Carbonate reservoirs pdf

We cannot guarantee that Carbonate Reservoirs Porosity Evolution and Diagenesis in a Sequence Stratigraphic Framework book is available in the library, click Get Book button to download or read online books. Join over The articles included explore the challenges associated with using well-log data, core data analysis, and their integration in the qualitative and quantitative assessment of.

The 2nd Edition of Carbonate Reservoirs aims to educate graduate students and industry professionals on the complexities of porosity evolution in carbonate reservoirs.

In the intervening 12 years since the first edition, there have been numerous studies of value published that need to be recognized and incorporated in the topics discussed. An accessible resource, covering the fundamentals of carbonatereservoir engineering Includes discussions on how, where and why carbonate areformed, plus reviews of basic sedimentological and stratigraphicprinciples to explain carbonate platform characteristics andstratigraphic relationships Offers a new, genetic classification of carbonate porosity thatis especially useful in predicting spatial distribution of porenetworks.

One main target in petroleum recovery is the description of the three-dimensional distribution of petrophysical properties on the interwell scale in carbonate reservoirs. Doing so. This second volume on carbonate reservoirs completes the two-volume treatise on this important topic for petroleum engineers and geologists. Together, the volumes form a complete, modern reference to the properties and production behaviour of carbonate petroleum reservoirs.

The book contains valuable glossaries to geologic and petroleum engineering terms providing exact. Eberli,Jose Luis Masaferro,J. The porosity of carbonates as compared to sandstones is vastly more complex with simple intergrain porosity dominates sandstones while carbonates commonly exhibit complex secondary pore systems that may evolve during burial.

Initial porosity of carbonates is much greater than that seen in sandstones due to common intragranular porosity. Note that the clasts are aligned in a fabric created by currents at the time of deposition. The width of the photo is 4 mm. The width of the photo is 2. The long axis of each pellet is about 2 mm. Grabau, The scale of these ripples varies from a few millimeters to tens of centimeters in crest-to-trough height and a few centimeters to a meter or more in crest-to-crest wavelength depending on the size and period of the waves that formed them.

Note that these ripples produce trough crossbeds visible in the lower right face of the diagram. Current direction is from left to right. Note the sinu- soidal shape of the advancing ripple crests and the scoured depressions in front of them. Parts a and c adapted from illustrations in Reineck and Singh ; part b adapted from an illustration in McKee He did not include a term for completely recrystallized or replaced rocks. Adapted from the illustration in Folk Depositional Texture Recognizable Depositional Original Components Not Bound Together During Deposition Texture Not Original components were bound together Recognizable Contains mud particles of clay and during deposition, as Grain-supported Subdivide according fine silt size, less than 20 microns shown by intergrown skeletal matter, to classifications lamination contrary to designed to bear Mud-supported Grain-supported on physical texture gravity, or sediment- floored cavities that are or diagenesis.

In order for that to be possible, reservoir and rock properties must have characteristics in common. Adapted from an illustration in Tucker and Wright 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. Meta- stable aragonitic or Mg calcitic grains are more susceptible to diagenesis than stable calcitic grains so that porosity may be the result of selective removal, recrys- tallization, or replacement of original minerals.

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. Folk and Dunham coined words to describe reef rocks. Folk chose biolithite and Dunham chose boundstone, 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.

Diagenetic properties are not included in the Folk and Dunham schemes either, except that Dunham included the term crystalline carbonate for a rock in which depositional texture is unrecognizable because it was obliterated by diagenesis. They are based on the mud-to-grain ratios in carbonate rocks and on the packing arrangement of the framework grains. 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.

Rocks with only grains and no mud are classed as sparites by Folk and grainstones by Dunham. The term sparite implies that sparry cement occu- pies intergranular pores. Between high and low mud content are the carbonates with variable proportions of mud and grains.

By doing so, he set the requirement that grain percentage deter- mines the rock name. He resorted to grain percentage as the determining factor for naming muddy rocks with grains but without a self- supporting grain fabric. Second, excluding diagenesis, fracturing, and special forms of intragranular porosity, mud content is inversely related to inter- granular porosity.

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 rela- tively easy to locate in repetitive sequences of these cycles. In those cases, diagenesis some- times 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.

That is, they must be made of sturdy skel- etal 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 stro- matolites. 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 as frame-built, or skeletal reefs, and all of those buildups without sturdy skeletal frameworks as reef mounds Tucker and Wright, 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.

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. Facies patterns associated with reefs vary as a function of the hydrologic regime.

Windward sides of reefs are characterized by massive and encrusted organic growth with boulder-to-gravel-sized particles as rudstones. 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 or buttress and chute develop on the windward sides of reefs Shinn, ; James, Deep-water buildups, or those that grew in protected shallows, do not exhibit windward and leeward sides, streamlining, or polarized facies geometry.

They cited papers from the s focusing on, if not lamenting, the problem. That is, a reef constructed of stout coral skeletons in a girder-like frame arrangement is called a framestone. Terms such as sparse and dense are used to describe the three-dimensional fabric of skeletal ele- ments in matrix supported reefs, and open, tight, and solid describe the architecture of the constructor assemblages in frame reefs.

It is reasonable to infer that depositional porosity and permeability are highest in frame reefs and lowest in micrite mounds and cement reefs.

Reser- voirs exist in those reef categories but usually owe their existence to enhanced porosity and permeability formed by diagenesis or fracturing.

Diagenetic porosity may be strongly bimodal in size. Microporosity is common in lime mud portions of reef rocks, for example. Each category has subheadings to distinguish the various rock properties that typify each mode. The terms intro- duced 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 con- tacts should include packstone along with grainstone. Dependent properties, especially porosity and permeability, are among the most important variables that determine reservoir quality.

Poros- ity is measured directly from core samples and indirectly with some types of borehole logs. It is measured directly from core samples and it is the yardstick by which many quality rankings are assigned to reservoirs. Not everyone agrees, especially those who work on carbonate reservoirs. Bulk density values can be used to aid in estimating porosity. Effective porosity is the ratio of the interconnected pore volume to the total rock volume.

Direct measurements of Vp in the laboratory are measurements of effective porosity. Not all pores are interconnected, however. Porosity varies with texture, fabric, and fracture geometry in the reservoir rock. Diagenesis may plug pores with cement, close pores with compaction, open pores with dissolution, or create new pores by recrystallization or replacement.

Depositional pore size is a function of grain size, packing, and sorting. High porosity grainstones with cubic packing and near-spherical grains will have pore sizes in the 0. Porosity does not vary with grain diameter, but it does vary with packing and sorting.

Cubic and rhombohedral packing arrangements for idealized, spherical grains have If a second size of spherical grains is introduced, that is, if sorting is poor Figure 2. Grain shape also affects porosity. A comparatively small number of irregularly shaped grains, Cubic packing Rhombohedral packing Figure 2. Cubic packing of spherical grains has In the cubic packing example it is clear that each pore is connected to others by three pore throats.

Later this will be known as a pore system with a coordination number of 3. When a second, smaller but uniform grain size is introduced, the original As we have seen earlier in this chapter, reef growth form and skeletal microstructure are types of biogenic rock fabric that have a major effect on effective porosity.

Some intraparticle pores are, in fact, totally disconnected. They make up part of the residual porosity in a reef reservoir. Conversely, if large interskeletal and intraskeletal pores are present and connected, a reef reservoir may have very high effective porosity.

Porosity reduction is complex and can involve cementation, compaction, or combinations of the two. Some studies show that porosity in carbonate reservoirs is reduced by a factor of 2 during burial to a depth of m and that burial depth has a greater effect on porosity reduction than the amount of time during burial Schmoker and Halley, They also found that porosity in dolos- tones was lower than that of limestones near the surface, but greater than limestones at depths greater than m, and that the rate of decrease in dolostone porosity was less than for limestones with increasing burial depth.

He also found that low permeability, lime-muddy rocks with median permeability values of 35 md millidar- cies or less did not show a clear trend of permeability change with depth, but limestones with median permeability of 69 to over md showed a clear trend of decreasing permeability with increasing depth of burial.

He concluded that the best limestone reservoir rocks in his study were those with grain-supported textures and higher permeability before burial. He found that depth-related permeability loss was due mainly to mechanical compaction in shallower depths and to chemical plus mechanical compaction at greater depths. For limestone reservoirs in general, poros- ity and permeability loss due to cementation is probably an early diagenetic phe- nomenon.

More pronounced porosity and permeability loss with depth is caused by mechanical and chemical compaction. Amthor et al. They concluded that dolostones undergo less porosity and permeability loss with depth than limestones because dolostones are more resistant to chemical and mechanical compaction than limestones. If one rock is composed of spherical, well-sorted grains of a given size, intergranular pores will be only a fraction of the grain size and the attendant pore throats will be even smaller but generally uniform in size.

Rocks with poorly sorted grains will have poorly sorted pore and pore throat sizes. Imagine molecules of oil migrating through pores and pore throats. If pore throat sizes are uniformly distributed, the oil molecules will encoun- ter more-or-less the same size pathway for migration and the threshold force pres- sure required to move through the rock will be more-or-less uniform for any given pore throat size.

If the pore throats are poorly size-sorted, the migrating oil mole- cules will encounter different sized pathways with different pressure thresholds to pass through.

In most terrigenous sandstones and in some carbonate grainstones, intergranular pore sizes have a strong statistical correlation to grain size. In such cases, pore throat sizes may correlate well with pore sizes, as illustrated by a plot of the log of permeability as a function of porosity.

Some schemes aid in interpreting the origin of pore types. However, a weakness in much of our traditional approach is that we treat rocks and reservoirs as separate entities. A simple method for grouping pore characteristics is all that is required for a basic reservoir rock description.

Gus Archie , who pioneered the study of electrical resistivity in rocks, developed the principles that led to the Archie saturation equation, and investigated methods to integrate geological data with laboratory petrophysical data and bore- hole log signatures. His objective was to illustrate relationships between rock and petrophysical properties in reservoirs. Class D includes large visible pores such as solution vugs larger than cuttings samples.

The Solenhofen Limestone is a good example of this type of rock. He avoided terms that denote rock composition or that suggest a geological origin for the porosity. He did not consider it important to identify the geo- logical thread that binds pore origin to rock origin, although many carbonate pore types are altered or created long after the host rock was deposited.

Performing these tasks requires data on how porosity relates to other rock properties that serve as proxies or markers for effective porosity. They recognized 15 basic pore types and organized them into three classes depending on whether they are fabric selective, not fabric selective, and fabric selective or not. Intergranular pores in an oolite grainstone, intercrystalline pores in a crystalline dolomite, or grain-moldic pores in a skeletal packstone are examples of fabric-selective pores that have different origins.

Non-fabric-selective porosity includes fractures or dis- solution cavities of various sizes that cut across rock fabric. Fabric selective or not is a category that includes mainly penetrative features such as animal or plant borings and burrows and desiccation cracks. Breccias may be conglomeratic residue from solution collapse, the products of clast-producing erosion followed by resedi- mentation, or the result of tectonism.

Times of origin are designated as primary and secondary. Primary origin includes pores formed by depositional and early postdepositional processes. Adapted from an illustration by Choquette and Pray in Scholle Mesogenetic changes are those unaffected by surface condi- tions or processes.

In the end, one can represent indi- vidual pore categories by a code string that names the pore and indicates the degree of fabric selectivity, pore size, direction of diagenetic change, and estimated abundance. Since the work by Choquette and Pray , the literature on carbonate dia- genesis has grown and terms such as eogenetic, telogenetic, and mesogenetic are not widely used. Above the water table is the vadose zone, below the water table is the phreatic zone, and below the phreatic zone is the subsurface burial zone.

Qualitatively and subjectively, the burial domain can be divided into shallow- and deep-burial environments with the shallow-burial environment differentiated from the overlying meteoric phreatic zone by its different water chemistry and somewhat greater overburden pressure and temperature.

Effects of pressure can be interpreted from the style of grain contacts, grain breakage, and stylolitiza- tion. Exotic crystal habits can also indicate high temperatures. Saddle crystal dolomite usually indicates deeper burial conditions. Normally, fractures cut across depositional and diagenetic fabrics, but in the case of some dolomite—limestone rock combinations, dolomite may fracture selectively because it behaves as a more brittle material than limestone.

As discussed earlier, rock fabric may represent mechanical sedimentation, biological growth processes, or diagenetically produced crystallinity. As such, they can appear at certain positions in stratigraphic cycles where the occurrences of facies-selective porosity are more predictable.

Fabric-selective porosity may not be mappable at reservoir scale, especially if it does not conform to facies boundaries as is commonly the case with diagenetic porosity. Vugs are pores larger than surrounding framework grains. Dissolution does not follow a predictable pattern in most cases; con- sequently, the size, shape, and spatial distribution of vugs may be quite irregular.

They may begin as fabric-selective dissolution or non-fabric-selective enlargement of fractures by leaching. Flow between separate vugs has to pass through matrix porosity and permeability to drain the vugs; therefore the contribution of separate vugs to total reservoir porosity and permeability can be estimated if matrix charac- teristics and total porosity are known.

Clearly, the only way to obtain that kind of information is by direct observation of rock samples—such as cores—that are large enough to display vugs that may be centimeter scale in size.

Because most vugs, particularly touching vugs, are larger than rotary drill cuttings, they may be overlooked during sample examination, which again emphasizes the importance of examining full-diameter cores when working with carbonate reservoirs. The relationships between porosity, permeability, and particle characteristics were further investigated by Lucia and expanded upon in his book Carbonate Reservoir Charac- terization.

Especially useful are the discussions on petrophysical attributes of the different rock and pore types. High correspon- dence between porosity and permeability alone does not offer clues for ways to correlate pore types at reservoir scale.

That information must come from rock properties that co-vary with porosity. They co-vary because the rock properties and pore types were formed at the same time by the same geological processes. It is those signatures—the rock or stratigraphic charac- teristics—that can be correlated at reservoir scale. That is a big order. Rock texture, fabric, and mode of pore origin are obtained from petrographic study of samples, usually from cores.

Because cores reveal fundamental rock properties of larger scale than cuttings, cored intervals can be correlated directly with borehole logs and with the stratigraphic column.

Cores provide a greater volume of rock for more representative measurements of petrophysical properties in reservoirs that have widely varying pore categories and pore sizes. Without rock samples, pore characteristics remain unseen, and their origin, geometrical properties, and distribution within the rock remain unknown. Petrophysical attributes of different pore types are not visible in thin sections or on sample surfaces but can be determined from capillary pressure measurements that provide information about pore throat size distribution and aperture size sorting.

If pores and rock matrix formed contemporaneously, as in reservoirs with purely depositional porosity, the common and synchronous origin of rocks and pores allows depositional facies to become proxies for porosity. Pores not formed contemporaneously with deposition, as in diagenetic and fracture poros- ity, must be interpreted differently. Those traces are geological clues that can help in correlating genetic pore types from borehole to borehole.

Relative timing of pore origins is established by interpreting cross-cutting diagenetic features or frac- tures. It is intuitive to plot those processes as end members on a triangular diagram Figure 2.

Pores associated with mechanically sedimented detrital deposits will conform to original grain texture and fabric such that depositional facies maps are proxies for reservoir porosity maps. If more than about half of pores visible in thin section are determined to have been altered by diagen- esis diagenetic attributes dominate , the types of diagenesis that created the hybrid pores must be determined because depositional facies are less reliable as proxies for porosity and facies maps will not be reliable guides to the spatial distribution of porosity at reservoir scale.

In this case, it is necessary to determine the types of dia- genesis that caused the alteration and at what times diagenesis was active during the burial history of the rocks. Diagenesis alters depositional porosity by dissolution, cementation, compaction and pressure solution, recrystallization, and replacement. It may enhance or reduce original porosity or it may create totally new pore types. Pores are created by three end-member processes that include depositional, diagenetic, and fracture mechanisms.

The end-member processes are independent but hybrid pore types exist between them because more than one mechanism can affect the formation of a given pore system at different times during its genetic history.

Complete porosity analysis must include the total amount percent of porosity present, and ideally the amounts of separate versus touching vugs.

Alternatively, diagenesis may enhance or create new porosity by dissolution or by some types of replacement or, rarely, by recrystalliza- tion.

For example, dissolution creates caves, connected vugs, and karst features including solution-collapse breccias that do not correspond to depositional facies boundaries. Instead, they may correspond to positions of ancient water tables, positions on antecedent structure, or to the locations of ancient mixing zones. Diagenetic-fracture hybrids are those fractures that form preferentially in rocks with diagenetically altered mineralogy and texture.

Dolostones behave as more brittle material than limestones such that fracture intensity is higher in dolostones than in limestones with the same crystal size and bed thickness. It varies with the mechanical properties of the rock and the magnitude, type, and direction of the differential stresses. Mechanical behavior of rocks can be grouped into a variety of classes, three of which are the most common in most situations: brittle, ductile, and plastic.

Brittle behavior is associated with fractures, faults, and joints. It occurs when the elastic limit of the brittle rock is exceeded and failure by rupture— brittle failure—occurs. Ductile behavior can be modeled by a soft metal rod e. The center of the rod continually becomes thinner and thinner under stress until it fails. Plastic behavior can be imagined as the behavior of bread dough or putty. Plastic deformation requires little stress to start deformation and once it begins, it continues with little additional stress.

Ductile and plastic behaviors are not generally associated with fracture porosity. Most fracture porosity is associated with tectonic fractures, as will be discussed in Chapter 7. Fractures occur in predictable patterns and orientations on faults and folds, making it possible to estimate the extent and orientation of fractures in reservoirs associated with such tectonic features.

However, there are special problems with fractured reservoirs that will be discussed later. Characteristics of depositional, diagenetic, and fracture porosity are discussed in more detail in Chapters 5, 6, and 7, respectively. Their purpose was to explain these principles as aids to planning and managing water distribution for Dijon and other cities in France Darcy, The experiments involved pure water and atmospheric pressure such that the principal variables were sand and gravel textural characteristics.

Flow rates and pressure differences were small in the origi- nal Darcy—Ritter experiments as compared to those in hydrocarbon reservoirs. Today the Darcy—Ritter expression is written with different letter designations for parameters and measurements than in their paper but the method and the outcome are unchanged. Consider a laboratory apparatus Figure 2.

Under steady-state conditions, the upstream pressure is P, and the downstream pressure is P — dP. Permeability varies greatly in carbonate reservoirs from values of less than 0.

Evaporites are the least permeable rocks, being impermeable to water. Shales are permeable to water but not generally permeable to oil. Very high permeability through connected vugs and fractures is relatively common in carbonate rocks, notably in limestones rather than dolostones. It is measured on core samples, commonly by commercial laboratories. Unlike porosity, permeability varies with grain size, as well as packing, sorting, and fabric.

Fine-grained detrital rocks with comparatively high intergranular porosity have low permeability. In ideal reservoirs with intergranular porosity and uniform grain size, permeability varies approximately as the fourth power of the average pore radius, or approximately as the square of the grain diameter North, Most reservoirs, especially carbonate reservoirs, are not represented by this ideal model.

Permeability depends on the geome- try of pore throats rather than on the largest pore dimensions, although in some cases, pore dimensions may vary predictably with pore throat dimensions. This task requires information from petrographic studies on reservoir rocks. Neither borehole logs nor the seismograph can make direct measurements of fundamental rock properties such as depositional texture, sedimentary structures, mineralogical composition, or rock fabric.

In the case of borehole logs, the geoscientist or engineer makes inferences about fundamental properties from log characteristics or from calculations based on log data. Tertiary properties are twice removed from fundamental rock proper- ties and once removed from dependent or secondary properties such as porosity, permeability, and bulk density.

One log that does make direct measurements of borehole properties is the caliper log. It records the diameter of the borehole. Because measurements of some tertiary properties require external energy sources for their measurement e. As they are raised, the tools transmit data through cables to recording devices at the surface.

Some tools are designed for open-hole logging; others are used in cased holes. Responses of logging tools vary with an array of parameters such as size of the borehole, mud properties, speed of tool movement up the borehole, and temperature; consequently, the novice interpreter should not assume that log measurements always provide accurate and representative values. Uncorrected logs usually require corrections before they are interpreted or com- pared with direct measurements such as those made during core analyses.

Detailed descriptions of the wide variety of logging devices, logging principles, and methods of interpretation are beyond the scope and purpose of this book, but a list of typical logs in use today, along with a brief list of their applications to the study of carbonate reservoirs, are shown in Table 2.

Distinctive patterns on analog wiggle traces and trends in numerical values that can be read from log traces are commonly compared with lithological descriptions to establish log signatures that correspond to certain rock types. For the nonspecialist, a brief review of the types of modern wireline logs, their applications, and their limitations is presented in Morton-Thompson and Woods Hodgkins and Howard present an illustrated discussion on NMR logging in Gulf of Mexico sandstone reservoirs and the hand- book by Asquith and Krygowski presents a variety of methods for calcula- tions from borehole logs including image and NMR logs.

Both Asquith and Krygowski and Rider include discussions on acoustic and nuclear magnetic resonance imaging, which are not included in the list of logs in Table 2. Traditional methods of interpretation, particularly on older analog records, involve reading values from analog wiggle traces and then making calculations to determine rock and reservoir properties.

Graphical methods involve cross- referencing the values read from wiggle traces on nomograms to obtain estimates of rock or reservoir properties. In the case of terrigenous sandstones, additional inferences can be made about depositional environments based on the shape of the resistivity and SP or gamma ray traces.

Gamma ray, or SP, and resistivity readings are sometimes interpreted to represent grain-size trends in siliciclastic sandstones; therefore, by extension, to represent depositional facies characteristics. The validity of these interpretations depends on the assumption that shapes of the gamma ray and resistivity curves are proxies for grain size trends, and that the logging engineer made no errors while running and recording the log.

In fact, gamma ray and resistivity devices do not measure grain size; they measure natural radioactivity and electrical resistivity. The shapes of gamma ray and resistivity log traces from carbonate reservoirs do not indicate anything about depositional environment, particle characteristics, or pore types.

When paired with resistivity log traces, these typical gamma ray or SP log curves can be imagined to describe bell, cylinder, and funnel shapes. Log shapes are not reliable indicators of texture or facies character in carbonate rocks; consequently, electrofacies mapping is generally limited to sand—shale successions.

These methods have to be cali- brated against real rocks and pore characteristics before the operator can be reason- ably certain about the results. These methods notwithstanding, depositional and diagenetic pore types in carbonates are not generally detectable by wireline log traces because most carbonate porosity is not simply depositional and interparticle in nature.

It is more useful to focus on methods such as the NMR log that can measure pore geometry in carbonates. Other challenges for the log interpreter include determin- ing reliable petrophysical calculations in carbonates that have a variety of pore types and sizes. Calculating a reliable Sw depends on knowing which m Archie cementation exponent value to use. In res- ervoirs with vuggy or moldic porosity, m may be 3 or 4, but in fractured reservoirs it may be close to 1.

The low density of gypsum compared to the surrounding dolomite and anhydrite dramatically increased the uncertainty of what values to use in density calculations. Correcting Swt in bimodal porosity and choosing m values for different pore types are discussed in Chapter 3. This task is not easy nor does it always produce reliable results, especially when several minerals are present in a single reservoir rock.

Lacking cores or cuttings, the teams had to create lithological logs synthetic rock descriptions based primarily on wireline log data. Determining the proportions of carbonate and evaporite minerals when they occur together is a well-known problem Hashmy and Alberty, In such cases, cuttings or core samples are necessary to make direct determinations of mineral composition to compare with log readings. If samples are unavailable, data from different logs can be crossplotted to derive values that are indicative of a particular rock type.

The use of an M-N plot to determine lithology is described and illustrated in Asquith and Krygowski Included among many examples of such interpretive programs in use today are those developed by such companies as Petcom, Landmark, and GeoQuest. Problems result from having to choose which minerals to exclude. For example, consider the case where two petrophysicists are in competition during litigation and have to make choices about which minerals to exclude from a list that includes quartz, clay minerals, anhydrite, dolomite, and calcite.

If the petrophysicists exclude different minerals from their calculations, the two outcomes will be differ- ent. Consider a situation where reservoir quality is related to dolomite content of the producing formation. In the presence of shale, the results are unpredictable, however Rider, This problem can be tackled by generating computer output of volume percent of each ideal, end-member component.

Statistical multilog analysis involves taking all log responses from a single depth and combining them into a multidimensional set in n-dimensional space. The sets are then subjected to multivariate statistical analyses to identify sets that can be grouped into populations of numbers that have some internal similarity and that can be differentiated from other populations of numbers. Think of cluster analysis dendrograms.

The next step is to try to relate the different number popu- lations to rock types or synthetic lithofacies. But in carbonates where distinctions between constituent components, fabrics and textures, and pore types are not readily distinguishable by borehole log measurements, it is a genuine problem—a problem that can only be resolved with certainty by direct observa- tion of the rocks. A borehole log that offers great potential for geologists is the NMR nuclear magnetic resonance log.

The total liquid volume represents total porosity. Additionally, the liquid volume represents the pore volume that, if samples of the reservoir rock are recovered and examined under the microscope, can be compared with measurements of pore geometry. Modern data processing techniques for analyzing seismic wave characteristics such as frequency, amplitude, polarity, spatial distribution, and shear wave characteristics enable geophysicists to make vastly more sophisticated interpretations than were possible only a decade ago.

The advent of 3D seismology has greatly advanced our ability to interpret subsurface structure, stratigraphy, and even reservoir character- istics. The vivid technological displays of reservoir charac- teristics require contrast in seismic velocities, or differences in acoustic impedance, between the reservoir and its enclosing strata. It can not distinguish between depositional, dia- genetic, or fracture porosity. The thickness of the porous zone in such a case is below the limit of separability or one-quarter the seismic wavelength, as illustrated by Brown Anselmetti and Eberli studied seismic compressional and shear wave velocities Vp and Vs in minicores and found, much as Wang did in his laboratory study, that different seismic veloci- ties in rocks of equal porosity were the result of different pore types.

But as Lorenz et al. Asquith and D. This edition includes new material on magnetic resonance imaging and borehole imaging logs. A thorough discussion of logging methods, how the logging tools work, and how to interpret log data is given in M. Additional references on seismology include W. Telford, L.

Geldart, and R. Sheriff , Applied Geophysics, 2nd edition; J. Milsom , Field Geophysics, 3rd edition; and R. Sheriff , Encyclopedic Dictionary of Exploration Geophysics, 4th edition. A widely cited book on 3D seismology is by A. Palaz and K. Marfurt , Carbonate Seismology. For those wishing to see color images of carbonate particles, pore types, and effects of diagenesis on carbonate rocks, A Color Guide to the Petrog- raphy of Carbonate Rocks: Grains, Textures, Porosity, Diagenesis, by P.

In the intervening 12 years since the first edition, there have been numerous studies of value published that need to be recognized and incorporated in the topics discussed. A chapter on the impact of global tectonics and biological evolution on the carbonate system has been added to emphasize the effects of global earth processes and the changing nature of life on earth through Phanerozoic time on all aspects of the carbonate system.

The centerpiece of this chapter—and easily the most important synthesis of carbonate concepts developed since the edition—is the discussion of the CATT hypothesis, an integrated global database bringing together stratigraphy, tectonics, global climate, oceanic geochemistry, carbonate platform characteristics, and biologic evolution in a common time framework.

Another new chapter concerns naturally fractured carbonates, a subject of increasing importance, given recent technological developments in 3D seismic, reservoir modeling, and reservoir production techniques. Detailed porosity classifications schemes for easy comparison Overview of the carbonate sedimentologic system Case studies to blend theory and practice. An accessible resource, covering the fundamentals of carbonatereservoir engineering Includes discussions on how, where and why carbonate areformed, plus reviews of basic sedimentological and stratigraphicprinciples to explain carbonate platform characteristics andstratigraphic relationships Offers a new, genetic classification of carbonate porosity thatis especially useful in predicting spatial distribution of porenetworks.

Includes a solution manual. One main target in petroleum recovery is the description of the three-dimensional distribution of petrophysical properties on the interwell scale in carbonate reservoirs. Doing so would improve performance predictions by means of fluid-flow computer simulations. A closing chapter deals with reservoir models as an input into flow simulators.

This second volume on carbonate reservoirs completes the two-volume treatise on this important topic for petroleum engineers and geologists.

Together, the volumes form a complete, modern reference to the properties and production behaviour of carbonate petroleum reservoirs. The book contains valuable glossaries to geologic and petroleum engineering terms providing exact definitions for writers and speakers.

Lecturers will find a useful appendix devoted to questions and problems that can be used for teaching assignments as well as a guide for lecture development.

In addition, there is a chapter devoted to core analysis of carbonate rocks which is ideal for laboratory instruction. Managers and production engineers will find a review of the latest laboratory technology for carbonate formation evaluation in the chapter on core analysis. The modern classification of carbonate rocks is presented with petroleum production performance and overall characterization using seismic and well test analyses.

Separate chapters are devoted to the important naturally fractured and chalk reservoirs. Throughout the book, the emphasis is on formation evaluation and performance. This two-volume work brings together the wide variety of approaches to the study of carbonate reservoirs and will therefore be of value to managers, engineers, geologists and lecturers. The porosity of carbonates as compared to sandstones is vastly more complex with simple intergrain porosity dominates sandstones while carbonates commonly exhibit complex secondary pore systems that may evolve during burial.

Initial porosity of carbonates is much greater than that seen in sandstones due to common intragranular porosity. Fractures, both natural and induced, are much more important in carbonates. Diagenesis is a major factor in the development of ultimate pore systems in carbonates. The geologically based Choquette—Pray carbonate porosity classification is the most commonly used scheme.

Their 15 different pore types are based on fabric selectivity. A major feature of the classification is its recognition of the potential of porosity evolution through time and burial. Three porosity development zones are recognized: eogenetic, dealing with surface processes; mesogenetic, dealing with burial processes; and telogenetic, exhumed rocks dealing again with surface processes.

This classification is best used during exploration, while other engineering-based classifications such as the one developed by Lucia should be used in reservoir characterization and as input for reservoir modeling.

Examples of all 15 pore types are given. The biological influence over the origin, distribution, composition, texture, and mineralogy of carbonate sediments is stressed. Environmental factors such as light, temperature, and water depth directly affect these biological processes. Abiotic carbonate precipitation is discussed. Three carbonate factories are identified: shallow water tropical; deep water mud mound; cool-water factory developed in high and low latitudes.

Basic attributes of each factory are developed. The rimmed shelf and ramp facies models of the tropical factory are detailed with the Belize shelf and Middle East Abu Dhabi as examples. The facies tract of the mud mound factory is detailed and the Devonian Canning Basin used as an example.

The role of sea-level changes and carbonate sedimentation in platform development is discussed. High sea-level carbonate sediment shedding combined with lowstand sediment starvation is opposite to what is seen in regions of siliciclastic sedimentation.

The dominance and importance of the Dunham rock classification is stressed. Finally, lacustrine carbonates are discussed using the African rift lakes as modern examples and developing a simple model of continental rift lake carbonate sedimentation emphasizing potential source rock and reservoir facies. The Brazil Cretaceous subsalt play of the south Atlantic rift and the potential of its African counterpart are discussed.

This book integrates those critical geologic aspects of reservoir formation and occurrence with engineering aspects of reservoirs, and presents a comprehensive treatment of the geometry, porosity and permeability evolution, and producing characteristics of carbonate reservoirs.

The intention of the volume is to fully aquaint professional petroleum geologists and engineers with an integrated geologic and engineering approach to the subject. As such, it presents a unique critical appraisal of the complex parameters that affect the recovery of hydrocarbon resources from carbonate rocks. The book may also be used as a text in petroleum geology and engineering courses at the advanced undergraduate and graduate levels.

Reservoir engineers today are challenged in the design and physical mechanisms behind low salinity injection projects, and to date, the research is currently only located in numerous journal locations.

This reference helps readers overcome these challenging issues with explanations on models, experiments, mechanism analysis, and field applications involved in low salinity and engineered water. Covering significant laboratory, numerical, and field studies, lessons learned are also highlighted along with key areas for future research in this fast-growing area of the oil and gas industry.

The overarching diagenetic drive during progressive burial of carbonate rocks is toward the loss of porosity through mechanical and chemical compaction the latter consisting of pressure solution plus related cementation. The passive margin diagenetic regime is marked by relatively rapid burial with steadily rising temperatures and pressures. Once a mechanically stable grain framework is achieved, the effective stress from sediment loading can eventually suffice to cause chemical compaction.