Carbonate reservoir geology

Carbonate sediments are commonly formed in shallow, warm oceans either by direct precipitation out of seawater or by biological extraction of calcium carbonate from seawater to form skeletal material. The result is sediment composed of particles with a wide range of sizes and shapes mixed together to form a multitude of depositional textures. The sediment may be bound together by encrusting organisms or, more commonly, deposited as loose sediment subject to transport by ocean currents. A basic overview of carbonate-reservoir model construction was presented by Lucia, and much of what is presented herein is taken from that book.

Depositional classification of carbonates

Depositional textures are described using a classification developed by Dunham. The Dunham classification divides carbonates into organically bound and loose sediments (see Fig. 1). The loose sediment cannot be described in simple terms of grain size and sorting because shapes of carbonate grains can vary from spheroid ooids to flat-concave and high-spiral shells having internal pore space. The grain content of a grain-supported sediment composed of shells can be as little as 30% of the bulk volume because the shells occupy less space than spheroids. Loose sediment is, therefore, described on the basis of the concept of mud vs. grain support. Mud refers to mud-size carbonate particles, not to mud composed of clay minerals. Grain-supported textures are:

• Grainstone, which lacks carbonate mud
• Packstone, which contains mud

Mud-supported textures are referred to as:

• Wackestone, which contains more than 10% grains
• Mudstone, which contains less than 10% grains

To complete the description, generic names are modified according to grain type, such as:

• “Fusulinid wackestone”
• “Ooid grainstone”

• 

  Fig. 1– Dunham and Embry and Klovan classification of carbonate sediments.

Dunham’s boundstone class was further divided by Embry and Klovan because carbonate reefs are commonly composed of large reef-building organisms, such as corals, sponges, and rudists, which form sediments composed of very large particles. To describe autochthonous (in-place) boundstone reef material, they introduced the terms:

• Bafflestone
• Bindstone
• Framestone

To describe allochthonous, (transported) reef sediment with particles larger than 2 mm in diameter, the following terms are used:

• Floatstone
• Rudstone

Rudstone is grain-supported, whereas floatstone is mud-supported sediment.

Enos and Sawatsky measured the porosity and permeability of modern carbonate sediments.

• The average porosity and permeability of grainstone are approximately 45% and 10 darcies, respectively.
• The average porosity and permeability of a wackestone are approximately 65% and 200 md, respectively.

The higher porosity in mud-supported sediments is caused by the needle shape of small aragonite crystals that make up the carbonate mud, and the decrease in permeability is caused by the small pore size found between mud-sized particles. An important observation based on this data is that all carbonate sediments have sufficient porosity and permeability to qualify as reservoir rocks.

With modifications, the Dunham approach can be used to characterize the petrophysical properties of carbonate rocks. The classification must be modified, however, because diagenesis significantly alters the depositional texture, and a rock-fabric classification that incorporates diagenetic overprints and that can be linked to petrophysical properties is required. The classification proposed by Lucia is designed for this purpose (see Fig. 2). All pore space is divided into the following classifications:

• Interparticle (intergrain and intercrystal)
• Vuggy (pore space within grains/crystals and much larger than grains/crystals)

Interparticle pore space is classified with the Dunham classification approach. Instead of grain support vs. mud support, however, grain- and mud-dominated are used as a basic division. Grain-dominated fabrics include grainstone and grain-dominated packstone. Mud-dominated fabrics include mud-dominated packstone, wackestone, and mudstone. The packstone class is divided into grain- and mud-dominated packstone because the petrophysical properties of grain-dominated packstone are according to grain size, whereas mud size controls the properties of mud-dominated packstone. Diagenetic reductions in porosity by cementation and compaction are reflected in the amount of interparticle porosity.

• 

  Fig. 2 - Lucia classification of carbonate pore space.

Dolostones are classified similarly if the precursor limestone fabric can be determined. The principal petrophysical difference between limestones and dolostones occurs in mud-dominated fabrics. Limestone-mud-dominated fabrics have mud-sized particles (< 20 μm) and very small pores. Dolomitized mud-dominated fabrics have crystal sizes ranging from 10 μm or less to more than 200 μm, with corresponding pore sizes. Dolomitization must, therefore, improve reservoir quality by increasing particle and pore size.

The classification of vuggy pore space is an important aspect of rock-fabric classification that is not found in the classification of depositional textures. Vuggy pore space is divided into two groups on the basis of how the pore space is connected. Separate vugs are connected to each other through interparticle pore space, and touching vugs are connected directly to one another. Selective dissolution of grains, such as ooids or skeletal material, and intrafossil porosity are types of separate vugs. Because separate-vug porosity is poorly connected, it contributes less to permeability than would be expected if the porosity were located between the particles. Touching vugs are commonly formed by mass dissolution and fracturing. These processes can form reservoir-scale vuggy pore systems that dominate the performance of carbonate reservoirs.

Depositional environments

Carbonate sediments accumulate in depositional environments that range from tidal flats to deepwater basins. Most carbonate sediments originate on a shallow-water platform, shelf, or ramp and are transported landward and basinward. "Platform" is a general term for the shallow-water environment, whereas "shelf" and "ramp" refer to topography—shelves with flat platform tops and steep foreslopes and ramps having gently dipping platform tops and slightly steeper foreslopes.

The lateral distribution of depositional environments reflects:

• Energy levels
• Topography
• Organic activity

These changes can be related to the geometry of the carbonate platform. Ocean currents are produced by tides and waves and are concentrated at major topographic features, such as:

• Ramp and shelf margins
• Islands
• Shorelines

Grainstones and boundstones are concentrated in the areas of highest energy, commonly at ramp and shelf margins. Sediment is transported from the shelf edge onto the shelf slope and into the basin environment. This transport occurs primarily during highstand and results in progradation of the shelf margin. Calcareous plankton is deposited in the basinal environment as well. Sediment is also transported landward onto the shoreline, creating tidal-flat deposits that prograde, primarily during regression. Transgressive sediments are generally wackestones and mudstones at all locations because rising sea level typically creates a low-energy depositional environment.

The combination of organic activity, ocean currents, topography, and eustasy produces a typical facies progression from land to basin during highstand, as shown in Fig. 3:

• Peritidal
• Middle ramp
• Ramp crest
• Ramp shelf or slope
• Basin

The peritidal facies, composed of tidal-flat-capped cycles, normally defines the most landward position of an HFC. The cycles are formed by filling accommodation space and depositing sediment above sea level by transporting carbonate sediment onto the mud flat with tidal and storm currents. Tidal-flat sediments are key indicator facies because they define sea level. The tidal-flat environment is divided into the intertidal zone overlain by the supratidal zone. Sediment in the intertidal zone is characterized by sediment that is:

• Burrowed
• Pelleted
• Muddy

Algal laminates are concentrated at the boundary between the intertidal and supratidal zones. The supratidal zone is easily identified by its characteristic:

• Irregular lamination
• Pisolites
• Mud cracks
• Intraclasts
• Fenestral fabrics

The supratidal zone is sometimes called a “sabkha” environment, referring to the extensive evaporitic flats on the western shore of the Persian Gulf.

• 

  Fig. 3 – Generalized block diagram illustrating carbonate-ramp facies patterns and topography.

In arid climates, evaporite deposits may form by precipitation of gypsum (CaSO₄ •2H₂O) or anhydrite (CaSO₄) from evaporation of seawater trapped on or in the supratidal zone. Halite (NaCl) is normally found in isolated basins similar to the Dead Sea. Sulfate minerals are found as deposits in hypersaline lakes and as beds and crystals within the peritidal sediments. Sulfates found within carbonate sediments are properly classified as diagenetic minerals and cannot be used to describe the depositional environment, but sulfate deposited out of a standing body of water, is properly classified as sediment and is characteristic of the depositional environment as well as the climate. For sulfate to precipitate from seawater, three conditions must be met:

1. The body of seawater must be highly restricted from the ocean
2. The hypersaline water must be able to escape either by returning to the ocean or by seeping into the underlying sediment (seepage reflux), otherwise large volumes of Halite will precipitate forming a bed of salt
3. The climate must be sufficiently arid to allow the seawater to evaporate to at least one-third its original volume

The middle-ramp facies is characterized by quiet-water deposits typically composed of skeletal wackestones and mudstones. Burrowing organisms churn the muddy sediment and produce fecal pellets that, together with skeletal material, comprise the grain fraction of the sediment. During highstand, accommodation space may be reduced and water depth lessened to the point at which wave and storm energy increase, lime mud is winnowed out, and a packstone texture is produced. The increase in grain content, possibly capped by packstone, is used to define sea-level changes in this environment.

The ramp-crest facies is characterized by high-energy deposits, typically grainstones and packstones. The classic upward-shoaling succession of wackestone to packstone and grainstone typifies this environment. Typical high-energy deposits are as follows:

• Shelf-margin, tidal-bar, and marine-sand belts
• Back-reef sands associated with landward transport of sediment for fringing reefs
• Local middle-shelf deposits associated with gaps between islands or tidal inlets forming lobate tidal deltas

Packstones are typically churned by burrowing organisms and show no evidence of current transport. Grainstones are commonly crossbedded, often in multiple directions, indicating deposition out of tidal currents. Reefs are also found in the ramp-crest facies. The term reef has been much misused in the petroleum industry. At one time, all carbonate reservoirs were referred to as reefs, and the term is commonly used today to describe any carbonate buildup. However, the term should be restricted to carbonate bodies composed of:

• Bindstone
• Bafflestone
• Associated float- and rudstones

The outer-ramp, or slope, facies is formed by transport of shelf-margin and inner-shelf sediment onto the shelf slope. Sediments are typically wackestones and mudstones, along with occasional packstones and grainstones, in channels associated with density flows into the basin. On steep slopes, sediments may be dominated by sedimentary breccias and debris flows produced by the collapse of a steep shelf margin. The basin facies is typically composed of thin-bedded, quiet-water lime muds that contain planktonic organisms. Wackestones are often punctuated by debris and grain flows. Classic turbidite textures and cycles are also found in basinal carbonate deposits.

Diagenetic environments

Because all carbonate-reservoir rocks have undergone significant diagenesis, understanding their diagenetic history can be as important as understanding their depositional history. Modern carbonate sediments have sufficient porosity and permeability to qualify as reservoir rocks. Many ancient carbonates, however, lack the porosity and permeability needed to produce hydrocarbons economically. Loss of reservoir quality occurs when sediment changes after deposition. The processes that cause these changes are referred to as diagenetic processes, and the resulting fabric is often referred to as the diagenetic overprint.

Carbonate diagenetic processes include:

• Calcium-carbonate cementation
• Mechanical and chemical compaction
• Selective dissolution
• Dolomitization
• Evaporite mineralization
• Massive dissolution, cavern collapse, and fracturing

Whereas sedimentation is a one-time event, diagenesis is a continuing process, and diagenetic processes interact with one another in time and space. Thus, a sequence of diagenetic events may be extremely complicated and the pattern of diagenetic products difficult to predict if they are not related to depositional patterns.

The process of diagenetic overprinting of depositional textures must be understood to predict the distribution of petrophysical properties in a carbonate reservoir. To this end, diagenetic processes are grouped according to their conformance to depositional patterns.

The first group is made up of:

• Calcium-carbonate cementation
• Compaction
• Selective dissolution form

These processes have the highest conformance to depositional patterns.

The second group is made up of:

• Reflux dolomitization
• Evaporite mineralization

Although these processes depend greatly on geochemical and hydrological considerations, they are often predictable because they can be related to tidal-flat and evaporite depositional environments.

The third group is made up of:

• Massive dissolution
• Collapse brecciation and fracturing
• Late dolomitization form

These processes have the lowest conformance to depositional patterns, and their products are quite unpredictable.

The following processes can often be linked to depositional textures:

• Calcium-carbonate cementation
• Compaction
• Selective dissolution

Because calcium-carbonate cementation begins soon after deposition, it is often connected to the depositional environment. It continues as the sediment is buried, so the distribution of late cements is often unpredictable. Cementation fills pore space from the pore walls inward, reducing both pore size and porosity in proportion to the amount of cement. Compaction and associated cementation are a function of depositional texture and the time-overburden history. Compaction is both a physical and a chemical process resulting from increased overburden pressure caused by burial. Textural effects include:

• Porosity loss
• Pore-size reduction
• Grain penetration, breaking, and deformation
• Microstylolites

Compaction does not require the addition of material from an outside source and is often related to depositional textures. Experiments and observations have shown that mud-supported sediments compact more readily than those that are grain-supported.

Selective dissolution occurs when one fabric element is selectively dissolved in preference to others. Carbonate sediments are composed of three varieties of calcium carbonate:

• Low-magnesium calcite
• High-magnesium calcite (magnesium substituted for some calcium in the crystal lattice)
• Aragonite

Aragonite, in particular, is an unstable form and is rarely found in carbonate rocks. Grains composed of aragonite tend to be dissolved, and the carbonate is deposited as calcite cement. This distribution of aragonite grains can be predicted on the basis of depositional models.

Dolostone (a rock composed of dolomite) is an important reservoir rock. The composition of dolomite is CaMg(CO₃)₂ , and it is formed by replacement of calcite and by occlusion of pore space. In the following dolomitization equation, x = the amount of carbonate added to the rock in excess of the amount in the sediment.

....................(1)

A main source of magnesium is thought to be modified seawater circulating through the sediment in response to various hydrodynamic forces, including density, elevation, and temperature differences. Many pore volumes of dolomitizing fluid are needed to convert a limestone to a dolostone. Therefore, the hydrologic system must be understood for the distribution of dolostone to be predicted. The hypersaline reflux model can be used to predict dolomite patterns because it can be linked to an evaporitic environment. In an arid climate, seawater is trapped in tidal-flat sediment and hypersaline lakes and is concentrated through evaporation, producing a dolomitizing fluid. A hydrodynamic potential is created because the evaporated fluid is denser than seawater or groundwater and the tidal flats are at a slightly higher elevation than sea level. As a result, the hypersaline fluid will reflux down through the underlying sediment, converting it to dolomite. The geometries of dolostone bodies formed by this mechanism can be predicted if the distribution of evaporitic tidal-flat facies is known.

The hypersaline reflux model also accounts for the addition of CaSO₄, commonly an evaporite mineral in carbonate reservoirs. CaSO₄ is most commonly formed near the Earth’s surface in its hydrous form, gypsum (CaSO₄•2H₂O). However, at higher temperatures, the stable form is anhydrite CaSO₄, which is the form most commonly found in carbonate reservoirs. In some locations, tectonics has uplifted carbonate strata into a cooler temperature, and anhydrite has hydrated, forming gypsum.

Four types of anhydrite are commonly found in dolostone reservoirs. Pore-filling anhydrite is typically composed of large crystals filling interparticle and vuggy pore space. Poikilotopic anhydrite is found as large crystals with inclusions of dolomite scattered throughout the dolostone. They are both replacive and pore filling. Nodules of anhydrite are composed of microcrystalline anhydrite, often showing evidence of displacing sediment. They make up a small percentage of the bulk volume and have little effect on reservoir quality. Bedded anhydrite is found as beds composed of both coalesced nodules and laminations. Anhydrite beds are flow barriers and seals in reservoirs.

The most unpredictable diagenetic processes are:

• Massive dissolution
• Collapse brecciation and fracturing
• Late dolomitization

Massive dissolution refers to nonfabric selective dissolution, including:

• Cavern formation at any scale
• Collapse brecciation and fracturing
• Solution enlargement of fractures
• Dissolution of bedded evaporates

This process is thought to be most commonly related to the flow of near-surface groundwater, referred to as meteoric diagenesis but often included under the general heading of karst. The products of this diagenetic environment are controlled by precursor diagenetic events, tectonic fracturing, and groundwater flow and show little relationship to depositional environments. Reservoirs of this type are, therefore, difficult to model.

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See also

Reservoir geology

Siliciclastic reservoir geology

PEH:Reservoir_Geology