-supported carbonate|grain-supported]] sediments may be concentrated in the upper part of the subtidal section in the form of offshore bars and high-energy [[shoreface]] deposits. [[Intertidal]] and supratidal sediments are typically muddy except in association with high-energy subtidal sediments. A typical vertical sequence would show intercalated [[Mud-supported carbonate|mud]]- and grain-supported sediments in the subtidal interval overlain by [[algal mat]]s in the intertidal interval, and mud-cracked and desiccated wackestones and mudstones in the supratidal interval.

The subtidal-supratidal sequence is commonly [[Dolomitization|dolomitized]] and contains [[anhydrite]] and [[gypsum]]. In the subtidal interval, dolomitized grainstones retain their [[Intergranular porosity|intergranular pore space]], except where cemented by anhydrite, and form permeable units. Dolomitization of the subtidal mud-supported sediments converts the tight, mud-supported [[limestone]]s to permeable units because of the larger [[dolomite]] crystals and [[intercrystalline porosity|intercrystalline pore space]]. This produces two types of flow units in the subtidal interval: a [[dolomud]]-supported flow unit and a [[dolograin]]-supported flow unit. Each will have a unique [[porosity]]-[[permeability]] transform.

The subtidal-supratidal sequence is commonly [[Dolomitization|dolomitized]] and contains [[anhydrite]] and [[gypsum]]. In the subtidal interval, dolomitized grainstones retain their [[Intergranular porosity|intergranular pore space]], except where cemented by anhydrite, and form permeable units. Dolomitization of the subtidal mud-supported sediments converts the tight, mud-supported [[limestone]]s to permeable units because of the larger [[dolomite]] crystals and [[intercrystalline porosity|intercrystalline pore space]]. This produces two types of [[Flow units for reservoir characterization|flow units]] in the subtidal interval: a [[dolomud]]-supported flow unit and a [[dolograin]]-supported flow unit. Each will have a unique [[porosity]]-[[permeability]] transform.

===Karst-collapse reservoir model===

===Karst-collapse reservoir model===

The geological [[reef]] model is a composite of the [[upward-shoaling]] [[subtidal]]-[[supratidal]] and [[karst]]-collapse reservoir models. The difference is that the [[facies tracts]] are compressed onto a carbonate [[shelf]] of limited areal extent with high relief above the seafloor and with steeply sloping sides. The [[interior shelf]] or [[lagoon]] facies ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig2.png|Figure 2]]) located landward of the shelf edge normally contains a high percentage of mud. [[Grainstone]]s, [[packstone]]s, and [[boundstone]]s associated with the reef facies are typically found along the shelf edge.

The geological [[reef]] model is a composite of the [[upward-shoaling]] [[subtidal]]-[[supratidal]] and [[karst]]-collapse reservoir models. The difference is that the [[facies tracts]] are compressed onto a carbonate [[shelf]] of limited areal extent with high relief above the seafloor and with steeply sloping sides. The [[interior shelf]] or [[lagoon]] facies ([[:file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig2.png|Figure 2]]) located landward of the shelf edge normally contains a high percentage of mud. [[Grainstone]]s, [[packstone]]s, and [[boundstone]]s associated with the reef facies are typically found along the shelf edge.

[[Reservoir quality#Compaction|Compaction]] and [[Postaccumulation cementation|cementation]] typically destroy the permeability of the lagoonal muds, leaving the grain-dominated sediments and boundstones of the reef edge as reservoir rocks. However, selective [[leaching]], [[dolomitization]], and [[karst]]ing can significantly alter the [[permeability]] patterns, as discussed in previous sections. The reservoir flow units can be very complex due to the numerous possible combinations of depositional and [[Carbonate reservoir models: facies, diagenesis, and flow characterization#Diagenesis|diagenetic]] events.

[[Reservoir quality#Compaction|Compaction]] and [[Postaccumulation cementation|cementation]] typically destroy the permeability of the lagoonal muds, leaving the grain-dominated sediments and boundstones of the reef edge as reservoir rocks. However, selective [[leaching]], [[dolomitization]], and [[karst]]ing can significantly alter the [[permeability]] patterns, as discussed in previous sections. The reservoir [[Flow units for reservoir characterization|flow units]] can be very complex due to the numerous possible combinations of depositional and [[Carbonate reservoir models: facies, diagenesis, and flow characterization#Diagenesis|diagenetic]] events.

==See also==

==See also==