Changes

==Carbonates are different from sandstones==

==Carbonates are different from sandstones==

Carbonate sediments have several features that set them apart by comparison with siliciclastics. Carbonate sediments tend to form and be deposited in situ, with enormous volumes of calcareous material provided by the death, disintegration, or digestion of plant and animal matter.<ref>Ginsburg, R. N., and N. P. James, 1974, Holocene carbonate sediments of continental shelves, in C. A Burk and C. L. Drake, eds., The geology of continental margins, New York, Springer-Verlag, p.137–155.</ref> The coarser material tends not to be widely spread or abraded by waves and currents. Consequently, uniform grain sorting is not a major characteristic of carbonates. There can be a great diversity of grain sizes and shapes in most carbonate sediments compared to sandstones.

Carbonate sediments have several features that set them apart by comparison with siliciclastics. Carbonate sediments tend to form and be deposited in situ, with enormous volumes of calcareous material provided by the death, disintegration, or digestion of plant and animal matter.<ref>Ginsburg, R. N., and N. P. James, 1974, Holocene carbonate sediments of continental shelves, in C. A Burk and C. L. Drake, eds., The geology of continental margins, New York, Springer-Verlag, p.137–155.</ref> The coarser material tends not to be widely spread or abraded by waves and currents. Consequently, uniform grain sorting is not a major characteristic of carbonates. There can be a great diversity of [[grain size]]s and shapes in most carbonate sediments compared to sandstones.

There are some similarities to siliciclastic environments. Various sedimentary bodies such as beaches, barrier islands, shelf sediments, gravity flows, and dune sands are also found in carbonate settings.

There are some similarities to siliciclastic environments. Various sedimentary bodies such as beaches, barrier islands, shelf sediments, [[gravity]] flows, and dune sands are also found in carbonate settings.

==Many carbonate reservoirs offer a challenge to the production geologist==

==Many carbonate reservoirs offer a challenge to the production geologist==

Carbonate reservoirs can be difficult to develop for a variety of reasons. They generally have poorer recoveries than siliciclastic sediments (e.g., Sun and Sloan).<ref name=S&S>Sun, S. Q., and R. Sloan, 2003, Quantification of uncertainty in recovery efficiency predictions: Lessons learned from 250 mature carbonate fields: Presented at the Society of Petroleum Engineers Annual Technical Conference and Exhibition, October 5–8, 2003, Denver, SPE Paper 84459, 15 p.</ref> A combination of depositional geometry and diagenesis creates highly heterogeneous reservoirs (Table 1). They can have lower primary recoveries as connected volumes may be areally limited with no contact to a large aquifer. The lower energy drive mechanisms such as solution gas drive are common. Heterogeneity at all the reservoir scales can make them a challenge to model, and it is not an easy task to make reliable predictions about their production performance. Reservoir management is difficult because the accurate targeting of production and injection wells is problematic, and sweep may be inefficient as a result of this.

Carbonate reservoirs can be difficult to develop for a variety of reasons. They generally have poorer recoveries than siliciclastic sediments (e.g., Sun and Sloan).<ref name=S&S>Sun, S. Q., and R. Sloan, 2003, Quantification of uncertainty in recovery efficiency predictions: Lessons learned from 250 mature carbonate fields: Presented at the Society of Petroleum Engineers Annual Technical Conference and Exhibition, October 5–8, 2003, Denver, SPE Paper 84459, 15 p.</ref> A combination of depositional geometry and [[diagenesis]] creates highly heterogeneous reservoirs (Table 1). They can have lower primary recoveries as connected volumes may be areally limited with no contact to a large aquifer. The lower energy drive mechanisms such as solution gas drive are common. Heterogeneity at all the reservoir scales can make them a challenge to model, and it is not an easy task to make reliable predictions about their production performance. Reservoir management is difficult because the accurate targeting of production and injection wells is problematic, and sweep may be inefficient as a result of this.

{| class=wikitable

{| class=wikitable

| Tend toward oil-wet behavior || || Early water breakthrough and high water production rates

| Tend toward oil-wet behavior || || Early water breakthrough and high water production rates

|-

|-

| Brittle rocks and commonly fractured || Fractures can create widespread connectivity in an otherwise heterogenous matrix rock || Can form thief zones with rapid water breakthrough

| Brittle rocks and commonly [[fracture]]d || Fractures can create widespread connectivity in an otherwise heterogenous matrix rock || Can form thief zones with rapid water breakthrough

|-

|-

| Common high-frequency cycles on a meter cycle || || Numerous hydraulic units and highly layered reservoirs

| Common high-frequency cycles on a meter cycle || || Numerous hydraulic units and highly layered reservoirs

[[File:M91FG197.JPG|thumb|300px|{{figure number|3}}High-frequency carbonate cycle on a meter scale from the Mississippian Madison Formation in the Wind River Basin of Wyoming (after Westphal et al.).<ref name=Westphal>Westphal, H., G. P. Eberli, L. B. Smith, G. M. Grammer, and J. Kislak, 2004, [http://archives.datapages.com/data/bulletns/2004/04apr/0405/0405.HTM Reservoir characterization of the Mississippian Madison Formation, Wind River basin, Wyoming]: AAPG Bulletin, v. 88, no. 4, p. 405–432</ref>]]

[[File:M91FG197.JPG|thumb|300px|{{figure number|3}}High-frequency carbonate cycle on a meter scale from the Mississippian Madison Formation in the Wind River Basin of Wyoming (after Westphal et al.).<ref name=Westphal>Westphal, H., G. P. Eberli, L. B. Smith, G. M. Grammer, and J. Kislak, 2004, [http://archives.datapages.com/data/bulletns/2004/04apr/0405/0405.HTM Reservoir characterization of the Mississippian Madison Formation, Wind River basin, Wyoming]: AAPG Bulletin, v. 88, no. 4, p. 405–432</ref>]]

Carbonate sediments tend to show a ribbon-like geometry and are less commonly developed as widespread sheets. Examples of both geometries are shown by two of the major carbonate reservoir intervals in the Middle East.<ref>Ehrenberg, S. N., P. H. Nadeau, and A. A. M. Aqrawi, 2007, [http://archives.datapages.com/data/bulletns/2007/03mar/BLTN06054/BLTN06054.HTM A comparison of Khuff and Arab reservoir potential throughout the Middle East]: AAPG Bulletin, v. 91, no. 3, p. 275–286</ref> Sediments of the Permian–Triassic Khuff Formation were deposited on a very low relief shelf, sheltered from the open ocean by a barrier reef. These show a layer-cake geometry consisting of interbedded mudstones and fine-grained grainstones.<ref>Alsharhan, A. S., 2006, Sedimentological character and hydrocarbon parameters of the middle Permian to Early Triassic Khuff Formation, United Arab Emirates: GeoArabia, v. 11, p. 121–158.</ref> By contrast, sedimentation in the Jurassic Arab Formation occurred on a shelf differentiated into shallow shoals and intrashelf basins. These exhibit a progradational geometry.<ref>Meyer, F. O., and R. C. Price, 1992, A new Arab-D depositional model, Ghawar field, Saudi Arabia: Presented at the Society of Petroleum Engineers 8th Middle East Oil Show, SPE Paper 25576, 10 p.</ref>

Carbonate sediments tend to show a ribbon-like geometry and are less commonly developed as widespread sheets. Examples of both geometries are shown by two of the major carbonate reservoir intervals in the Middle East.<ref>Ehrenberg, S. N., P. H. Nadeau, and A. A. M. Aqrawi, 2007, [http://archives.datapages.com/data/bulletns/2007/03mar/BLTN06054/BLTN06054.HTM A comparison of Khuff and Arab reservoir potential throughout the Middle East]: AAPG Bulletin, v. 91, no. 3, p. 275–286</ref> Sediments of the Permian–Triassic Khuff Formation were deposited on a very low relief shelf, sheltered from the open ocean by a barrier reef. These show a layer-cake geometry consisting of interbedded [[mudstones]] and fine-grained grainstones.<ref>Alsharhan, A. S., 2006, Sedimentological character and hydrocarbon parameters of the middle Permian to Early Triassic Khuff Formation, United Arab Emirates: GeoArabia, v. 11, p. 121–158.</ref> By contrast, sedimentation in the Jurassic Arab Formation occurred on a shelf differentiated into shallow shoals and intrashelf basins. These exhibit a progradational geometry.<ref>Meyer, F. O., and R. C. Price, 1992, A new Arab-D depositional model, Ghawar field, Saudi Arabia: Presented at the Society of Petroleum Engineers 8th Middle East Oil Show, SPE Paper 25576, 10 p.</ref>

Carbonate sediments with ribbon geometries show a complex lateral facies progression in map view. A tendency for lateral accretion in successive cycles creates a subtle shingled geometry, which can make accurate correlation difficult ([[:File:M91FG67.JPG|Figure 2]]). For example, laterally accreting grainstones show a shingled geometry on a kilometer scale in Albian carbonates in northern Mexico ([[:File:M91FG196.JPG|Figure 1]]).<ref name=Osleger /> It can be a mistake to fit a layer-cake geometry to these systems because this results in reservoir models where lateral connectivity is predicted to be more extensive than is the case.<ref>Tinker, S. W., 1996, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0080/0004/0450/0460.htm Building the 3-D jigsaw puzzle, applications of sequence stratigraphy to 3-D reservoir characterization, Permian Basin]: AAPG Bulletin, v. 80, no. 4, p. 460–484.</ref> Facies belts may be difficult to define as lithofacies variation in carbonates is frequently transitional rather than sharp.

Carbonate sediments with ribbon geometries show a complex [[lateral]] facies progression in map view. A tendency for lateral accretion in successive cycles creates a subtle shingled geometry, which can make accurate correlation difficult ([[:File:M91FG67.JPG|Figure 2]]). For example, laterally accreting grainstones show a shingled geometry on a kilometer scale in Albian carbonates in northern Mexico ([[:File:M91FG196.JPG|Figure 1]]).<ref name=Osleger /> It can be a mistake to fit a layer-cake geometry to these systems because this results in reservoir models where lateral connectivity is predicted to be more extensive than is the case.<ref>Tinker, S. W., 1996, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0080/0004/0450/0460.htm Building the 3-D jigsaw puzzle, applications of sequence stratigraphy to 3-D reservoir characterization, Permian Basin]: AAPG Bulletin, v. 80, no. 4, p. 460–484.</ref> Facies belts may be difficult to define as [[lithofacies]] variation in carbonates is frequently transitional rather than sharp.

Carbonate sedimentation is very rapid and the build-up of carbonate sediment can exceed sea-level rise in a short period of time. For example, Neumann and Land<ref>Neumann, A. C., and L. S. Land, 1975, Lime mud deposition and calcareous algae in the Bight of Abaco, Bahamas: A budget: Journal of Sedimentary Petrology, v. 45, no. 4, p. 763–786.</ref> estimated that the carbonate sediment accumulation rate in the Bight of Abaco in the Bahamas is 120 mm (5 in.) per thousand years. This is about three times the estimated subsidence rate of 38 mm (1.4 in.) per thousand years. The phrase carbonate factory is commonly used to describe the manner in which large volumes of sediment are produced on tropical shelfs.

Carbonate sedimentation is very rapid and the build-up of carbonate sediment can exceed sea-level rise in a short period of time. For example, Neumann and Land<ref>Neumann, A. C., and L. S. Land, 1975, Lime mud deposition and calcareous algae in the Bight of Abaco, Bahamas: A budget: Journal of Sedimentary Petrology, v. 45, no. 4, p. 763–786.</ref> estimated that the carbonate sediment accumulation rate in the Bight of Abaco in the Bahamas is 120 mm (5 in.) per thousand years. This is about three times the estimated subsidence rate of 38 mm (1.4 in.) per thousand years. The phrase carbonate factory is commonly used to describe the manner in which large volumes of sediment are produced on tropical shelfs.

The diagenetic history of a carbonate reservoir can be complex, involving various phases of cementation, dissolution, compaction, and mineral transformation.<ref name=TW1990>Tucker, M. E., and V. P. Wright, 1990, Carbonate sedimentology: London, Blackwell, 496 p.</ref> Early oil migration can inhibit further diagenesis and preserve porosity in carbonate reservoirs.<ref>Neilson, J. E., N. H. Oxtoby, M. D. Simmons, I. R. Simpson, and N. K. Fortunatova, 1998, The relationship between petroleum emplacement and carbonate reservoir quality: Examples from Abu Dhabi and the Amu Darya Basin: Marine and Petroleum Geology, v. 15, p. 57–72.</ref>

The diagenetic history of a carbonate reservoir can be complex, involving various phases of cementation, dissolution, compaction, and mineral transformation.<ref name=TW1990>Tucker, M. E., and V. P. Wright, 1990, Carbonate sedimentology: London, Blackwell, 496 p.</ref> Early oil migration can inhibit further diagenesis and preserve porosity in carbonate reservoirs.<ref>Neilson, J. E., N. H. Oxtoby, M. D. Simmons, I. R. Simpson, and N. K. Fortunatova, 1998, The relationship between petroleum emplacement and carbonate reservoir quality: Examples from Abu Dhabi and the Amu Darya Basin: Marine and Petroleum Geology, v. 15, p. 57–72.</ref>

Dolomitization is the process by which calcium carbonate is altered to the magnesium-rich carbonate mineral dolomite. It has been estimated that about 80% of the reserves in the carbonates of the United States are in dolomite with 20% in limestone.<ref>North, F. K., 1985, Petroleum geology: Boston, Allen and Unwin, 619 p.</ref> Dolomitization materially affects the pore distribution of carbonate sediments. Dolomitization can act to eliminate heterogeneities in minor lithofacies that would otherwise form barriers or extensive baffles. Muddy carbonates can be transformed into porous dolomites with good intercrystalline connectivity. Dolomites tend to show higher porosities at increased depths of burial by comparison to limestones.<ref name=Ehrenberg2006 />

Dolomitization is the process by which calcium carbonate is altered to the magnesium-rich carbonate mineral [[dolomite]]. It has been estimated that about 80% of the reserves in the carbonates of the United States are in dolomite with 20% in limestone.<ref>North, F. K., 1985, Petroleum geology: Boston, Allen and Unwin, 619 p.</ref> Dolomitization materially affects the pore distribution of carbonate sediments. Dolomitization can act to eliminate heterogeneities in minor lithofacies that would otherwise form barriers or extensive baffles. Muddy carbonates can be transformed into porous dolomites with good intercrystalline connectivity. Dolomites tend to show higher porosities at increased depths of burial by comparison to limestones.<ref name=Ehrenberg2006 />

The process of dolomitization requires a large source of magnesium ions and a fluid transport path for the magnesium to move through the pore space. Several mechanisms have been proposed to explain dolomitization.<ref>Machel, H. G., 2004, Concepts and models of dolomitization: A critical reappraisal, in C. J. R. Braithwaite, G. Rizzi, and G. Darke, eds., The geometry and petrogenesis of dolomite reservoirs: Geological Society (London) Special Publication 235, p. 7–63.</ref> For instance, in the reflux model of dolomitization, dolomite can form where hypersaline conditions exist in peritidal, lagoonal, and restricted basinal environments. Intense evaporation in the tropical heat will result in brine concentrations. The precipitation of gypsum and anhydrite removes calcium from the saline fluids, leaving a magnesium-rich residual brine. The dense, concentrated brine solution will subsequently filter down, reacting with the underlying sediments to form dolomite.<ref>Adams, J. E., and M. L. Rhodes, 1960, [http://archives.datapages.com/data/bulletns/1957-60/data/pg/0044/0012/1900/1912.htm Dolomitization by seepage refluxion]: AAPG Bulletin, v. 44, p. 1912–1920.</ref>

The process of dolomitization requires a large source of magnesium ions and a fluid transport path for the magnesium to move through the pore space. Several mechanisms have been proposed to explain dolomitization.<ref>Machel, H. G., 2004, Concepts and models of dolomitization: A critical reappraisal, in C. J. R. Braithwaite, G. Rizzi, and G. Darke, eds., The geometry and petrogenesis of dolomite reservoirs: Geological Society (London) Special Publication 235, p. 7–63.</ref> For instance, in the reflux model of dolomitization, dolomite can form where hypersaline conditions exist in peritidal, lagoonal, and restricted basinal environments. Intense evaporation in the tropical heat will result in brine concentrations. The precipitation of [[gypsum]] and [[anhydrite]] removes calcium from the saline fluids, leaving a magnesium-rich residual brine. The dense, concentrated brine solution will subsequently filter down, reacting with the underlying sediments to form dolomite.<ref>Adams, J. E., and M. L. Rhodes, 1960, [http://archives.datapages.com/data/bulletns/1957-60/data/pg/0044/0012/1900/1912.htm Dolomitization by seepage refluxion]: AAPG Bulletin, v. 44, p. 1912–1920.</ref>

==Rock types in carbonates==

==Rock types in carbonates==

It is an established procedure to characterize the rock properties of carbonates by rock types instead of lithofacies.<ref name=Lucia1995 /><ref name=Lucia1999 /> These are textural classes that are related to both depositional and diagenetic processes. Sandstone rock properties are dominated by intergranular pore systems, which exhibit a strong lithofacies control on grain size, shape, and sorting. By contrast, carbonate pore systems are much more complex.<ref>Choquette, P. W., and L. C. Pray, 1970, [http://archives.datapages.com/data/bulletns/1968-70/data/pg/0054/0002/0200/0207.htm Geologic nomenclature and classification of porosity in sedimentary carbonates]: AAPG Bulletin, v. 54, no. 2, p. 207–250.</ref> The primary intergranular porosity is more variable because of the greater range in grain sizes and shapes. In addition, skeletal materials common in carbonates will show intraparticle porosity. The primary rock texture will often then be overprinted by postdepositional leaching, replacement, and cementation to form an even more complex pore network.<ref name=JW1982 />

It is an established procedure to characterize the rock properties of carbonates by rock types instead of lithofacies.<ref name=Lucia1995 /><ref name=Lucia1999 /> These are textural classes that are related to both depositional and diagenetic processes. Sandstone rock properties are dominated by intergranular pore systems, which exhibit a strong lithofacies control on [[grain size]], shape, and sorting. By contrast, carbonate pore systems are much more complex.<ref>Choquette, P. W., and L. C. Pray, 1970, [http://archives.datapages.com/data/bulletns/1968-70/data/pg/0054/0002/0200/0207.htm Geologic nomenclature and classification of porosity in sedimentary carbonates]: AAPG Bulletin, v. 54, no. 2, p. 207–250.</ref> The primary intergranular porosity is more variable because of the greater range in grain sizes and shapes. In addition, skeletal materials common in carbonates will show intraparticle porosity. The primary rock texture will often then be overprinted by postdepositional leaching, replacement, and cementation to form an even more complex pore network.<ref name=JW1982 />

==Typical settings for carbonate reservoirs==

==Typical settings for carbonate reservoirs==

The shelf interior in carbonate systems commonly shoals to a tidal flat environment that may be extensive in area ([[:File:M91FG199.JPG|Figure 5]]). The highest porosities and permeabilities are found in the subtidal to intertidal facies with the best reservoir quality in tidal channel sediments. Supratidal sediments show the poorest reservoir quality and are typically barriers to vertical flow.<ref name=Shinn /> In arid environments, supratidal sabkha may be found. The evaporites can act as internal seals.<ref>Wilson, J. L., 1980, A review of carbonate reservoirs, in A. D. Miall, ed., Facts and principles of world petroleum occurrence: Canadian Society of Petroleum Geologists Memoir 6, p. 95–115.</ref>

The shelf interior in carbonate systems commonly shoals to a tidal flat environment that may be extensive in area ([[:File:M91FG199.JPG|Figure 5]]). The highest porosities and permeabilities are found in the subtidal to intertidal facies with the best reservoir quality in tidal channel sediments. Supratidal sediments show the poorest reservoir quality and are typically barriers to vertical flow.<ref name=Shinn /> In arid environments, supratidal sabkha may be found. The evaporites can act as internal seals.<ref>Wilson, J. L., 1980, A review of carbonate reservoirs, in A. D. Miall, ed., Facts and principles of world petroleum occurrence: Canadian Society of Petroleum Geologists Memoir 6, p. 95–115.</ref>

Tidal flat mudstones can be extensively dolomitized to form significant reservoir intervals. Examples of this are found in reservoirs of the Ordovician Ellenburger Formation in the United States, the Ordovician Red River Formation of the Williston basin, the Permian Basin carbonates of Texas, and the Cretaceous offshore of west Africa.

Tidal flat mudstones can be extensively dolomitized to form significant reservoir intervals. Examples of this are found in reservoirs of the Ordovician Ellenburger Formation in the United States, the Ordovician Red River Formation of the Williston basin, the [[Permian Basin]] carbonates of Texas, and the Cretaceous offshore of west Africa.

==Karstification and paleocave systems==

==Karstification and paleocave systems==

Karstified landscapes and paleocave systems form an important class of carbonate reservoirs. Caves present within a limestone bedrock are liable to collapse on compaction, creating a collapse breccia and with associated fracturing of the roof rock. Not all caves fall in with increasing burial; some can survive. When these are penetrated during drilling, the bit can suddenly drop several meters and large losses of drilling mud into the cave system can ensue.

Karstified landscapes and paleocave systems form an important class of carbonate reservoirs. Caves present within a limestone bedrock are liable to collapse on compaction, creating a collapse [[breccia]] and with associated fracturing of the roof rock. Not all caves fall in with increasing burial; some can survive. When these are penetrated during drilling, the bit can suddenly drop several meters and large losses of drilling mud into the cave system can ensue.

Numerous cycles of cave formation and subsequent collapse can result in coalescing collapsed cave systems of considerable size, typically hundreds to several thousands of meters across. These systems may be mappable on 3-D seismic data. Collapse and sag structures form circular karst features that may be discernable from amplitude displays.<ref>Loucks, R. G., 1999, [http://archives.datapages.com/data/bulletns/1999/11nov/1795/1795.htm Paleocave carbonate reservoirs: Origins, burial-depth modifications, spatial complexity and reservoir implications]: AAPG Bulletin, v. 83, no. 11, p. 1795–1834.</ref>

Numerous cycles of cave formation and subsequent collapse can result in coalescing collapsed cave systems of considerable size, typically hundreds to several thousands of meters across. These systems may be mappable on 3-D seismic data. Collapse and sag structures form circular karst features that may be discernable from amplitude displays.<ref>Loucks, R. G., 1999, [http://archives.datapages.com/data/bulletns/1999/11nov/1795/1795.htm Paleocave carbonate reservoirs: Origins, burial-depth modifications, spatial complexity and reservoir implications]: AAPG Bulletin, v. 83, no. 11, p. 1795–1834.</ref>

Chalk is very fine-grained carbonate sediment, comprising skeletal calcitic debris of algae platelets. Porosity in chalk can be high, sometimes as high as 40–50%. Nevertheless, given the very fine-grained nature of the rock, permeabilities are low; 1–7 md is typical of the productive intervals. Factors influencing porosity preservation in chalk are overpressure, early oil migration, burial depth, chalk lithofacies, mud content, and grain size.<ref>Scholle, P. A., 1977, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0061/0007/0950/0982.htm Chalk diagenesis and its relation to petroleum exploration: Oil from chalks, a modern miracle?]: AAPG Bulletin, v. 61, no. 7, p. 982–1009.</ref><ref>Nygaard, E., K. Lieberkind, and P. Frykman, 1983, Sedimentology and reservoir parameters of the Chalk Group in the Danish central graben: Geologie en Mijnbouw, v. 62, no. 1, p. 177–190.</ref><ref name=DHeur>D'Heur, M., 1986, The Norwegian chalk fields, in A. M. Spencer, ed., Habitat of hydrocarbons on the Norwegian Continental Shelf: London, Graham amp Trotman, p. 77–89.</ref><ref>Brasher, J. E. and K. R. Vagle, 1996, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0080/0005/0700/0746.htm Influence of lithofacies and diagenesis on Norwegian North Sea chalk reservoirs]: AAPG Bulletin, v. 80, no. 5, p. 746–768.</ref> A correlation is found between the clay content of the chalk and the degradation of reservoir quality; clay hinders early lithification. As a result, clay-rich chalks are less rigid and will tend to undergo more compaction.<ref name=Kennedy>Kennedy, W. J., 1987, Sedimentology of Late Cretaceous–Paleocene Chalk reservoirs, North Sea central graben, in J. Brooks and K. Glennie, eds., Petroleum geology of northwest Europe 1987: London, Graham amp Trotman, p. 469–481.</ref> It is a common pattern in chalk oil fields to find the highest porosity in the crest of the field, decreasing incrementally toward the oil-water contact.<ref name=DHeur /> This character may result from the race for space between oil migration and cementing fluids. The permeability in the water leg can be so poor that chalk fields are unlikely to have significant aquifers.

Chalk is very fine-grained carbonate sediment, comprising skeletal calcitic debris of algae platelets. Porosity in chalk can be high, sometimes as high as 40–50%. Nevertheless, given the very fine-grained nature of the rock, permeabilities are low; 1–7 md is typical of the productive intervals. Factors influencing porosity preservation in chalk are overpressure, early oil migration, burial depth, chalk lithofacies, mud content, and grain size.<ref>Scholle, P. A., 1977, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0061/0007/0950/0982.htm Chalk diagenesis and its relation to petroleum exploration: Oil from chalks, a modern miracle?]: AAPG Bulletin, v. 61, no. 7, p. 982–1009.</ref><ref>Nygaard, E., K. Lieberkind, and P. Frykman, 1983, Sedimentology and reservoir parameters of the Chalk Group in the Danish central graben: Geologie en Mijnbouw, v. 62, no. 1, p. 177–190.</ref><ref name=DHeur>D'Heur, M., 1986, The Norwegian chalk fields, in A. M. Spencer, ed., Habitat of hydrocarbons on the Norwegian Continental Shelf: London, Graham amp Trotman, p. 77–89.</ref><ref>Brasher, J. E. and K. R. Vagle, 1996, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0080/0005/0700/0746.htm Influence of lithofacies and diagenesis on Norwegian North Sea chalk reservoirs]: AAPG Bulletin, v. 80, no. 5, p. 746–768.</ref> A correlation is found between the clay content of the chalk and the degradation of reservoir quality; clay hinders early lithification. As a result, clay-rich chalks are less rigid and will tend to undergo more compaction.<ref name=Kennedy>Kennedy, W. J., 1987, Sedimentology of Late Cretaceous–Paleocene Chalk reservoirs, North Sea central graben, in J. Brooks and K. Glennie, eds., Petroleum geology of northwest Europe 1987: London, Graham amp Trotman, p. 469–481.</ref> It is a common pattern in chalk oil fields to find the highest porosity in the crest of the field, decreasing incrementally toward the oil-water contact.<ref name=DHeur /> This character may result from the race for space between oil migration and cementing fluids. The permeability in the water leg can be so poor that chalk fields are unlikely to have significant aquifers.

Chalk reservoirs can show strong permeability layering. Pelagic chalk is usually non-net reservoir although under favorable circumstances it can be productive.<ref>Megson, J., and T. Tygesen, 2005, The North Sea Chalk: An underexplored and underdeveloped play, in A. G. Dore and B. A. Vining, eds., Petroleum geology: Northwest Europe and global perspectives: Proceedings of the 6th Petroleum Geology Conference, Geological Society (London), v. 1, p. 159–168.</ref> Pelagic or autochthonous chalk results from the slow settling of sediment on the sea floor. Pervasive early cementation and extensive bioturbation significantly reduce the porosity and permeability from an early stage.

Chalk reservoirs can show strong permeability layering. Pelagic chalk is usually non-net reservoir although under favorable circumstances it can be productive.<ref>Megson, J., and T. Tygesen, 2005, The North Sea Chalk: An underexplored and underdeveloped play, in A. G. Dore and B. A. Vining, eds., Petroleum geology: Northwest Europe and global perspectives: Proceedings of the 6th Petroleum Geology Conference, Geological Society (London), v. 1, p. 159–168.</ref> Pelagic or autochthonous chalk results from the slow settling of sediment on the sea floor. Pervasive early cementation and extensive [[bioturbation]] significantly reduce the porosity and permeability from an early stage.

Pelagic chalk on the seabed is easily disturbed and remobilized. Clean chalk lacks any significant sediment cohesion as it has no unbalanced interparticle electric charges or platy interlocking grains to hold it together.<ref>Bramwell, N. P., G. Caillet, L. Meciani, N. Judge, M. Green, and P. Adam, 1999, Chalk exploration, the search for a subtle trap, in A. J. Fleet and S. A. R. Boldy, eds., Petroleum geology of northwest Europe: Proceedings of the 5th Conference, Geological Society (London), p. 911–937.</ref> Processes tending to redeposit chalk include debris flows, turbidity currents, slumps, and slides ([[:File:M91FG200.JPG|Figure 6]]).<ref name=Kennedy />

Pelagic chalk on the seabed is easily disturbed and remobilized. Clean chalk lacks any significant sediment cohesion as it has no unbalanced interparticle electric charges or platy interlocking grains to hold it together.<ref>Bramwell, N. P., G. Caillet, L. Meciani, N. Judge, M. Green, and P. Adam, 1999, Chalk exploration, the search for a subtle trap, in A. J. Fleet and S. A. R. Boldy, eds., Petroleum geology of northwest Europe: Proceedings of the 5th Conference, Geological Society (London), p. 911–937.</ref> Processes tending to redeposit chalk include debris flows, turbidity currents, slumps, and slides ([[:File:M91FG200.JPG|Figure 6]]).<ref name=Kennedy />