Ahr W1 H. Geologyof Carbonate Reservoirs

Ahr W1 H. Geologyof Carbonate Reservoirs

(Parte 7 de 8)

Rounded and abraded

Subequal spar and lime mud

Over 2/3 lime mud matrix

0 - 1%1 - 10%10 - 50%Over 50% Over 2/3 spar cement

MicriteSparry calcite cement
Figure 2.7R. J. Dunham ’ s classifi cation of carbonate rocks. Note that the classifi cation

includes detrital carbonates as mudstones through grainstones, biogenic (reef) carbonates as boundstones, and diagenetically altered carbonates as crystalline carbonate. (Adapted from the classifi cation scheme illustrated in Dunham (1962) .)

Depositional Texture Recognizable Original Components Not Bound Together During Deposition

Contains mud (particles of clay and fine silt size, less than 20 microns) Grain-supported

Mud-supported Grain-supported

Less than 10 percent grains

More than 10 percent grains

More than * 10 percent mud

Less than * 10 percent mud

Mudstone Wackestone Packstone Grainstone Boundstone

Crystalline Carbonate

Depositional Texture Not Recognizable

* Modification of original Dunham classification by changing percent mud from 1 to 10%

Original components were bound together during deposition, as shown by intergrown skeletal matter, lamination contrary to gravity, or sedimentfloored cavities that are roofed over by organic or questionably organic matter and are too large to be interslices.

(Subdivide according to classifications designed to bear on physical texture or diagenesis.)

CLASSIFICATION OF CARBONATE ROCKS 25

26 CARBONATE RESERVOIR ROCK PROPERTIES

importance, their petrophysical attributes
Figure 2.8The skeletal reef classifi cation of Embry and Klovan (1971) . Note that there is
mounds. ” Dunham (1970) and Ahr (1971)addressed the use of the term “ reef ” for nonskeletal

depositional rock properties and fractures or diagenetic attributes. For example, dolomicrites are more brittle than pure lime micrites and fracture more readily; therefore fracture intensity should be greater in the former than in the latter. Metastable (aragonitic or Mg calcitic) grains are more susceptible to diagenesis than stable calcitic grains so that porosity may be the result of selective removal, recrystallization, or replacement of original minerals. We will see that several carbonate porosity classifi cations do not include the mode of origin of rocks; consequently, it is diffi cult to use those classifi cations to distinguish pore types that formed as results of depositional processes from those that were modifi ed or created by postdepositional diagenesis or fracturing. It is equally diffi cult to distinguish fl ow units by their geological origin, without which it is diffi cult if not impossible to predict their spatial distribution at stratigraphic scale. This book presents a genetic classifi cation of porosity linked to the complete geological history of reservoir rocks as an alternative. It is based on the idea that there are three end - member pore types in carbonate reservoirs: depositional, diagenetic, and fracture pores. These different processes impart distinctive characteristics to both rock matrix and pores. Because the distinctive characteristics were imparted to pores and rocks at the same time and by the same processes, key rock properties may act as “ markers ” or proxies for pore types that can be identifi ed and traced at stratigraphic scale. To the extent that the proxies are identifi able and mappable, so will be the accompanying pore types and, of capital no provision for nonskeletal mounds such as microbialite buildups, “ mud mounds, ” or “ algal buildups and Riding (2002) developed a classifi cation for various nonframebuilt mounds. (Adapted from an illustration in Tucker and Wright (1990) .)

Allochthonous Autochthonous

Matrix supported

Supported by >2 m component

By organisms that act as baffles

By organisms that encrust and bind

By organisms that build a rigid framework

Floatstone Rudstone Bafflestone Bindstone Framestone

> 10% grains > 2 m

Original components not organically bound during deposition

Original components organically bound during deposition

2.3.1Classifi cation of Detrital Carbonates

The Folk (1959, 1962) and Dunham (1962) classifi cations work well for detrital carbonates but they are not as useful to describe reef rocks or diagenetic textures and fabrics. Folk and Dunham coined words to describe reef rocks. Folk chosebiolithite and Dunham choseboundstone , but those terms treat all reefs alike and as if the entire reef mass were homogeneous. Porosity and permeability vary greatly in reefs depending on the type of reef organisms present, the reef growth forms, the ratio of skeletal framework to loose detritus, and the internal microstructure of the reef - building organisms. Embry and Klovan (1971) and Riding (2002) devised reef classifi cations that include more detailed systems for describing the variability found in reef reservoirs. Diagenetic properties are not included in the Folk and Dunham schemes either, except that Dunham included the termcrystalline carbonate for a rock in which depositional texture is unrecognizable because it was obliterated by diagenesis.

The Folk and Dunham classifi cations share a common theme. They are based on the mud - to - grain ratios in carbonate rocks and on the packing arrangement of the framework grains. These similarities exist because Dunham and Folk shared discussions on carbonate classifi cations (Folk, 1962 ) and how the concept of textural maturity used for terrigenous sandstones could be applied to carbonates. Textural maturity in terrigenous sandstones refers to the amount of matrix (clay or mud) that has been removed by winnowing and the extent to which sorting and rounding are visible in framework grains. When applied to carbonates, rocks with high lime mud content (more than 90% lime mud) are classed asmudstones by Dunham and asmicrites by Folk. Rocks with only grains and no mud are classed as sparites by Folk andgrainstones by Dunham. The term sparite implies that sparry cement occupies intergranular pores. Unconsolidated sediments were excluded by Folk because his scheme was devised for lithifi ed limestones. Between high and low mud content are the carbonates with variable proportions of mud and grains. Folk chose the terms “ sparse ” and “ packed ” to modify micrites with 10% – 50% and over 50% grains, respectively. By doing so, he set the requirement that grain percentage determines the rock name. Dunham chose a different approach, probably because he saw that less than 50% of irregularly shaped grains could create a self - supporting fabric; hence the origin of the termpackstone . He resorted to grain percentage as the determining factor for naming muddy rocks with grains but without a self - supporting grain fabric. These muddy, grainy mixtures in which the grains are widely dispersed ( “ fl oating ” ) in the mud matrix are termed wackestones . Most industry professionals use the Dunham classifi cation today because the terms are shorter, and easier to log when working on large quantities of rock, they do not require tedious counting or percentage estimates, and they seem to evoke mental images of rock properties that can be related to reservoir properties.

For the reservoir analyst, detrital rock classifi cations based on depositional texture are the most practical and easiest to use. First, the muddy rocks can be assumed to have formed in environments where winnowing was insignifi cant and rocks with high grain content represent environments with extensive winnowing, the “ high - energy ” environments. Second, excluding diagenesis, fracturing, and special forms of intragranular porosity, mud content is inversely related to intergranular porosity. Grainstones and packstones have the highest percentage of

CLASSIFICATION OF CARBONATE ROCKS27

28CARBONATE RESERVOIR ROCK PROPERTIES depositional porosity, they usually have comparatively simple intergranular pore systems, and porosity is predictably related to facies geometry. However, because they may have high intergranular porosity, they are susceptible to early cementation and compaction that reduce pore and pore throat size. As rocks with high grain content commonly occur near the tops of shallowing - upward cycles, they are relatively easy to locate in repetitive sequences of these cycles. Some cycles terminate in evaporite “ capping facies ” with pores in the underlying grainstones and packstones plugged with gypsum, anhydrite, or halite. In those cases, diagenesis sometimes compensates for pore plugging at the cycle tops, because dolomitization commonly accompanies evaporite formation and it may be linked with enhanced porosity in midcycle wackestone and packstone facies.

2.3.2Classifi cation of Reef Rocks

The word “ reef ” still prompts animated discussion and disagreement among geologists. Much of the older terminology on reefs centers on the academic issue of whether reefs are “ ecological ” or “ stratigraphic, ” as described by Dunham (1970) . An ecological reef is built by constructor organisms that have the “ ecological potential ” to form wave - resistant frameworks. That is, they must be made of sturdy skeletal structures that grew presumably in the midst of breaking waves. Many reefs throughout time grew in environments that were not exposed to breaking waves and many biogenic buildups lack sturdy skeletal frameworks, especially buildups constructed of micrite, or carbonate cement, or microbial thrombolites and stromatolites. Terminology is not a major issue for reservoir studies or for carbonate sedimentologists who follow the more modern style of classifying all sturdy skeletal buildups asframe - built , or skeletal reefs , and all of those buildups without sturdy skeletal frameworks asreef mounds (Tucker and Wright, 1990 ). However, additional detail is needed in reef classifi cation schemes to describe the fundamental rock properties of the reef as they relate to reservoir porosity, permeability, and connectivity.

Reservoir characteristics in reefs vary with the type of constructor organisms, with the relationship between constructor organisms and associated reef detritus, and with growth patterns of reef complexes in response to prevailing hydrologic conditions. Reefs built by calcifi ed microbes, for example, have high proportions of lime mud and cement but few skeletal framebuilders. Skeletal framework reefs, reefs built up as repeated layers of pavement - like organic encrustations, and reefs formed by the current - baffl ing and sediment - trapping action of benthic organisms such as sea grasses and algae present unique rock fabrics and pore characteristics. Dense encrustations by calcareous algae exhibit internal microstructures that differ from those of porous sponge or coral skeletons. Patterns of reef growth vary in response to the depth of the photic zone, to oxygenation and nutrient content, to turbidity, and to water agitation by waves and currents. For example, modern corals grow in sheet - like or dome - like fashion in deeper water because they need light for their photosynthesizing symbionts, the zooxanthellae. Stromatoporoids, major constructor organisms in Silurian and Devonian reefs, took on specifi c growth forms in response to higher or lower levels of wave and current activity.

Facies patterns associated with reefs vary as a function of the hydrologic regime. Shallow - water (in most modern oceans this is less than about 10 - m depth) reefs have distinct seaward and leeward sides because they are shaped or “ polarized ” by the prevailing wind – wave direction. Windward sides of reefs are characterized by massive and encrusted organic growth with boulder - to - gravel - sized particles as rudstones. Leeward sides of reefs are characterized by higher percentages of detrital sediments such as fl oatstones, grainstones, and more delicate growth forms of reef organisms. Shallow - water patch reefs and shelf - edge reefs tend to be streamlined in plan view with buttress - like structures and more massive skeletal frameworks on the windward side. In modern coral – algal reefs, structures called spur and groove orbuttress and chute develop on the windward sides of reefs (Shinn, 1963 ; James, 1983 ). Deep - water buildups, or those that grew in protected shallows, do not exhibit windward and leeward sides, streamlining, or polarized facies geometry. In short, reservoir characteristics of reef rocks are related to variables different from those that form the basis for detrital rock classifi cations. Embry and Klovan (1971) found it hard to map reef reservoir porosity zones with nothing more defi nitive than Dunham ’ s boundstone or Folk ’ s biolithite terminology. They cited papers from the 1960s focusing on, if not lamenting, the problem. As an improvement, they adapted Dunham ’ s classifi cation to reef reservoirs and developed a more detailed scheme to account for different organic growth forms and for the associated detrital carbonates that surround and fi l - in open spaces within reefs.

The Embry – Klovan terms framestone , bindstone , andbaffl estone refer to growth patterns of reef organisms. That is, a reef constructed of stout coral skeletons in a girder - like frame arrangement is called a framestone. One in which the reef is constructed by pavement - builders or encrusters is known as a bindstone, and reefs that exhibit detrital carbonate accumulations in the midst of organic thickets such as seagrass beds are known as baffl estones. There is some controversy over whether reefs can truly be formed by baffl ing action of sessile benthonic organisms because sessile benthonic animals are mainly fi lter feeders that would be smothered by a rain of carbonate sediment. If the baffl ers were plants, true grasses can be eliminated for much of geological history, as they have existed only since the Mesozoic Era. Baffl estones tend to have high proportions of lime mud, and there is evidence, especially in mud mounds of Lower Carboniferous age, that muds are formed in place by biological or biochemical processes rather than having been trapped by organisms. The term baffl estone is considered by Tucker and Wright (1990) to be “ rather subjective. ” Those authors also point out, that reef rocks are subject to extensive diagenesis and bioerosion that may dramatically alter the original rock fabric. Diagenetic micritization of reef rocks is common and may account for the loss of 20 – 70% of the original reef framework (Tucker and Wright, 1990 ). Finally, the Embry – Klovan terms rudstone and fl oatstone refer to detrital rocks associated with reefs. Rudstones are the reef - derived, gravel equivalent of grainstones and packstones; fl oatstones are the gravel and sand equivalent of wackestones.

Riding (2002) developed an alternative classifi cation for reef rocks. He defi nes reef as “ in - place calcareous deposits with topographic relief, created by sessile organisms. ” The different types of reef rocks are classifi ed on the basis of whether there is matrix (mud) support (thecarbonate mound category), skeletal support (the frame reef category), or cement support (the cement reef category). Terms such as sparse and dense are used to describe the three - dimensional fabric of skeletal elements in matrix supported reefs, and open, tight, and solid describe the architecture of the constructor assemblages in frame reefs. These terms refer to spatial patterns

CLASSIFICATION OF CARBONATE ROCKS29

30CARBONATE RESERVOIR ROCK PROPERTIES and evoke mental images of framework/detritus ratio, which translates into type and spatial distribution of pore categories, assuming that diagenesis has not radically altered them. It is reasonable to infer that depositional porosity and permeability are highest in frame reefs and lowest in micrite mounds and cement reefs. Reservoirs exist in those reef categories but usually owe their existence to enhanced porosity and permeability formed by diagenesis or fracturing. Predicting reservoir connectivity is diffi cult in all reef categories but is especially hard to predict in diagenetically altered, complex mixtures of frame and detritus. Diagenetic porosity may be strongly bimodal in size. Microporosity is common in lime mud portions of reef rocks, for example.

2.3.3Wright’s Genetic Classifi cation

grated, genetic classifi cations

The Folk and Dunham classifi cations for detrital carbonates were introduced nearly a half - century ago when our understanding of diagenetic processes and their products was in its infancy and before much effort was made to develop classifi cations for reef rocks. Recognizing these defi ciencies in descriptive terminology, Wright (1992) proposed an integrated scheme that links the depositional classifi cation of Dunham (1962) and the biological classifi cation of Embry and Klovan (1971) with a new classifi cation for diagenetic rocks. In concept, this classifi cation is logical and more utilitarian for the reservoir geoscientist than any existing single classifi cation. This is agenetic classifi cation system in which carbonate rocks are grouped by mode of origin — depositional, biological, and diagenetic. Each category has subheadings to distinguish the various rock properties that typify each mode. The terms introduced for diagenetic carbonates draw attention to whether or not the diagenetic process has obliterated the original texture and fabric. This distinction requires examination and interpretation of thin sections under the polarizing microscope, however. In addition, the terms for compacted rocks with microstylolitic grain contacts should include packstone along with grainstone. It is diffi cult to have a perfect classifi cation for all applications, but this genetic scheme represents an advance. Later in this chapter we will discuss classifi cations of porosity and we will see that purely descriptive classifi cations of porosity, like nonintegrated classifi cations for carbonate rocks, may be less useful in analyzing carbonate reservoirs than inte-

2.4DEPENDENT OR DERIVED ROCK PROPERTIES

Porosity, permeability, and bulk density depend on fundamental properties such as texture, mineralogical composition, and fabric. Dependent properties, especially porosity and permeability, are among the most important variables that determine reservoir quality. While rocks are classifi ed according to their fundamental properties and inferences are made from rock classifi cations about depositional environments, porosity is classifi ed according to physical attributes that may not be related to mode of origin. But unless mode of origin is included in porosity classifi cations, it is not possible to deduce the environment in which the porosity was formed, when it was modifi ed, and which genetic pore types correspond to highest permeability. Comprehensive reservoir description depends on identifi cation and description of correspondence between rock matrix and pore characteristics, how they are genetically and temporally related, and how they infl uence petrophysical attributes. Porosity is measured directly from core samples and indirectly with some types of borehole logs. Permeability is measured as the coeffi cient of proportionality in Darcy ’ s equation for fl uid fl ow through porous media. It is measured directly from core samples and it is the yardstick by which many quality rankings are assigned to reservoirs. Special wireline testers and pressure buildup tests can measure fl ow rates and provide meaningful estimates of permeability and petrophysical experts argue that permeability can be estimated from wireline log data. Not everyone agrees, especially those who work on carbonate reservoirs. Bulk density is a measure of the solid/void ratio in reservoir rocks and is measured directly in core analyses or indirectly with wireline logs. Bulk density values can be used to aid in estimating porosity.

2.4.1 Porosity

(Parte 7 de 8)

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