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The flow characteristics of [[carbonate]] reservoirs are controlled by a combination of [[Carbonate reservoir models: facies, diagenesis, and flow characterization#Carbonate reservoir models|depositional]] and [[Carbonate reservoir models: facies, diagenesis, and flow characterization#Diagenesis|diagenetic]] processes. Depositional processes control the initial [[Pore and pore throat sizes|pore size]] distribution and the geometry of the individual [[depositional facies]]. The diagenetic overprint modifies the pore size distribution and controls the productivity of depositional facies. In some cases, [[reservoir quality]] and flow characteristics are totally controlled by diagenesis, as in [[Karst|karsted]] reservoirs.

The flow characteristics of [[carbonate]] reservoirs are controlled by a combination of [[Carbonate reservoir models: facies, diagenesis, and flow characterization#Carbonate reservoir models|depositional]] and [[Carbonate reservoir models: facies, diagenesis, and flow characterization#Diagenesis|diagenetic]] processes. Depositional processes control the initial [[Pore and pore throat sizes|pore size]] distribution and the geometry of the individual depositional [[Lithofacies|facies]]. The diagenetic overprint modifies the pore size distribution and controls the productivity of depositional facies. In some cases, [[reservoir quality]] and flow characteristics are totally controlled by diagenesis, as in [[Karst|karsted]] reservoirs.

Carbonate reservoir descriptions are based on observations of depositional and diagenetic fabrics and pore space from [[Overview of routine core analysis|core]] and [[Mudlogging: drill cuttings analysis|cuttings]] samples. The descriptions are correlated with [[Basic open hole tools|wireline log]] responses and incorporated into geological facies and/or diagenetic models in order to map [[porosity]], [[Interpreting Sw distribution in a reservoir| water saturation]], and [[permeability]].

Carbonate reservoir descriptions are based on observations of depositional and diagenetic fabrics and pore space from [[Overview of routine core analysis|core]] and [[Mudlogging: drill cuttings analysis|cuttings]] samples. The descriptions are correlated with [[Basic open hole tools|wireline log]] responses and incorporated into geological facies and/or diagenetic models in order to map [[porosity]], [[Interpreting Sw distribution in a reservoir| water saturation]], and [[permeability]].

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]]). (For more information on carbonate depositional environments, see Scholle et al.<ref name=Scholleetal_1983>Scholle, P. A., D. G. Bebout, and C. H. Moore, eds., 1983, [http://store.aapg.org/detail.aspx?id=656 Carbonate depositional environments]: AAPG Memoir 33, 708 p.</ref>)

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]]). (For more information on carbonate depositional environments, see Scholle et al.<ref name=Scholleetal_1983>Scholle, P. A., D. G. Bebout, and C. H. Moore, eds., 1983, [http://store.aapg.org/detail.aspx?id=656 Carbonate depositional environments]: AAPG Memoir 33, 708 p.</ref>)

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 [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 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.

===Calcium carbonate cementation and compaction===

===Calcium carbonate cementation and compaction===

Calcium carbonate [[Postaccumulation cementation|cementation]] and [[Reservoir quality#Compaction|compaction]] are diagenetic processes that are initiated soon after deposition and by which [[Porosity#Carbonate pore systems|intergranular pore space]] is progressively reduced producing systematic changes in petrophysical properties (see [[Evaluating diagenetically complex reservoirs]] and [[Reservoir quality]]). Compaction is a physical/chemical process, while cementation requires fluid flow. [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=microporosity Microporosity] within grains or between lime mud particles may be retained even though intergranular pore space is lost.

Calcium carbonate [[Postaccumulation cementation|cementation]] and [[Reservoir quality#Compaction|compaction]] are diagenetic processes that are initiated soon after deposition and by which [[Porosity#Carbonate pore systems|intergranular pore space]] is progressively reduced producing systematic changes in petrophysical properties (see [[Evaluating diagenetically complex reservoirs]] and [[Reservoir quality]]). Compaction is a physical/chemical process, while cementation requires [[Fluid flow fundamentals|fluid flow]]. [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=microporosity Microporosity] within grains or between lime mud particles may be retained even though intergranular pore space is lost.

===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 most common evaporite mineral found with carbonate rocks is [[anhydrite]] and its hydrous form, [[gypsum]]. Gypsum is the common form at shallow depths, but it converts to anhydrite at depth in response to higher temperatures. Bedded anhydrite is commonly found in tidal flat environments and is an effective reservoir seal. Diagenetic anhydrite is found in reservoir rocks as nodules and [[poikilotopic crystal]]s, which have little effect on reservoir properties, and as pore-filling crystals, which reduce [[reservoir quality]].

The most common [[evaporite]] mineral found with carbonate rocks is [[anhydrite]] and its hydrous form, [[gypsum]]. Gypsum is the common form at shallow depths, but it converts to anhydrite at depth in response to higher temperatures. Bedded anhydrite is commonly found in tidal flat environments and is an effective reservoir seal. Diagenetic anhydrite is found in reservoir rocks as nodules and [[poikilotopic crystal]]s, which have little effect on reservoir properties, and as pore-filling crystals, which reduce [[reservoir quality]].

===Dissolution and associated processes===

===Dissolution and associated processes===

''Dissolution'' is the diagenetic process by which [[carbonate]] and [[Evaporites|evaporite]] minerals are dissolved and removed, thus creating and modifying pore space in reservoir rocks (see [[Reservoir quality]]). The effect of this process on [[permeability]] depends upon the geometry and location of the resulting voids relative to the rock fabric. In some cases, dissolution is [[Fabric selective disslution|fabric selective]] and results in formation of isolated [http://wiki.seg.org/wiki/Dictionary:Vug vugs]. In other cases, dissolution enlarges [[fracture]]s and [[Porosity#Carbonate pore systems|interparticle pores]] resulting in large, connected [[vug]]s. If the vugs are large enough, the roof may collapse, forming a floor [http://geology.com/rocks/breccia.shtml breccia] and fractured roof.

''Dissolution'' is the diagenetic process by which [[carbonate]] and evaporite minerals are dissolved and removed, thus creating and modifying pore space in reservoir rocks (see [[Reservoir quality]]). The effect of this process on [[permeability]] depends upon the geometry and location of the resulting voids relative to the rock fabric. In some cases, dissolution is [[Fabric selective disslution|fabric selective]] and results in formation of isolated [http://wiki.seg.org/wiki/Dictionary:Vug vugs]. In other cases, dissolution enlarges [[fracture]]s and [[Porosity#Carbonate pore systems|interparticle pores]] resulting in large, connected [[vug]]s. If the vugs are large enough, the roof may collapse, forming a floor [http://geology.com/rocks/breccia.shtml breccia] and fractured roof.

==Carbonate rock fabric and petrophysical relationships==

==Carbonate rock fabric and petrophysical relationships==

[[file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig3.png|300px|thumb|{{figure number|3}}Schematic diagrams of the upward-shoaling cementation and compaction reservoir model and the subtidat-supratidal dolomitlzation and sulfate emplacement reservoir model.]]

[[file:carbonate-reservoir-models-facies-diagenesis-and-flow-characterization_fig3.png|300px|thumb|{{figure number|3}}Schematic diagrams of the upward-shoaling cementation and compaction reservoir model and the subtidat-supratidal dolomitlzation and sulfate emplacement reservoir model.]]

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 [[Well_log_sequence_analysis#Parasequence_stacking_patterns|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 bars.

[[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.

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==

[[Category:Sedimentology and stratigraphy – Carbonates]]

[[Category:Sedimentology and stratigraphy – Carbonates]]

[[Category:Depositional environments]]

[[Category:Depositional environments]]