32 Oilfield Review

Dolomite: Perspectives on a Perplexing Mineral

Dolomite is a metastable carbonate. It forms in a variety of distinctly different settings

and can change as conditions change. The mode of formation influences dolomite

morphology and thus impacts exploration and production strategies. New approaches

to carbonate evaluation are helping geoscientists unearth reservoir-quality dolomite,

despite its heterogeneous and often enigmatic nature.

Mishari Al-AwadiKuwait Oil CompanyEast Ahmadi, Kuwait

William J. ClarkWilliam Ray MooreDenver, Colorado, USA

Michael HerronTuanfeng ZhangWeishu ZhaoCambridge, Massachusetts, USA

Neil Hurley Dhahran, Saudi Arabia

Djisan KhoEast Ahmadi, Kuwait

Bernard MontaronDubai, UAE

Fadhil SadooniQatar University Doha, Qatar

Oilfield Review Autumn 2009: 21, no. 3. Copyright © 2009 Schlumberger.For help in preparation of this article, thanks to Tony Smithson, Northport, Alabama, USA.Carbonate Advisor, CMR, EcoScope, ECS, ELAN, Litho-Density and MDT are marks of Schlumberger.

“I think you should be more explicithere in step two.”

Modified with permission from Sidney Harris, copyright ScienceCartoonsPlus.com.

26678schD5R1.indd 1 11/5/09 3:53 PM

Autumn 2009 33

Dolomite is a complex mineral. It can precipitate directly from solutions containing magnesium, calcium and carbonate ions to form cement or unlithified sediment. However, most dolomite forms through the chemical alteration of precur-sor carbonate rock or sediment—primarily lime-stone or calcareous muds. These carbonates tend to be unstable, composed chiefly of calcite or its more thermodynamically unstable polymorph, aragonite. When these precursor materials are exposed to magnesium-rich fluids, a portion of the calcium ions may be replaced by magnesium ions to form a more stable magnesium calcium carbonate known as dolomite.

Dolomite is found in a wide range of settings including hydrothermal veins, lakes, shallow oceans, lagoons and evaporative basins. Theories surrounding the origins of dolomite continue to evolve. Amid controversy and speculation, many modes of origin have been proposed over the years, and nearly as many have been discarded.1

A common sedimentary rock-forming min-eral, dolomite is not merely an assemblage of magnesium, calcium and carbonate (right). Rather, it is a metastable mineral of dubious lin-eage with a variable chemical composition and atomic structure. For a given span of geologic time, it may reside in one form, only to pass to a more stable state when its equilibrium is dis-turbed—primarily through changes in pressure, temperature or chemistry. The crystals may even grow in size. Thus, early generations of crystals may subsequently be recast into ever more stable forms.

This process can be repeated numerous times during burial and diagenesis, with each new phase forming through partial or complete disso-lution of an earlier dolomite. Recrystallization can be beneficial to reservoir formation when it generates intercrystalline porosity, but porosity gains can later be negated by the precipitation of pore-filling dolomite cement or by dolomite crys-tal growth that forms large interlocking crystals.

Because the morphology of a dolomite body is controlled by processes that created it, geoscien-tists usually try to integrate the mode of origin into their exploration strategies. Over time, however, the recrystallization of metastable dolomite can obliterate all traces of the mineral’s earliest mode of origin, with subsequent generations reflecting only the latest environment of recrystallization.2 By masking its mode of origin, dolomite recrystal-lization can hamper exploration efforts.

Some dolomites host exceptional reservoirs characterized by high porosity and permeability. E&P companies therefore endeavor to predict where their drill bit will stand the best chance of encountering reservoir-quality dolomite—despite its chemical complexities and hidden modes of origin. This article describes various modes and settings in which dolomite is formed, as well as processes that are responsible for enhancing or destroying its porosity. It also reviews problems encountered when interpreting data from con-ventional logging suites and provides a glimpse into advanced tools and methodologies used for evaluating reservoirs in this enigmatic rock.

A Metastable Lexicon Because it is a descriptive science based on observations made in the field, geology depends on a precisely tuned lexicon. When transferred from one analog to another, geological terminol-ogy tends to evolve. Dolomite geology is rife with such terms.

Dolomite is named in honor of Déodat Gratet de Dolomieu (1750–1801), a colorful and some-what controversial geologist who described cal-careous rock exposures in the southern Alps of northeastern Italy.3 Dolomieu observed that these rocks looked like limestone but failed to effer-vesce as limestone does when treated with weak

1. Despite more than 200 years of research, the origin of dolomite remains the subject of considerable controversy, partly because critical chemical, biological and hydro logical conditions are poorly understood and partly because petrographic and geochemical data permit more than one interpretation.

For more on this controversy: Machel HG: “Concepts and Models of Dolomitization: A Critical Reappraisal,” in Braithwaite CJR, Rizzi G and Darke G (eds): The Geometry and Petrogenesis of Dolomite Hydrocarbon Reservoirs. London: Geological Society, Special Publication 235 (2004): 7–63.

2. Warren J: “Dolomite: Occurrence, Evolution and Economically Important Associations,” Earth Science Reviews 52, nos. 1–3 (November 2000): 1–81.

> Solid-solution series. In its purest state, dolomite falls along the calcite-magnesite line in the solid-solution series of calcite, magnesite and siderite. Although the composition of dolomite is written as [CaMg(CO3)2], naturally occurring dolomite ranges from about Ca1.16 Mg0.84(CO3)2 to about Ca0.96 Mg1.04(CO3)2.

MattV_ORAUT09_Fig_1_2

CalciteCaCO3

DolomiteCaMg(CO3)2

AnkeriteCaFe(CO3)2

MgCO3

MagnesiteFeCO3

Siderite

3. de Dolomieu DG: “Sur un genre de pierres calcaires très peu effervescente avec les acides et phosphorescentes par la collision,” Journal de Physique 39 (October 1791): 3–10.

For an updated perspective: Zenger DH, Bourrouilh-Le Jan FG and Carozzi AV: “Dolomieu and the First Description of Dolomite,” in Purser BH, Tucker ME and Zenger DH (eds): Dolomites: A Volume in Honor of Dolomieu. Boston, Massachusetts, USA: Blackwell Scientific, International Association of Sedimentologists, Special Publication 21 (1994): 21–28.

26678schD5R1.indd 33 12/9/09 7:27 AM

34 Oilfield Review

acid. So although the dolomite label was first applied to the rock, it also names the chief min-eral constituent as well as the mountain range in which it was first described (above).

To distinguish between rock and mineral, the term dolostone was introduced in 1948.4 This name refers to rock formed of the mineral dolomite (more than 75%), along with other min-erals.5 Dolomicrite is formed when dolomite replaces very fine crystalline muds.

Other researchers felt it necessary to distin-guish between different types of dolomite and introduced new terms to account for variations in magnesium and calcium content. The ideal com-position of dolomite consists of equal amounts of Ca and Mg in alternating layers separated by CO3

layers. The designation of high-calcium dolomite is given to the mineral when calcium carbonate [CaCO3] increases by 10% or more above its ideal composition; alternatively, it may also be called calcitic dolomite or lime dolomite. With a decrease in magnesium carbonate [MgCO3] con-tent, such that CaCO3 ranges from 50% to 90%, the rock may be called dolomitic limestone. Further reducing MgCO3 to between 5% and 10% results in magnesian limestone, though some consider this term obsolete. With less than 5% MgCO3, the precursor rock is simply called limestone.

Protodolomite is a common metastable pre-cursor of dolomite. Although it approximates dolomite in chemical composition, it is said to be poorly ordered, or lacking the well-developed

crystal lattices found in mature, ordered, stoi-chiometric dolomite.6 Like other terms in this discussion, some would strike it from the dolo-mite vocabulary, while others find it useful.

The plural term dolomites may be used to col-lectively describe different types of dolomite that vary in texture, composition or genesis.7 When describing a carbonate that has been subjected to replacement, the adjective dolomitized may be used.

Dolomites can be divided into two major fami-lies. Penecontemporaneous dolomites form soon after deposition of carbonate precursors as a result of geochemical conditions that prevail within the precursor’s environment of deposition. Most penecontemporaneous dolomites are of Holocene age and are restricted to certain evaporitic lagoonal or lacustrine settings. Postdepositional dolomites form after carbonate sediment has been deposited and subsequently removed from the active zone of sedimentation. This may happen through progradation of the sedimentary surface, burial and subsidence, uplift and emergence, or eustatic sea-level fluc-tuations. Almost all examples of massive, region-ally extensive dolostones are postdepositional.8

Important but sometimes confusing distinc-tions have been drawn between various types of dolomite, based on how they form. A dolomite’s mode of origin is an important concept that can be tied to its general orientation and areal extent in the subsurface. Primary dolomite consists of particles that first formed as dolomite by direct precipitation from seawater or other aqueous solu-tion. This process creates unlithified dolomite sediment. However, later researchers assigned the designation of primary to dolomite on the basis of its position in the rock fabric.9 Primary, in this case, came to refer to dolomite that has directly precipitated above, at or within the sediment, while also forming at the same time as that sedi-ment. Thus, the geochemical term primary takes on a stratigraphic context.

> Smallest to largest. The dolomite label can be applied to the mineral (left ), rock (center ) and mountain range (right ).

MattV_ORAUT09_Fig_2

4. Shrock RR: “A Classification of Sedimentary Rocks,” The Journal of Geology 56, no. 2 (March 1948): 118–129.

5. The popularity of this term has waxed and waned over the years, mainly because the designation of dolomite has historical priority for the rock. However, dolostone may once more gain acceptance as researchers seek to avoid ambiguity.

6. Machel, reference 1. 7. Machel, reference 1. 8. Machel, reference 1. 9. Rodgers J: “Terminology of Limestones and Related

Rocks: An Interim Report,” Journal of Sedimentary Petrology 24, no. 4 (December 1954): 225–234.

10. Warren, reference 2.11. Sun SQ: “Dolomite Reservoirs: Porosity Evolution and

Reservoir Characteristics,” AAPG Bulletin 79, no. 2 (February 1995): 186–204.

12. Lucia FJ: “Origin and Petrophysics of Dolostone Pore Space,” in Braithwaite CJR, Rizzi G and Darke G (eds): The Geometry and Petrogenesis of Dolomite Hydrocarbon Reservoirs. London: Geological Society, Special Publication 235 (2004): 141–155.

Halley RB and Schmoker JW: “High-Porosity Cenozoic Carbonate Rocks of South Florida: Progressive Loss of Porosity with Depth,” AAPG Bulletin 67, no. 2 (February 1983): 191–200.

13. In 1837, Jean-Baptiste Élie de Beaumont used a model of mole-for-mole exchange of calcium by magnesium to account for vuggy porosity in the dolostones of the Tyrolean Alps. Élie de Beaumont J-B: “ L’application du calcul à l’hypothèse de la formation par épigenie des anhydrites, des gypses, et des dolomies,” Bulletin de la Société Géologique de France 8 (1837): 174–177.

14. Powers RW: “Arabian Upper Jurassic Carbonate Reservoir Rocks,” in Ham WE (ed): Classification of Carbonate Rocks—A Symposium. Tulsa: The American Association of Petroleum Geologists, AAPG Memoir 1 (1962): 122–192.

This relationship between dolomitization and porosity is also reviewed by Lucia, reference 12.

15. Murray RC and Pray LC: “Dolomitization and Limestone Diagenesis—An Introduction,” in Pray LC and Murray RC (eds): Dolomitization and Limestone Diagenesis: A Symposium. Tulsa: Society of Economic Paleontologists and Mineralogists, SEPM Special Publication 13 (1965): 1–2.

16. Murray and Pray, reference 15.17. Weyl PK: “Porosity Through Dolomitization:

Conservation-of-Mass Requirements,” Journal of Sedimentary Research 30, no. 1 (March 1960): 85–90.

26678schD5R1.indd 3 11/5/09 3:53 PM

Autumn 2009 35

Not all precipitates fall into the primary clas-sification. A different type of precipitate has more negative implications in the oil field. This dolomite precipitates from aqueous solutions in the form of pore-filling cement.

Secondary dolomites are formed through the replacement of CaCO3 by CaMg(CO3)2. Currently, the term replacive, or some variation thereof, appears to be eclipsing secondary.

Thus, precipitation is responsible for both primary dolomite and pore-filling cement. On the other hand, dolomitization forms secondary or replacive dolomite. Unfortunately, the latter term is frequently used to describe distinctly different processes. Many use this term loosely to describe either the process in which magnesium ions replace calcium ions or settings where precipita-tion leads to unlithified sediment or pore-filling cements. Some experts feel that too much lati-tude is granted by such usage. To them, dolomiti-zation should not be applied to dolomite cementation or to cases in which hydrothermal fluid leads to recrystallization of preexisting dolomites. They reserve the term solely for the replacement reaction.

This brief glimpse into the dolomite lexicon chronicles attempts by geoscientists to grasp the nature of a perplexing mineral. Despite the com-plexity inherent in dolomite, E&P companies have a history of success in exploiting these formations.

Mineralization and Reservoir Quality Exploration efforts specifically targeted at dolo-mite reservoirs have paid off in the form of numerous oil and gas fields around the world. It is estimated that up to 50% of the world’s carbon-ate reservoirs are in dolomite, and in North America that estimate ranges to 80%.10 Dolomite reservoirs also host significant volumes of hydro-carbons in Russia, northwestern and southern Europe, northern and western Africa, the Middle East and Far East (above right).

Dolomite formation has a marked effect on reservoir quality, though not all dolomites make good reservoirs. In some reservoirs, it is a detri-ment to production. The permeability, solubility and original depositional fabric of a carbonate rock or sediment, as well as the chemistry, tem-perature and volume of dolomitizing fluids, all influence dolomite reservoir quality. Given these variables, dolomitization can enhance, preserve or destroy porosity.11

There are at least two trains of thought con-cerning dolomite porosity: Some geologists main-tain that dolostone porosity is inherited from its limestone precursor.12 Others cite a long-held claim that the chemical conversion of limestone

to dolostone results in a 12% porosity increase because the molar volume of dolomite is smaller than that of calcite.13

Modern-day studies from different parts of the globe show interesting changes in porosity and permeability with increasing dolomite vol-ume. For example, an evaluation of Jurassic Arab-D carbonates in Ghawar field, Saudi Arabia, indicated a steady decrease in porosity and per-meability as dolomite volume increased from 10% to 80%.14 However, as 80% to 90% of the rock is replaced by dolomite, both intercrystalline poros-ity and permeability increased. Beyond 90%, porosity and permeability decreased again as more dolomite was added to the rock. A similar result from Mississippian carbonates of Saskatchewan, Canada, showed that maximum porosity was developed in carbonates that con-tained 80% to 90% dolomite.15 These examples highlight important processes that take place as dolomite is forming—processes that can affect reservoir quality.

Most dolomites are thought to form through the replacement of preexisting calcite or aragonite sediments. Dolomitization occurs more readily in lime muds than in coarser carbonate sands because muds have greater numbers of nucleation sites on which dolomite crystals can form.16 During early stages of dolomitization in mud-dominated carbonates, porosity decreases slightly as dolomite crystals encroach upon space previously occupied by mud. As dolomite is buried, mechanical com-paction caused by the steadily increasing weight of overburden will further reduce porosity.

However, as dolomitization continues, the dolomite crystals begin to develop a supporting framework. By the time a carbonate reaches 80%

bulk-volume dolomite, it has acquired a grain-dominated fabric in which dolomite crystals essentially support the overburden, thereby sub-stantially inhibiting compaction.17 This is one reason ancient or deeply buried dolomite is often much more porous than associated limestone (below). Higher porosity and permeability are

> Distribution of basins (blue dots) that host production from dolomite reservoirs. Most of these basins occupy a position along a broad belt between 60° north and south of the equator. (From Sun, reference 11.)

MattV_ORAUT09_Fig_3

60°N

30°N

30°S

60°S

0°

> Progressive loss of porosity with depth. As expected, limestone and dolomite units from the South Florida basin both show decreases in porosity with depth. The limestones tend to be more porous at shallower depths. Below 5,600 ft (1,700 m), however, the rate of porosity decline actually slows for dolomites (blue) as they become less susceptible to diagenesis and recrystallization than the more reactive limestones (green). (From Allan and Wiggins, reference 19.)

MattV_ORAUT09_Fig5_2

0 0

1,000

2,000

3,000

4,000

5,000

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

0 10

Dept

h, ft

Dept

h, m

20 30 40 50 60Porosity, %

75% to 100%dolomite

75% to 100%limestone

26678schD5R1.indd 4 11/5/09 3:53 PM

36 Oilfield Review

more likely to be preserved in dolostone than in limestone because the supporting framework of dolomite crystals provides greater compressive strength, thus the limestone is more susceptible to compaction.

Beyond 90% dolomitization, the loss of poros-ity can be attributed to the addition of carbonate and magnesium, through a process known as overdolomitization. After an initial replacement phase during which calcite is replaced by dolo-mite, a pore-filling phase may occur, whereby dolomite precipitates to form crystal overgrowths or pore-occluding cement. Thus, overdolomitiza-tion causes young dolostones to have less porosity than associated limestones.18

Dolomite crystal formation plays another role in reservoir quality. Dolomite frequently forms larger crystals than the calcite it replaces. Enlarged crystal size is associated with increases in pore-throat size and pore smoothness, which boost permeability in dolostones.19

Because the quality of a dolomite reservoir is characterized by its texture, this interrelation-ship of crystal shape and grain size, orientation and packing within a rock can also affect reser-voir quality. Textural classification schemes help geologists infer processes that controlled crystal nucleation and growth.20 One widely accepted dolomite classification scheme is based on crys-tal boundary relationships and divides textures into two types: planar and nonplanar. The planar crystals are further divided into euhedral and sub-hedral classes (above).

Planar dolomite forms in both shallow and burial diagenetic environments. Texture develops when crystals undergo faceted growth with pla-nar interfaces, characteristic of dolomite crystals formed during early diagenesis and, under cer-tain conditions, at elevated temperatures in the subsurface. Two porosity-permeability popula-tions exist for planar dolomite.

•Planar-e (euhedral) dolomite: This texture,often referred to as “sucrosic,” forms important reservoirs worldwide. Permeability varies strongly with porosity. Uniform pore-throat sizes and well-interconnected pore systems are found in planar-e dolomite, as seen in capillary pressure data and scanning electron microscope (SEM) pore-cast analysis.

•Planar-s (subhedral) dolomite: Permeability is lower than in planar-e dolomite and does not increase as rapidly with increasing poros-ity. Uniform throat sizes and well-connected pore systems are not seen in this dolomite, probably because of continued cementation during diagenesis.

Nonplanar dolomite occurs in the subsurface at temperatures greater than 50°C [122°F]. This dolo-mite exhibits no significant correlation between permeability and porosity (below). Permeability in > Dolomite textures. Dolomite can be divided into planar and nonplanar

textures (top). The planar texture is further subdivided into euhedral and subhedral classes. Euhedral (planar-e) dolomite is characterized by well-developed crystal faces with sharp boundaries, with the area between crystals being either porous or filled by another mineral. Subhedral (planar-s) dolomite grains are still planar but less distinct than planar-e grains and show compromised boundaries between crystals. Nonplanar dolomite consists of anhedral grains that lack well-developed crystal faces. These anhedral grains are closely packed, with curved, lobate, serrated or otherwise irregular crystalline boundaries. (Adapted from Sibley and Gregg, reference 20.) Actual examples of these textures are captured in polished thin-section micrographs obtained through a petrographic microscope under polarized light. Euhedral dolomite (bottom left ) from a Cretaceous reservoir of the Middle East exhibits well-developed faces associated with intercrystalline porosity. Subhedral dolomite (center bottom) was obtained from a Triassic reservoir of the northern Arabian Platform. Anhedral dolomite from a Jurassic reservoir of the Arabian basin (bottom right ) shows a lack of crystal faces and interlocked crystals that destroy porosity. (Photographs courtesy of Fadhil Sadooni.)

MattV_ORAUT09_Fig6_2

Planar texture

Increase in temperature

Nonplanar texture

Euhedral Subhedral Anhedral

> Porosity versus permeability. Quantitative analysis of different textural types indicates that permeability in dolomites is not directly related to total porosity or crystal size, but rather to the connectivity of pore throats. There is a strong relationship between increasing porosity and permeability in planar-e dolomites (top, green), and an apparent strong relationship in planar-s (blue). The correlation coefficient (r) between porosity and permeability in nonplanar dolomites (bottom, yellow) is low, as permeability in this type of dolomite is a function of secondary features such as connected vugs and fractures. Points plotted at 0.5 mD represent measurements that fell below the lower determination limit of the permeameter and are not part of a statistical trend. (From Woody et al, reference 21.)

MattV_ORAUT09_Fig_7

Total porosity, % by volume

105

104

103

102

101

r = 0.99

r = 0.99100

10–1

Perm

eabi

lity,

mD

105

104

103

102

101

100

10–1

0 5 10 15 20 25 30

Total porosity, % by volume

0 5 10 15 20 25 30Pe

rmea

bilit

y, m

D

r = 0.15

Planar-e dolomite

Planar-s dolomite

Nonplanar dolomite

26678schD5R1.indd 36 12/9/09 7:28 AM

Autumn 2009 37

nonplanar dolomite is often attributed to secondary porosity features such as fractures or intercon-nected vugs, rather than intergranular porosity found between crystals.21

Researchers continue to unravel the mysteries of dolomite mineralization. The discovery that dolomite is metastable was a revelation that helped geoscientists explain the variations in chemical proportions and structural order that are seen as the mineral evolves. Dolomitization is not a single event; it is a sequence of responses caused by changing geologic conditions.

Modes of Dolomite FormationMany environments of dolomitization have been identified. Some result in unique reservoir geom-etries that bear directly on exploration strategy.

Rather than describe every type of dolomite formation, the following discussion focuses pri-marily on modes that permit dolomites that are thick enough to be targeted for exploration. This also implies that the discussion mainly covers secondary, or replacive, dolomite. In some cases the distinction between modern and ancient con-ditions must be drawn, because current settings do not necessarily reflect the conditions in which ancient dolomites were formed. Three well-established hydrologic models and settings, along with some of their variants, are discussed first, and the section concludes with hydrothermal and bacterial cases.

Brine-Reflux Model—Perhaps the most popu-lar concept of dolomite formation is embodied in the brine-reflux model and similar variations. In this instance, seawater in a restricted lagoon evap-orates to form a hypersaline brine that sinks to the lagoon floor and seeps through underlying lime sediments as it escapes, or refluxes back to the sea (above). As it filters through the pores of the underlying rock, magnesium from the brine replaces part of the calcium contained within the aragonite and calcite components of the lime-stone, converting it to dolomite.

This scenario was proposed in 1960 to explain extensive lagoonal and reefal dolomites associ-ated with platform evaporites of the Permian basin in West Texas, USA.22 Reflux dolomitization has since been recognized in cores from other areas, where the intensity of dolomitization decreases with distance from the evaporite- carbonate contact. Today, hypersaline environ-ments—where water salinity rises above that of normal seawater—are widespread in a belt between about 30° north and south latitude. In the Permian basin, lagoons developed behind

barrier reefs on a broad shelf inundated by the shallow waters of Permian seas. The reefs impeded the surface exchange of water between lagoon and sea. Restricted circulation, combined with loss of water by evaporation, lowered water levels in the lagoon, raised the salinity of brines and promoted the precipitation of evaporites. As the specific gravity of the concentrated brine increased, it sank through the water column and migrated to the lowest depressions in the carbon-ate floor of the lagoon.

Displacing the connate water in the underly-ing rock, the dense hypersaline brine seeped downward along vertical migration pathways, fol-lowing bedding planes only when vertical paths were exhausted. In rocks with varying permeabil-ities, the seeping brines migrated mainly through porous zones while bypassing denser limestone lenses. Thus, coarse-grained and porous Permian dolomites are limited to beds previously com-posed of coarse-grained and porous limestones. By contrast, fine-grained dense dolomites occupy open-shelf positions, where extremely fine-grained, mud-based lithographic limestones would normally form. Dolomite textures were

seen to be caused by primary permeability and crystallinity, rather than by dolomitization.

In this model, the down-and-out migration of the hypersaline brine was responsible for dolomi-tizing broad expanses of carbonate rock in the Permian basin. Within the carbonates, the brine-reflux pathways shifted seaward as the shelves regressed. The lagoons, which sourced the brines, also followed progressively forestepping reef deposits. As established escape zones for the brine became sealed off by advancing evaporites, they would be replaced by similar outlets farther seaward. With each forestep, previously unin-vaded reef limestones were exposed to the dolomitizing brines. The pace of regression was geologic, and so slow that most of the limestones were dolomitized before the supply of brine was cut off.

Most modern dolomite is associated with hypersaline solutions.23 Modern brine-reflux con-ditions have been documented on a smaller scale in settings such as the San Andrés Islands offshore Colombia, the Canary Islands, Spain and the Caribbean island of Bonaire in Netherlands Antilles.24

> Brine reflux in an evaporitic setting. A sill to seaward restricts circulation of waters. Some of the seawater evaporates, causing water density to increase. The dense brines sink below the sediment, reflux through the basin or lagoon floor and dolomitize any carbonate sediments that they pass through. (Adapted from Allan and Wiggins, reference 19.)

MattV_ORAUT09_Fig_8

Evaporation

Increasing water densityFree flow

Open marineSill

Seepage reflux

18. Lucia, reference 12.19. Allan JR and Wiggins WD: Dolomite Reservoirs:

Geochemical Techniques for Evaluating Origin and Distribution. Tulsa: The American Association of Petroleum Geologists, AAPG Continuing Education Course Note Series 36 (1993).

20. Sibley DF and Gregg JM: “Classification of Dolomite Rock Textures,” Journal of Sedimentary Research 57, no. 6 (November 1987): 967–975.

21. Woody RE, Gregg JM and Koederitz LF: “Effect of Texture on Petrophysical Properties of Dolomite: Evidence from the Cambrian-Ordovician of Southeastern Missouri,” AAPG Bulletin 80, no. 1 (January 1996): 119–132.

22. Adams JE and Rhodes ML: “Dolomitization by Seepage Refluxion,” AAPG Bulletin 44, no. 12 (December 1960): 1912–1920.

23. Land LS: “The Origin of Massive Dolomite,” Journal of Geological Education 33, no. 2 (1985): 112–125.

24. Kocurko MJ: “Dolomitization by Spray-Zone Brine-Seepage, San Andrés, Colombia,” Journal of Sedimentary Research 49, no. 1 (March 1979): 209–213.

Müller G and Teitz G: “Dolomite Replacing “Cement A” in Biocalcarenites from Fuerteventura, Canary Islands, Spain,” in Bricker OP (ed): Carbonate Cements. Baltimore, Maryland, USA: Johns Hopkins Press, 1971.

Deffeyes KS, Lucia FJ and Weyl PK: “Dolomitization of Recent and Plio-Pleistocene Sediments by Marine Evaporite Water on Bonaire, Netherlands Antilles,” in Pray LC and Murray RC (eds): Dolomitization and Limestone Diagenesis: A Symposium. Tulsa: Society of Economic Paleontologists and Mineralogists, SEPM Special Publication 13 (1965): 71–88.

26678schD5R1.indd 6 11/5/09 3:53 PM

38 Oilfield Review

Perhaps an even better-known variation on lagoonal brine reflux is seen on a localized scale in the sabkha model. In this arid-climate sce-nario, storm surges or high tides push seawater landward, over the peritidal sediment of a sabkha flat. As the surface water sinks into the sediment, some of the pore waters are lost to capillary evap-oration, leaving a hypersaline brine. Here, hydro-dynamic pressure provides the hydrologic pump for moving Mg through the system. As it becomes more concentrated, the brine precipitates arago-nite and anhydrite or gypsum—minerals that sometimes form an updip seal in dolomite reser-voirs. Precipitation of these minerals removes calcium from the solution but leaves the magne-sium content unchanged, thus raising the Mg/Ca ratio and promoting dolomite precipitation or dolomitization.25 The dense brine continues to percolate downward into underlying lime sedi-ments and refluxes back to its source (above). Sabkha dolomite is commonly associated with supratidal sediments and features, such as algal stromatolites, nodular anhydrites and wind-driven interbedded deposits.

A prime example of the sabkha model is the Ordovician Red River dolomite in the Williston basin of the USA and Canada. Modern-day sabkhas are undergoing extensive study in the Trucial Coast of the United Arab Emirates. There, dolomitization takes place only in the storm recharge zone, and the amount of dolomite cor-relates with the frequency of recharge.26

Researchers have, however, raised doubts as to whether reflux can operate on a regional scale, as originally proposed. The hydrologies of modern

brine-reflux dolomites never approach the scale of processes that caused dolomitization of shelf carbonates adjacent to ancient evaporites. In modern settings, reflux dolomites have been found beneath evaporite crusts, but the areas of evaporite precipitation are both small in scale and localized.27 Modern analogs to ancient dolo-mite deposits are often hard to find. As with other modes, this popular conceptual construct must be applied judiciously, on a case-by-case basis.

Marine-Meteoric Mixing Model—The strati-graphic position, related fossil assemblages and lack of associated evaporite indicate that some dolomites do not form within a restricted-marine, supratidal setting. Instead, they are found in areas where Mg-rich saline waters mix with fresh meteoric water. Modern and ancient dolomite formations around the world support variations on this theme.

One such variation is grounded in widespread dolomitic facies associated with shallow epicon-tinental shelves or structural highs, where—unlike the previous models—evidence of saline brine evaporation is not seen.28 The origin of these dolomites is explained by the dorag model, in which dolomitization occurs in the brackish zone that forms when fresh groundwater is mixed with seawater.29 In this zone, seawater supplies Mg2+ ions, and dissolution of CaCO3 occurs as the two waters mix. Calculations show that mixing meteoric groundwaters with 5% to 30% seawater can cause undersaturation with respect to cal-cite, while dolomite becomes supersaturated. Within this range, calcite can be replaced by

dolomite. In general, the dolomitization process can be expressed by the chemical equation:

2CaCO3 + Mg2+ => CaMg(CO3)2 + Ca2+.This model is based on Mifflin carbonate

outcrops of the Platteville Formation, in the Middle Ordovician Champlain Series of Wisconsin, USA. Here, the carbonates appear homogeneous over a broad area, and the thickness of the unit and general structure suggest a broad, shallow open-marine environment. The open-marine fossils, along with a lack of mud cracks, algal mats and evaporites, preclude supratidal deposi-tion and dolomitization in a physically restricted lagoonal environment.

Dolomitization of the Mifflin Member was the result of a relatively early diagenetic process fol-lowing subaerial exposure of uplifted limestone and subsequent establishment of freshwater lenses. Dolomitization occurred in the brackish zone where seawater and fresh waters mixed, with a dolostone-limestone boundary established along the lower margin of the groundwater lens.30

In a somewhat different mode, dolomite may be created through the circulation of saline groundwaters deep within a carbonate platform. In southern Florida, USA, cold, dense seawater is drawn through the platform margin from the deep Straits of Florida. Geothermally driven circulation causes the Mg-rich seawater to rise into the inte-rior of the Florida carbonate platform, where it mixes with fresh meteoric water before discharg-ing through an extensive aquifer system.

The interplay of fresh and saline waters with geothermal heat flow is known as Kohout convection.31 In this scenario, the resulting pore waters become undersaturated with respect to calcite and aragonite but still saturated with respect to dolomite, which is precipitated in the permeable aquifers.

Another environment for mixing of fresh and saline waters is found along the coastal plains of southeastern Australia. From the present, and extending throughout the Quaternary Period, microcrystalline dolomite and other carbonate minerals have been forming in shallow ephemeral lakes of the Coorong region. These lakes develop along a 100-km [62-mi] belt, in an interdune cor-ridor located immediately inland from the present coastline, behind a calcareous sand barrier. The lakes are considered to be outcrops of the water table, and free water, resulting from rainfall and regional or local aquifer recharge, is found at their surface only during winter and spring.32

Modern dolomite is found only in lakes sub-jected to an annual desiccation phase. Those lakes occur mainly in areas receiving less than 500 mm

> Sabkha reflux environment. This schematic of peritidal sediments on a Qatar peninsula sabkha shows another variation on the reflux theme. Seawater is pushed onshore during storm surges, becomes concentrated through evaporation, then seeps into the underlying sediment to reflux to its source. (Adapted from Warren, reference 2.)

MattV_ORAUT09_Fig_9

Subtidal

Water level

Intertidal

Supratidal

High water

Low water

Evaporation

Seepage reflux

Storm flood

26678schD5R1.indd 7 11/5/09 3:53 PM

Autumn 2009 39

[19.7 in.] of rainfall per year, and typically fill to a water depth of 0.5 to 1 m [1.6 to 3.3 ft]. When filled, these lakes have a carbonate-mud bottom that contains algae and other organic matter. As lake levels fall, the waters become increasingly saline before eventually exposing the mud bottom to sunlight and consequent desiccation. Ensuing brines form during the drying phase and are refluxed out of the system into seaward-flowing groundwaters. Fine-grained dolomites and other carbonates remain behind, while saline and sul-fate evaporite minerals are flushed out of the sys-tem. This dolomite is thought to precipitate from a carbonate gel suspension, not through replace-ment of a preexisting carbonate.

The dolomites in this system accumulate above shallow continental groundwaters that flow toward the sea. During their coastward migration, the groundwaters traverse large volumes of predomi-nantly carbonate aquifer sediments. The source of the Mg is poorly understood but is believed either to be supplied by a local Quaternary volcanic prov-ince or to be scavenged by groundwater flow from other sources.

Burial Diagenesis Model—Dolomite can form in environments where pore-fluid chemistry is dominated by subsurface diagenetic processes or where interactions between water and rock have modified the original pore waters. Such environ-ments are removed from active surface sedimen-tation by intermediate to deep burial and are characterized by chemically reducing conditions.

Burial dolomites form in the subsurface after lithification of lime sediments. These dolomites can either directly precipitate as cement or form as replacements in permeable intervals flushed by warm to hot magnesium-enriched basinal and hydrothermal waters. Since burial dolomite replacement occurs after lithification of a car-bonate host, this dolomite may crosscut deposi-tional facies as well as formation boundaries.33 In addition to structural position, oxygen and stron-tium [Sr] isotopes are useful in determining how such dolomites originate. These dolomites tend to have negative δ18O oxygen isotope values, indicating precipitation from fluids at some- what higher temperatures than those of earlier platform dolomites. The recrystallization of previ-

ously formed dolomites by basinal fluids can reset the crystal characteristics, producing crys-tals with low δ18O values, modified 87Sr/86Sr ratios and saline high-temperature fluid inclusions.34

In these subsurface environments, dolomitiza-tion of limestone is facilitated by higher tempera-tures as burial depth increases. In turn, higher temperatures enable dolomitization by solutions with lower Mg/Ca ratios than the previously men-tioned hypersaline brines. Temperatures of 60° to 70°C [140° to 158°F] are sufficient for burial dolo-mites to form, and these conditions can usually be met within just a few kilometers of the surface. With sufficient temperature increase, many sub-surface waters can become dolomitizing solutions, including residual evaporite brines, seawater and shale-compaction waters. In the latter case, pore water is expelled from fine-grained sediments dur-ing burial and compaction. Clay minerals release Mg+2, which may pass through carbonates, result-ing in their dolomitization.

However, dolomitization in the deep subsur-face is not extensive because pore fluids and ions are progressively lost with continued compac-tion. The case for shale compaction is another contentious topic. Some experts hold that the precipitation of chlorite within shales may be a local sink for Mg. As with other models, large vol-umes of Mg-bearing fluids are necessary for this model to be viable.

Hydrothermal Model—One fairly popular model, hydrothermal dolomitization (HTD), stems from an older idea that has been reincar-nated in refined form. HTD commonly forms mas-sive dolomites that are localized around faults (above right). Hydrothermal dolomite is formed by deep basinal waters as they travel upward through relatively permeable conduits, such as faults and thrust planes, or even zones beneath impermeable seals. As waters circulate down-ward in basinal convection cells, they warm in accordance with the local geothermal gradient. With heating, they become more buoyant, move upward and flow outward along faults and bed-ding planes.

Buoyancy and viscosity affect the ascent rate and geometry of the rising fluid. Where buoyancy forces are stronger, the rising fluid forms a

concentrated, predominantly vertical plume. Within this plume, temperatures, flow rates and chemical potential may be expected to decrease from the center toward its margins. For relatively cool systems, in which viscosity dominates, fluids rise slowly and plume geometry is determined by the ratio of vertical to horizontal permeability.35

Deep waters become hydrothermal—meaning they are at least 5°C [9°F] higher than the ambi-ent formation temperature—as they are transmit-ted upward into cooler, shallower parts of the basin. Pressures of hydrothermal fluids also tend to be higher than ambient fluid pressures.

Hydrothermal fluids, therefore, are those that ascend to cooler strata before their heat has had time to dissipate appreciably into the formation. They flow rapidly upward through permeable conduits, rather than migrating slowly through low-permeability strata. Active faults make the best conduits because they have not been miner-alized. Some faults may even breach the seals of deeper aquifers, tapping geopressured fluids that flow at a high rate up the faults.36

A similar process—fault-related hydrother-mal alteration—has long been recognized by the mining industry as an important aspect of car-bonate diagenesis. However, until recently, this process was largely overlooked in the evaluation of carbonate reservoirs. As a result, some fea-tures that were probably produced by faulting and hydrothermal fluid flow have been inter-preted as having formed in meteoric mixing zones, deep burial and other environments.37

25. Warren, reference 2.26. Land, reference 23.27. Warren, reference 2. 28. Epicontinental shelves are flooded continents, created

through flooding by ancient seaways.29. The term “dorag” is said to be loosely translated from

the Farsi language, and is used to infer “mixed blood or hybrid.”

Badiozamani K: “The Dorag Dolomitization Model—Application to the Middle Ordovician of Wisconsin,” Journal of Sedimentary Research 43, no. 4 (December 1973): 965–984.

> Hydrothermal dolomitization. Fluids from deep within a basin can rapidly move up fault planes to dolomitize carbonates at shallower depths.

MattV_ORAUT09_Fig_10

Dolomite

Limestone

Fluids

flow up

fault

pla

ne

30. For more on this type of dolomite: Folk RL and Siedlecka A: “The “Schizohaline” Environment: Its Sedimentary and Diagenetic Fabrics as Exemplified by Late Paleozoic Rocks of Bear Island, Svalbard,” Sedimentary Geology 11, no. 1 (May 1974): 1–15.

31. Kohout FA: “Ground-Water Flow and the Geothermal Regime of the Floridian Plateau,” Transactions, Gulf Coast Association of Geological Societies 17 (1967): 339–354.

32. von der Borch CC and Lock D: “Geological Significance of Coorong Dolomites,” Sedimentology 26, no. 6 (December 1979): 813–824.

33. Allan and Wiggins, reference 19. 34. Warren, reference 2.35. Warren, reference 2.36. Allan and Wiggins, reference 19.37. Allan and Wiggins, reference 19.

26678schD5R1.indd 8 11/5/09 3:53 PM

40 Oilfield Review

A prime example of an ancient fault-related dolomite is found in the Ordovician Trenton–Black River limestones of Michigan, USA, and southwestern Ontario, Canada.38 There, dolomite defines zones of faulting and fracturing within the surrounding limestone.

Microbial Mediation Model—Present-day low-temperature dolomite most often forms in restricted-marine or hypersaline coastal environ-ments; however, these modern settings produce only a small fraction of the total dolomite found in the rock record. Although dolomite is abun-dant in rocks of the Paleozoic era (250 to 540 Ma), it becomes increasingly scarce in younger rock or sediment—particularly in recent (Holocene) settings. By contrast, ancient massive dolomites are believed to have formed in a wide variety of settings, described previously. This disparity leads some researchers to question whether present-day conditions actually reflect those that allowed the formation of massive ancient dolomites.

To understand the rarity of dolomite in the recent rock record, researchers sought to dis-cover how dolomite forms. Until recently, they have struggled to synthesize the mineral in their laboratories. Reasoning that seawater contained the right ingredients needed for the creation of dolomite, geochemists used brine concentrations and pressure-temperature conditions thought to exist in nature during the formation of dolomite.39 The inability to produce dolomite in the labora-tory goes to the very heart of the problem that has plagued geoscientists for years (see “The Dolomite Problem,” page 1). Although magne-sium, calcium and carbonate ions are common in seawater, the conditions necessary to arrange them in the neatly ordered, alternating layers that formed stoichiometric dolomite have appar-ently changed. Once geoscientists understand how dolomite forms in a controlled environment, they may come closer to learning how it forms in nature and why it was once so prevalent and yet is so uncommon today.

The dolomite problem is tied to a number of interrelated processes involving thermodynam-ics, chemical kinetics, hydrology, host-rock texture and mineralogy. Discoveries in the 1990s revealed that another process—microbial action—should be factored into the equation (above right). Microbes became the focus of attention in the sulfate-rich sludges of shallow isolated lagoons, when it was discovered that calcium-rich dolomite precipitates under anoxic, hypersaline conditions.

Sulfate-reducing bacteria in the Brazilian Lagoa Vermelha play an important role in the for-mation of primary dolomite in lagoons along the

coast east of Rio de Janeiro.40 There, lagoonal hydrological cycles vary with alternating wet and dry seasons. During the wet season, precipitation and continental groundwater raise water levels; during the dry season, seawater recharges the lagoon, which becomes increasingly saline as evaporation intensifies. This dynamic system helps supply the ions needed for dolomite pre-cipitation and anaerobic microbial activity. Dolomite precipitation requires Mg2+, Ca2+ and CO3

2– ions, whereas a continuous supply of SO42–

ions provides oxygen required to sustain the metabolic activity of the sulfate-reducing bacte-ria. The most favorable time for dolomite precipi-tation is the dry season, when the main source of groundwater recharge is seawater, which delivers the ions necessary for both dolomite precipita-tion and sulfate reduction.

In some geochemical models, sulfate is thought to inhibit dolomite production. Experiments have shown that in a purely inorganic system without benefit of bacterial action, the sulfate does indeed inhibit dolomite precipitation. However, this is just the opposite of the Lagoa Vermelha case, in which sulfate is necessary to maintain the microbial activity required to produce dolomite. The hydro-logic system furnishes sulfate ions to the zone of active sulfate reduction where sediments become enriched with dolomite, which, once nucleated, continue to grow with burial. The right strain of bacteria is also a key to dolomite precipitation,

as evidenced by the fact that dolomite is not pre-cipitating in most other anoxic, organic-carbon-rich marine sediments.

Laboratory experiments were able to simulate the chemistry of the dry-season anoxic hypersaline lagoonal waters. Bacteria taken from the lagoonal sludge were used to inoculate a cultural medium. They were incubated for one year in a refrigerator at 4°C [39°F]. After incubation, a dolomite pre-cipitate was recovered. Scanning electron micro-scope (SEM) and X-ray diffraction (XRD) analysis showed that a ferroan dolomite with a fairly high degree of cation order had been precipitated.

Subsequent laboratory experiments using two aerobic bacteria cultures, Halomonas meridiana and Virgibacillus marismortui, were shown to precipitate dolomite in just 30 days at 25°C and 35°C [77°F and 95°F], respectively.41 These experi-ments also showed that the time required for ini-tiation and precipitation of dolomite decreased with increasing temperature, while the quantity of crystals increased with greater incubation time. Here, bacterial metabolic activity involves produc-tion of ammonia [NH3], which creates an alkaline microenvironment around the bacteria cells. The bacteria also produce CO2, which dissolves and transforms into either HCO–

3 or CO–23 at higher pH.

In the presence of Ca2+ and Mg2+, the culture medium becomes supersaturated with respect to dolomite. These physiochemical changes influ-ence the geochemical environment and promote

> Scanning electron microscope photomicrograph of rod-shaped microbial cells inhabiting the surface of a basalt sample. These microbes have precipitated dolomite after three months in anaerobic groundwater. Differences in crystal encrustation may be due to microbial residence time on the basalt surface or may simply reflect differences in metabolic activity. Each cell is approximately 1 µm long. (From Roberts et al, reference 43.)

MattV_ORAUT09_Fig_11

0.5 µm

26678schD5R1.indd 40 12/9/09 7:29 AM

Autumn 2009 41

dolomite precipitation. Other related experiments are helping researchers develop oxygen isotope paleothermometers to evaluate conditions of ancient dolomite formation.42

These analyses proved that microbial media-tion of dolomite production can be achieved under low-temperature anoxic conditions, and in a relatively short time. When dolomite is associ-ated with sediments that are rich in organic carbon, biological influences should therefore be investigated.

A different type of biomineralization was reported in 2004 when methanogens, rather than sulfate reducers, were found to be responsible for dolomite nucleation and precipitation. Rather than examining a hypersaline lagoon, groundwa-ter researchers conducted a long-term evaluation of a petroleum-contaminated freshwater aquifer in Minnesota, USA. There they discovered dolo-mite on the cells of methanogenic microbes that colonized a subsurface basalt layer in a highly reducing environment.43

In this setting, dolomite formation is seen as part of a two-step process in which microorgan-isms first weather the basalt and incidentally release Mg, Ca and Fe. Microbial consumption of CO2 then results in dolomite crystal nucleation on their cell walls.

Field observations showed low-temperature dolomite precipitation by microbial action after three months. Subsequent experiments were conducted in a controlled laboratory environ-ment in which methanogenic bacteria were incu-bated in an anaerobic chamber at 25°C for eight months. Once again, dolomite crystals nucleated on microbe cells that colonized the basalt sur-faces. This study expands the range of environ-ments in which dolomite precipitation is found to occur at low temperature, opening the possibility for new models to explain the origin and diage-netic history of ancient dolomites.

The preceding review of dolomite formation provides only a general orientation and does not cover the full range of environments that support the creation of dolomite. Variations and combina-tions of different modes are espoused by many researchers. Furthermore, a survey of dolomite literature would reveal that each model is only as good as the latest technical paper, and all models have been roundly debated, criticized and in some cases, rebuked.

Each dolomite reservoir is created under unique circumstances, and some reservoirs may consist of multiple generations of dolomite formed by different flow systems and mecha-nisms. All dolomite reservoirs should be investi-gated and characterized on a case-by-case basis.

Reservoir EvaluationEvaluation of dolomite reservoirs is never straightforward. The heterogeneous pore systems in dolomitic rock can easily confound petrophysi-cal evaluation efforts. Even the quantification of dolomite can be difficult. The carbonate precur-sors of dolomite, deposited primarily as a result of biological activity and composed of fossil frag-ments and assorted rock grains, tend to create rocks with very complex textures and a wide range of pore shapes and sizes. These rocks may be further beset by multiple physical, biological and chemical processes, each operating at differ-ent scales. Once converted, dolomite may later be subjected to multiple stages of dissolution, precipitation and recrystallization.

Dolomite reservoir evaluation must account for heterogeneity in lithology, rock pores, grains and textures. The Carbonate Advisor petrophys-ics and productivity analysis process was devel-oped to help geoscientists evaluate these complex reservoirs. Carbonate Advisor analysis relates logging data to producibility using texture-sensi-tive logs and borehole imaging to characterize pore geometry.44 The interpretation methodology involves an integrated sequence to determine lithology, porosity, pore type, permeability, rela-tive permeability and saturation (above).

Lithology and porosity are derived by combin-ing measurements from various tools, each with sensitivity to different factors, including rock matrix, fluid properties and porosity. Neutron cap-ture spectroscopy and photoelectric factor (PEF) data are used to quantify rock mineralogy. Bulk density and neutron porosity measurements are sensitive to both the lithology and the fluids

contained in their pore spaces. Nuclear magnetic resonance (NMR) porosity and bound-fluid volume are sensitive to fluid type and pore-space geome-try, but less sensitive to the rock matrix. Relative permeability, which pertains to the effective flow of oil or gas and water, affects shallow resistivity measurements more than deep resistivity mea-surements. All these measurements are inte-grated with others into a simultaneous solution.

Porosity, in particular, is a focal point of any reservoir evaluation. However, calculating poros-ity values in carbonates, which include calcite

38. Hurley NF and Budros R: “Albion-Scipio and Stoney Point Fields, U.S.A., Michigan Basin,” in Beaumont EA and Foster NH (eds): Stratigraphic Traps I. Tulsa: American Association of Petroleum Geologists, AAPG Treatise of Petroleum Geology, Atlas of Oil and Gas Fields (1990): 1–37.

39. Land LS: “Failure to Precipitate Dolomite at 25°C from Dilute Solution Despite 1000-Fold Oversaturation After 32 Years,” Aquatic Geochemistry 4, nos. 3–4 (September 1998): 361–368.

40. Vasconcelos C and McKenzie JA: “Microbial Mediation of Modern Dolomite Precipitation and Diagenesis Under Anoxic Conditions (Lagoa Vermelha, Rio de Janeiro, Brazil),” Journal of Sedimentary Research 67, no. 3 (May 1997): 378–390.

41. Sánchez-Román M, Vasconcelos C, Schmid T, Dittrich M, McKenzie JA, Zenobi R and Rivadeneyra MA: “Aerobic Microbial Dolomite at the Nanometer Scale: Implications for the Geologic Record,” Geology 36, no. 11 (November 2008): 879–882.

42. Vasconcelos C, McKenzie JA, Warthmann R and Bernasconi SM: “Calibration of the δ18O Paleo-thermometer for Dolomite Precipitated in Microbial Cultures and Natural Environments,” Geology 33, no. 4 (April 2005): 317–320.

43. Roberts JA, Bennett PC, González LA, Macpherson GL and Milliken KL: “Microbial Precipitation of Dolomite in Methanogenic Groundwater,” Geology 32, no. 4 (April 2004): 277–280.

44. Ramamoorthy R, Boyd A, Neville TJ, Seleznev N, Sun H, Flaum C and Ma J: “A New Workflow for Petrophysical and Textural Evaluation of Carbonate Reservoirs,” Transactions of the SPWLA 49th Annual Logging Symposium, Edinburgh, Scotland, May 25–28, 2008, paper B.

> Carbonate Advisor sequential workflow. The first step incorporates results from tools that provide lithology and porosity information: spectroscopy, density, thermal neutron, epithermal neutron, photoelectric factor, NMR and gamma ray. The data are examined by petrophysicists and serve as inputs to the next step, which involves evaluation of the pore system and permeability using NMR T2 distribution or image logs. Next, relative permeability and saturation are obtained from array laterolog or array induction resistivity measurements. Also, core data, such as grain density, porosity and permeability, can be added to the analysis.

MattV_ORAUT09_Fig_12

Total porosity, vol %

105

104

103

102

101

r=0.99

r=0.99

100

10-1

Perm

eabi

lity,

md

105

104

103

102

101

100

10-1

0 5 10 15 20 25 30

Perm

eabi

lity,

md

r=0.99

A

Planar-s dolomite

Planar-e dolomite

B

Nonplanar dolomite

SpectroscopyDensity

Thermal neutronEpithermal neutronPhotoelectric factor

NMRGamma ray

Lithology

and porosityCore data

NMR T2 distribution

Image log

Laterolog resistivity

Induction resistivity

Grain density

Porosity

Permeability

Pore system

and permeability

Relative permeability

and saturation

26678schD5R1.indd 10 11/5/09 3:53 PM

42 Oilfield Review

and dolomite, can be a rather convoluted pro-cess. Neutron porosity measurements must be corrected for the rock matrix. If the matrix con-tains only dolomite or only calcite, the porosity transform is fairly simple. But if the rock contains a mixture of both minerals, then the cor-rect proportions of each must be determined to accurately calculate porosity values.

Matrix complexity also affects the computa-tion of density porosity because the equation used to convert porosity from bulk density mea-surements requires matrix density as an input. Should the rock be a mix of dolomite and calcite, the porosity calculations will be incorrect unless an accurate matrix density is obtained. Thus, underestimating or ignoring the presence of dolomite can lead to low computed porosity val-ues that mask potentially productive zones.

In some cases, calcite and dolomite can be readily distinguished using PEF data from a

Litho-Density tool.45 The PEF matrix value for pure sandstone is 1.81; for dolomite it is 3.14 and for limestone it is 5.08. From the PEF measure-ment, the percentage of dolomite can be directly calculated if the matrix contains only two miner-als; unfortunately, rocks often contain a mixture of minerals. Adding to the complexity is the fact that even small concentrations of relatively com-mon minerals, such as siderite (with a PEF of 14.7), pyrite (with its PEF of 16.97) or anhydrite (with a PEF of 5.03), distort the measured PEF values and shift the value toward calcite. There are too many unknowns in this case to determine the matrix type and the matrix porosity from a standard logging suite.

An additional problem with using PEF for lithology determination is the effect of barite, which is commonly added as a weighting material to drilling mud systems. Barite, with its PEF of 266.82, overwhelms other PEF measurements in these mud systems.

The ECS elemental capture spectroscopy tool can help to fill some of the gaps in the interpreta-tion process. Neutron capture spectroscopy mea-sures elemental yields of minerals found in the formation. Recent advances in elemental capture spectroscopy have resulted in improved magne-sium yield measurements to help petrophysicists quantify the amount of dolomite and other miner-als contained in reservoir rocks. ECS measure-ments also provide yields of calcium and sulfur, which are critical for most carbonate lithology determination. In addition, ECS spectroscopy data provide relative yields of elements such as iron, silicon, barium, hydrogen and chlorine. ECS data thus reduce uncertainty in porosity mea-surements derived from basic logging suites.

Pore geometry comes into play when evaluat-ing reservoir quality and fluid-flow properties. For the Carbonate Advisor system, the pores are parti-tioned into different types based on pore-throat size. Partitioning is based on NMR transverse relaxation time (T2) distributions augmented by borehole images. Even though NMR is sensitive to pore-body size distribution, the Carbonate Advisor system calibrates the results to appear as pore- throat size distribution. Two cutoffs are applied to T2 distributions relating relaxation time to pore- size distribution (above left).

The short cutoff defines the microporosity fraction, and the long cutoff defines the macropo-rosity fraction, while the mesoporosity fraction falls between the two. The macroporosity compo-nent is also determined from borehole images by converting the resistivity image into a porosity image and extracting the fraction of large pores present. From the three porosity partitions, eight petrophysical pore system classes are identified.

Matrix permeability is also estimated using transforms optimized for each pore class. Permeability estimates can be validated or cali-brated using data from formation testing tools or core measurements.

Simultaneous solutions of saturation and rel-ative permeability are obtained through forward modeling. The full model accounts for radial vari-ations in resistivity caused by the distribution of drilling fluids that invaded the formation, which influences resistivity tool response. Both array induction and array laterolog measurements can be used for the analysis. With their multiple depths of investigation, the resistivity tools can accurately characterize the invasion front, which is inverted to determine imbibition relative- permeability curves. The saturation front and salinity front are simultaneously solved to deter-mine fractional flow, relative permeability versus saturation and true formation resistivity.

The Carbonate Advisor system was recently put to the test in a reservoir in northern Kuwait. Reservoir evaluation in this area can be compli-cated by drilling fluids weighted with barite, used to increase drilling safety in fields known for high concentrations of hydrogen sulfide and high res-ervoir pressures.46 Geoscientists with the opera-tor Kuwait Oil Company (KOC) found that zones of improved porosity and permeability were asso-ciated with dolomitization in this field. The quan-tification of dolomite content was therefore important in classifying reservoir quality.

However, the estimation of dolomite content from conventional measurements can be hindered by a variety of factors, such as barite mud effects, complex lithologies and sensitivity of logging tool measurements to dolomite, as well as differences in each tool’s vertical resolution and depth of investigation. To overcome these formation evalua-tion challenges, an ECS tool was used to obtain elemental relative yields for mineralogy computa-tion. Magnesium measured by this tool was a key element for dolomite quantification in this com-plex reservoir. The CMR combinable magnetic resonance tool was also run to obtain pore geome-try information. The Carbonate Advisor system provided formation evaluation results that closely agree with core data (next page).

> Pore geometries. Total porosity (top) can be divided into different types of pores based on NMR and image log data. Micropores, with pore-throat diameters less than 0.5 μm, usually contain mostly irreducible water and little hydrocarbon. Mesopores, with pore-throat diameters between 0.5 and 5 μm, may contain significant amounts of oil or gas in pores above the free-water level (FWL). Macropores, with throats measuring more than 5 μm in diameter, are responsible for prolific production rates in many carbonate reservoirs but often provide pathways for early water breakthrough, leaving considerable gas and oil behind in the mesopores above the FWL. The three different types of pores can be further divided into eight pore system classes (bottom).

MattV_ORAUT09_Fig_13

Total porosity

Micro-porosity

All pores< 50 to 100 µm

have the same T2

Blind to poressmaller thantool buttons

100%macroporosity

100%microporosity

100%mesoporosity

Imageresponse Nonvug porosity

Vugporosity

NMRresponse

φ fordistribution

< shortT2 cutoff

φ fordistribution

> longT2 cutoff

Meso-porosity

Macro-porosity

~ 5 µm~ 0.5 µm

Macro-porous

Micro-macro

Macro-micro

Macro-meso

Micro-porous

Micro-meso

Meso-micro

Meso-porous

45. The PEF log is recorded as part of the density measurement. The PEF measurement is unitless, but because it is proportional to the photoelectric cross section per electron, it is sometimes expressed in barns/electron.

46. Kho D, Al-Awadi M and Acharya M: “Application of Magnesium Yield Measurement from Elemental Capture Spectroscopy Tool in Formation Evaluation of Northern Kuwait Fields,” presented at the SPWLA 50th Annual Logging Symposium, The Woodlands, Texas, June 21–24, 2009.

26678schD5R1.indd 11 11/5/09 3:53 PM

Autumn 2009 43

> Appraisal of a complex carbonate reservoir. In Track 1, a strong correlation is seen between the relative dry weight Mg measurement obtained by the ECS tool (green curve) and grain density measurements obtained through core analysis (red dots). The array induction resistivity data (Track 2) and conventional density (pink curve), neutron (blue curve) and sonic (green curve) data in Track 3 are used for computing porosity and water saturation. Track 4 shows the resulting lithology, porosity and fluid volumes computed by the Carbonate Advisor service. A good match between core data (red) and computed grain density (blue curve) and computed porosity (black curve) are seen in Track 5. The T2 distribution measurement from the CMR combinable magnetic resonance tool (Track 6) is used for porosity partitioning, pore system classification and permeability

computations. When the porosity partitioning result (Track 7) is compared with the computed lithology (Track 4), dolomite content shows a close correlation with the mesoporosity (green shading) and macroporosity (red shading). This formation shows a general correspondence between increases in dolomite content and pore size. Porosity is further related to permeability (Track 8). High permeability values generally correspond to zones of macroporosity (red shading). The computed permeability can be compared to core and MDT modular formation dynamics tester data (Track 9). Carbonate Advisor estimated permeability (black curve) matches the core permeability (red dots) and is confirmed by the MDT mobility reading (blue dots).

MattV_ORAUT09_Fig_14

ohm.m0.2 2000

20-in. Resistivity

ohm.m0.2 2000

30-in. Resistivity

ohm.m0.2 2000

60-in. Resistivity

ohm.m0.2 2000

90-in. Resistivity

ohm.m0.2 2000

10-in. Resistivity

µs/ft109 29

DT Compressional

%0.45 - 0.15

Neutron Porosity

%0.5 0

Core Porosity

mD0.1 10,000Core Permeability

mD/cP0.1 10,000MDT Mobility

mDmD 0.1 10,0000.1 10,000

Carbonate AdvisorEstimated Permeability

%0.5 0

Computed Porosity

g/cm32.5 3.5

Core Grain Density

g/cm32.5 3.5

Computed Grain Density

0.015

–0.015

0.5 ft3/ft3 0

0.5 ft3/ft3 0

T2 Distribution

Microporosity

Carbonate AdvisorTotal Porosity

Carbonate AdvisorPermeability

%100 0

ELAN VOLUMES

g/cm31.95 2.95

Density

Dolomite Grain Density

lbf/lbf

Rel. Dry Weight Mg

Core Grain Density Depth,ft

X,050

X,100

X,150

X,200

X,250

2.7 3

g/cm3

g/cm3

g/cm3

2.7 3

0 0.25

Limestone Grain Density2.7 3

Moved Water

Moved Hydrocarbon

Dolomite

Calcite

Kaolinite

Chlorite

Illite

Pyrite

Macroporosity

Mesoporosity

Macroporosity

Mesoporosity

Micro-macro

Micro-meso

Meso-micro

Microporosity

Macro-micro

Macro-meso

Anhydrite

Oil

Water

26678schD5R1.indd 43 12/9/09 7:30 AM

44 Oilfield Review

Core analysis confirmed the relationship between dolomite content and reservoir qual-ity in this northern Kuwait field. The 3D crossplot shows a general trend of increasing dolomite content with increases in porosity and permeability (below left).

Expanding the ScopeDespite efforts to determine environmental set-tings, modes of origin and conditions that impact dolomite quality, exploration and production of these formations are fraught with uncertainty. Refinements in distinctly different approaches to formation evaluation technology are helping E&P companies to reduce some of these unknowns.

As previously discussed, petrophysical evalua-tions of dolomite reservoirs require detailed min-eralogy and matrix properties to correct density and neutron porosity calculations. These inputs depend, in part, on the ability to distinguish cal-cite from dolomite. The first step in obtaining these inputs rests with the selection of logging

tools used to investigate the reservoir; standard capture spectroscopy tools are not sensitive to pro-portions of Mg and Ca in a formation. Although photoelectric factor measurements can be used for this purpose, the shallowness of the PEF mea-surement makes it sensitive to borehole condi-tions, barite muds, and invasion by drilling fluids.

However, ECS measurements obtained by the EcoScope multifunction LWD service are sensi-tive to the proportion of Mg in a formation. This capability is key to determining calcite and dolo-mite content in a carbonate formation. This LWD collar obtains a broad array of measurements. Designed around a pulsed neutron generator, the EcoScope tool measures resistivity, neutron porosity, azimuthal gamma ray, density, neutron gamma density and formation sigma, in addition to elemental capture spectroscopy.

Another approach, based on wellbore imag-ing and high-resolution computed tomography (CT) scans, is helping E&P companies to better predict fractures and high-permeability trends

in highly heterogeneous formations. With the aid of sophisticated conditional simulation algo-rithms, this approach analyzes wellbore images to determine where pores and conductive patches lie in relation to rock matrix.47 Gaps in the wellbore image—an inherent feature of pad coverage provided by imaging tools—are filled using a multipoint statistical (MPS) conditional simulation to create a fullbore image of the wellbore (below right). The multipoint condi-tional simulation incorporates micron-scale CT scans of actual core to create digital rock sam-ples that train the MPS program.48 This pattern-based approach honors all data obtained by the pad device; it also extends patterns from within the pad measurement into the gaps, thereby creating a 3D pseudocore.49 The new fullbore image can then be divided into different petro-physical facies that are used for estimating porosity and permeability.

> Effect of dolomitization on reservoir quality. Core analysis data from a field in Kuwait were used to plot dolomite volume, core porosity and core permeability. These data showed strong correlations between increasing dolomite volumes and increases in porosity and permeability. The scatter within this 3D crossplot reflects the heterogeneous nature of the pore system within the dolomitic rock.

MattV_ORAUT09_Fig_15

Cor

e po

rosi

ty, %

Core permeabilit

y, mD

Volume of dolomite C

ore porosity, %

Core permeability, mD Volume of dolomite

0.300

0.270

0.240

0.210

0.180 0.150

0.120 0.090

0.060 0.030 0.000

1,000 100 10

1

0.1

0.01

0.01

0.1

1

10

100

1,000

0.300

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.270

0.240

0.210

0.180

0.150

0.120

0.090

0.060

0.030

0.000 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

> Filling the gaps. Images through a layered, fractured interval were obtained in a single pass (left ) of a pad-type borehole imaging tool. Data from one pad have been removed to emphasize the area normally measured by each pad (green dotted line, left ). Gaps between pads have been filled in the fullbore image using MPS pattern-based geostatistical modeling (center ). Dark conductive patches are outlined by green contours (right ). These contours help identify complex 3D fluid-flow pathways in heterogeneous carbonates.

MattV_ORAUT09_Fig_16

X00

Depth,ft

X02

X04

X06

X08

26678schD5R1.indd 13 11/5/09 3:54 PM

Autumn 2009 45

For example, it can be used to evaluate vugs—large, irregular pores visible to the naked eye—commonly seen in carbonate rocks. Zones of enhanced porosity and permeability exist in the vicinity of vugs, as confirmed by thin sections, SEM images and minipermeability measure-ments. Swarms of small vugs are commonly seen in the vicinity of large vugs. On borehole-imaging logs, small vugs that fall below the resolution of

imaging pads appear as dark high-conductivity regions, rather than as discrete pores.

Fullbore images allow closed contours to be drawn around resistive or nonresistive regions in the image. Such regions provide important mea-sures of reservoir heterogeneity and are gener-ally much larger than the core plugs or digital models generated from CT scans of rocks. Thus, borehole images are critical for identifying flow

model heterogeneities ranging from centimeters to meters in scale. By defining regions of high or low resistivity, the imaging technique can help determine whether the vugs form a connected and therefore permeable network.

Capillary pressure and relative-permeability curves can be assigned to different petrophysical facies, based on laboratory special core analysis and mercury-injection capillary pressure tests run on actual reservoir rock core samples. Numerical simulations using these results provide the key to quantifying the impact of carbonate rock hetero-geneity on fluid flow during primary production, waterflooding or gasflooding. Such simulations are carried out on the previously constructed numeri-cal pseudocores to estimate important effective parameters such as water cut, oil recovery factor and recovery efficiency on a pseudocore or well logging scale (left).

CT scans and microscale observations can help geoscientists predict attribute characteris-tics on a macroscale. The size, shape and height of the numerical pseudocore are limited only by the amount of computer memory that is avail-able. This allows researchers to quickly perform numerical experiments on large samples that could not be duplicated in a laboratory, given any amount of time or money.

Although formation evaluation techniques can readily distinguish sandstones from carbon-ates, the capability to identify and quantify dolo-mite in reservoir rocks poses a distinct challenge. While laboratory-based measurements may not address ongoing controversies regarding dolo-mite formation, they are able to accurately char-acterize the wellbore to provide valuable insights that will help E&P companies develop these notoriously heterogeneous reservoirs. —MV

> Flow simulation. These results have been produced after 0.72 pore-volumes of water were injected through a numerical pseudocore in an oil-wet dolomite. Bulk remaining oil saturation is 58%; water cut is 77%. Water is injected through the pseudocore from outside to inside. Colors represent oil saturations. Heterogeneity is obvious in the nonuniform breakthrough of water (B) shown in some parts of the flow pseudocore, whereas in other areas the flood front (F) has barely moved into the rock.

MattV_ORAUT09_Fig_17

Reduce image 75% after placeing

Mike- place 20-40-60-80-100% along the top.

F

0.17 Oil saturation 0.86

B

Dia

m

eter, 8

.5 in. [22 cm]

Diameter, 4 in. [10 cm]

1 ft

[0.

3 m

]

47. For more on core evaluation using X-ray computed tomography: Kayser A, Knackstedt M and Ziauddin M: “A Closer Look at Pore Geometry,” Oilfield Review 18, no. 1 (Spring 2006): 4–13.

48. Zhang T, Hurley NF and Zhao W: “Numerical Modeling of Heterogeneous Carbonates and Multi-Scale Dynamics,” paper JJJ, presented at the SPWLA 50th Annual Logging Symposium, The Woodlands, Texas, June 21–24, 2009.

49. Hurley NF and Zhang T: “Method to Generate Fullbore Images Using Borehole Images and Multi-Point Statistics,” paper SPE 120671, presented at the SPE Middle East Oil and Gas Show and Conference, Bahrain, March15–18, 2009.

26678schD5R1.indd 14 11/5/09 3:54 PM