Assignment 1:

Topic: Carbonates (carbonate formation)Context: some of the largest oil reservoirs are in carbonates.

Question: How did they form? (carbonates and reservoirs – Nico: this is 100% biology!!). Understand formation in depth… then, clearly show the causal link between formation history and today’s properties.

Length: no restrictions

Expected: Depth of thought, clarity and physical insight

Note: plagiarism is absolutely unacceptable in any form (http://www.plagiarism.org/)

Due: Sunday 23 @ 7 am

OUTLINE:

1- INTRODUCTION

2- FORMATION

3- LINK BETWEEN FORMATION HISTORY AND TODAY’S PROPERTIES

Basic concepts:

Lithification (from the Greek word lithos meaning 'rock' and the Latin-derived suffix -ific) is the

process in which sediments compact under pressure, expel connate fluids, and gradually become

solid rock. Essentially, lithification is a process of porosity destruction

through compaction and cementation. Lithification includes all the processes which convert

unconsolidated sediments into sedimentary rocks. Petrification, though often used as a synonym, is

more specifically used to describe the replacement of organic material bysilica in the formation

of fossils.

Diagenesis /ˌdaɪəˈdʒɛnəsɪs/ is the change of sediments or existing sedimentary rocks into a

different sedimentary rock during and after rock formation (lithification), at temperatures and

pressures less than that required for the formation of metamorphic rocks.[1] It does not include

changes from weathering.[1] It is any chemical, physical, or biological change undergone by

a sediment after its initial deposition, after its lithification. This process excludes surface alteration

(weathering) and metamorphism. These changes happen at relatively low temperatures and

pressures and result in changes to the rock's original mineralogy and texture. There is no sharp

boundary between diagenesis and metamorphism, but the latter occurs at

higher temperatures and pressures than the former. Hydrothermal solutions, meteoric groundwater,

porosity, permeability, solubility, and time are all influential factors.

When animal or plant matter is buried during sedimentation, the constituent

organic molecules (lipids, proteins, carbohydrates and lignin-humic compounds) break down due to

the increase in temperature and pressure. This transformation occurs in the first few hundred meters

of burial and results in the creation of two primary products: kerogens and bitumens.

It is generally accepted that hydrocarbons are formed by the thermal alteration of these kerogens

(the biogenic theory). In this way, given certain conditions (which are largely temperature-

dependent) kerogens will break down to form hydrocarbons through a chemical process known

as cracking, or catagenesis.

Grains in Carbonate Rocks - The grains that occur in carbonate rocks are calledallochemical particles or allochems.   They are grains often precipitated by organisms that formed elsewhere and became included in the carbonate sediment.  Because calcite and aragonite, the main biochemical precipitates, are soft and soluble in water, the distance of transport is usually not very far.  Unlike clastic sediments, the degree of rounding and sorting of the grains may not be a reflection of the energy of the transporting medium, but may be biologically determined.  For example some organisms produce particles that already have a rounded shape. If many of the same size organisms die at the same place, then the grains may be well sorted.  Grains found in carbonate rocks are as follows:

 

1. Whole or broken skeletons of organisms (fossils).  These may range in size from gravel to fine sand, depending on the organism and the degree to which the grains are broken by waves or during transport.

 

2. Ooids.  These are spherical sand sized particles that have a concentric or radial internal structure.  The central part of each particle consists of a grain of quartz or other carbonate particle surrounded by thin concentric layers of chemically precipitated calcite.  The layers or coatings are formed in agitated waters as the grain rolls around.

3. Peloids.  These are spherical aggregates of microcrystalline calcite of coarse silt to fine sand size.  Most appear to be fecal pellets from burrowing benthic organisms.  As these organisms burrow through the muddy carbonate-rich sediment, they ingest material in search of nutritional organic compounds resulting in waste products containing microcrystalline calcite.  The peloids are much easier seen in thin section than in hand specimen because of their small size.

4. Limeclasts.  These are fragments of earlier formed limestone or partially lithified carbonate sediment.  Most are intraclasts, originating within the basin of deposition.  They may be pieces of partially cemented carbonate mud that were ripped from the seafloor by storms.  Some appear to be fragments of partially cemented carbonate mud that originated in intertidal mudflats.  Some may also be pieces of limestone carried into the basin from nearby carbonate outcrops.

Matrix-   The matrix of carbonate rocks consists of either fine grained carbonate mud, called micrite. Or coarser grained calcite crystals formed during diagenesis, calledsparite.

 

o The micrite results from recrystallization of carbonate mud during diagenesis or from direct precipitation of calcite, and causes lithification of the sediment.  The micrite gives the dull opaque appearance of most limestones as seen in hand specimen. If the rock consists entirely of fine-grained mud matrix, it implies deposition in a low energy environment just like in siliclastic mudstones.  Some of the mud may start out as aragonite needles 5 to 10 m in length produced by calcareous algae. But, again this becomes recrystallized to a microspar 5 to 15 m in diameter during diagenesis.  

o Larger sparry calcite matrix results from diagenesis in the same way that calcite cement originates in sandstones.

StructuresSince most limestones are formed by clastic processes, many of the same types of structures observed in siliciclastic rocks also occur in limestones.   

Current-Generated Structures.  Structures like cross-bedding, ripple marks, dunes, graded bedding, and imbricate bedding are common in carbonate rocks, although they may not be as evident as in siliclastic rocks because of the lack of contrasting colors of individual beds in carbonates.  Since many shells of organisms have curved outlines in cross-section (brachipods, pelecypods, ostracods, and trilobites, especially), when the organism dies it may settle to the bottom with the outline being concave downward, and latter become filled with carbonate mud.  When such features occur they can be used as top/bottom indicators.

Lamination.   The most common type of lamination in carbonate rocks is produced by organisms, in particular blue-green algae that grow in the tidal environment.  These organism grow as filaments and produce mats by trapping and binding microcrystalline

carbonates, as incoming tides sweep over the sand.  This leads to the formation of laminated layers that consist of layers of organic tissue interbedded with mud.  In ancient limestones, the organic matter has usually been removed as a result of decay, leaving cavities in the rock separated by layers of material that was once mud.  These cavities are called fenestrae (for a photo, see Figure 16-14, page 305, in Blatt & Tracy).  

Another type of lamination occurs as bulbous structures, termed Stomatolites (for photo, see Figure 16-15, p. 306, in Blatt & Tracy).  These are produced in a similar fashion, i.e. by filamentous blue-green algae, but represent mounds rather than mats.

Stylolites.  Stylolites are irregular surfaces that result from pressure solution of large amounts of carbonate.  In cross-section they have a saw tooth appearance with the stylolites themselves being made of insoluble residues or insoluble organic material.  Some studies have suggested that the stylolites represent anywhere from 25% to as much as 90% of missing rock that has been dissolved and carried away by dissolution (for a photo, see Figure 16-22, page 314, in Blatt & Tracy).

Carbonate Depositional EnvironmentsMost modern, and probably most ancient, carbonates are predominantly shallow water (depths <10-20 m) deposits.  This is because the organisms that produce carbonate are either photosynthetic or require the presence of photosynthetic organisms.  Since photosynthesis requires light from the Sun, and such light cannot penetrate to great depths in the oceans, the organisms thrive only at shallow depths.  Furthermore, carbonate deposition in general only occurs in environments where there is a lack of siliciclastic input into the water.  Siliclastic input increases the turbidity of the water and prevents light from penetrating, and silicate minerals have a hardness much greater than carbonate minerals, and would tend to mechanically abrade the carbonates.  Most carbonate deposition also requires relatively warm waters which also enhance the abundance of carbonate secreting organisms and decrease the solubility of calcium carbonate in seawater.  Nevertheless, carbonate rocks form in the deep ocean basins and in colder environments if other conditions are right. 

For this course, our discussion of carbonate depositional environments will be brief.   See your text for more detailed discussion.  The principal carbonate depositional environments are as follows: 

Carbonate Platforms and Shelves.  Warm shallow seas attached the continents, or in the case of epiric seas, partially covering the continents, are ideal places for carbonate deposition.  Other shelves occur surrounding oceanic islands after volcanism has ceased and the island has been eroded (these are called atolls).  Carbonate platforms are buildups of carbonate rocks in the deeper parts of the oceans on top of continental blocks left behind during continent - continent separation.

Reef building organisms from the framework of most of these carbonate buildups.  

Tidal Flats. Tidal flats are areas that flood during high tides and are exposed during low tides.  Carbonate sands carried in by the tides are cemented together by carbonate secreting organisms, forming algal mats and stromatolites.

Deep Ocean.  Carbonate deposition can only occur in the shallower parts of the deep ocean unless organic productivity is so high that the remains of organisms are quickly buried.  This is because at depths between 3,000 and 5,000 m (largely dependent on latitude - deeper near the equator and shallower nearer the poles) in the deep oceans the rate of dissolution of carbonate is so high and the water so undersaturated with respect to calcium carbonate, that carbonates cannot accumulate. This depth is called the carbonate compensation depth (CCD). The main type of carbonate deposition in the deep oceans consists of the accumulation of the remains of planktonic foraminifera to form a carbonate ooze.  Upon burial, this ooze undergoes diagenetic recrystallization to form micritic limestones.  Since most oceanic ridges are at a depth shallower than the CCD, carbonate oozes can accumulate on the flanks of the ridges and can be buried as the oceanic crust moves away from the ridge to deeper levels in the ocean. Since most oceanic crust and overlying sediment are eventually subducted, the preservation of such deep sea carbonates in the geologic record is rare, although some have been identified in areas where sediment has been scraped off the top of the subducting oceanic crust and added to the continents, such as in the Franciscan Formation of Jurassic age in California.

Non-marine Lakes. Carbonate deposition can occur in non-marine lakes as a result of evaporation, in which case the carbonates are associated with other evaporite deposits, and as a result of organisms that remove CO2 from the water causing it to become oversaturated with respect to calcite.

Hot Springs. When hot water saturated with calcium carbonate reaches the surface of the Earth at hot springs, the water evaporates and cools resulting in the precipitation of calcite to form a type of limestone called travertine.

DolostonesDolostones are carbonate rocks composed almost entirely of dolomite - (Ca,Mg)CO3.  Although there used to be a common perception that the abundance of dolostones increased with age of the rock, it is now recognized that although no primary dolomite bearing rocks are being directly precipitated in modern times, dolostones have formed throughout geologic time. This is true despite the fact that modern sea water is saturated with respect to dolomite. Still, most dolostones appear to result from diagenetic conversion of calcite or high-Mg calcite to dolomite, after primary deposition of the original calcium carbonate bearing minerals. 

Dolomite, and therefore rocks containing large amounts of dolomite, like dolostones, is easily distinguished by the fact that it only fizzes in dilute HCl if broken down to a fine powder. Also, dolostones tend to weather to a brownish color rock, whereas limestones tend to weather to a white or gray colored rock.  The brown color of dolostones is due to the fact that Fe occurs in small amounts replacing some of the Mg in dolomite.

In thin section it is more difficult to distinguish from calcite, unless it is twined.  Unfortunately, sedimentary dolomite is rarely twinned.  In order to facilitate its identification in thin section, the sections are often stained with alizarin red S.  This turns calcite pink, but leaves the dolomite unstained.

Dolostones are almost entirely composed of euhedral and subhedral rhombs of dolomite.  Although dolostones contain allochems, like limestones, the allochems are generally recrystallized to dolomite, and rhombs of dolomite can be seen to cut across the boundaries of allochemical particles.  Some dolostones show no evidence of allochems, but only contain rhombs of dolomite.  These could either represent limestones that have completely recrystallized to dolomite, leaving no trace of the original fragments that made up the limestone, or could represent primary crystals of dolomite. 

Two mechanisms of dolomitization of limestones have been proposed based on field and laboratory studies.

Evaporative Reflux.  This mechanism involves the evaporation of seawater to form a brine that precipitates gypsum.  After precipitation of gypsum, the brine is both enriched in Mg relative to Ca and has a higher density.  If the brine then enters the groundwater system and moves downward into buried limestones.   This Mg-rich brine then reacts with the calcite in the limestone to produce dolomite.  

Mixing of Seawater and Meteoric Water.  This mechanism involves the mixing of groundwater derived from the surface with saline groundwater beneath the oceans.  Dolomitization is thought to occur where the two groundwater compositions mix with each in the porous and permeable limestone within a few meters of the surface.   

http://www.tulane.edu/~sanelson/eens212/carbonates.htm

Classification table:

Dunham Classification

The Dunham classification (1962) is based on concept of grain/mud support therefore on the proportion mud-particle and the depositional textures. The concept of "support" assume continuity of either the mud matrix or that of the grains. If the carbonate is mud-supported the grains float into a continuum of mud matrix. In grain-supported carbonate rocks the grains from an interconnected skeleton in which the mud fills the gap. The percentage mud/cement at which there is a switch between mud-supported and grain-supported depends on the fabric (preferential agencement) of the particles.

DEPOSITION TEXTURE RECOGNIZABLE UNRECOGNIZABLEORIGINAL COMPONENTS NOT BOUND TOGETHER DURING DEPOSITION ORIGINAL BOUND

CONTAIN MUD, CLAY, AND FINE SILT-SIZE CARBONATE NO MUD

MUD SUPPORTED GRAIN-SUPPRTD

GRAINS < 10% GRAINS > 10%

MUDSTONES WAKESTONE PACKSTONE GRAINSTONE BOUNDSTONE CRYSTALLINE

What are sedimentary rocks? Sedimentary rocks are the product of the creation, transport,

deposition, and diagenesis of detritus and solutes derived from pre-existing rocks.

Additional readings:

Other texts on carbonate reservoirs, including those by Chilingar et al. (1992) , Lucia (1999), and

Moore (2001) , concentrate on engineering aspects of carbonate reservoirs (Chilingar and Lucia)

or on sequence stratigraphy as it relates to carbonates (Moore), but they are not textbooks on the

general geology of carbonate reservoirs.

Ahr, W. M. (2008). Geology of Carbonate Reservoirs : The Identification, Description and

Characterization of Hydrocarbon Reservoirs in Carbonate Rocks. Hoboken, NJ, USA: Wiley.

Retrieved from http://www.ebrary.com

Primary recovery methods have produced only about one-third of the world’s original oil in

place, leaving an estimated 891 billion barrels or more (Ahlbrandt et al., 2005) . If

unconventional sources of oil and natural gas are included, the figure will be even larger. If

reservoir flow units could be mapped with a higher degree of precision than was available

previously, then a significant percentage of those 1 trillion barrels of remaining oil could be

within reach with novel methods of improved recovery.

To understand carbonate rocks at reservoir scale, one first has to understand them at pore scale.

Carbonate reservoirs are porous and permeable rocks that contain hydrocarbons.

The book is organized around a genetic classification of carbonate porosity and ways it can be

employed in exploration and development. The genetic categories include three end members—

depositional pores, diagenetic pores, and fractures. Genetic pore categories are linked with

geological processes that created, reduced , or enlarged pores during lithification and burial.

1-INTRODUCTION:

1.1 DEFINITION OF CARBONATE RESERVOIRS

1.1.1 Carbonates

Carbonates are anionic complexes of (CO 3 ) and divalent metallic cations such as

Ca, Mg, Fe, Mn, Zn, Ba, Sr, and Cu, along with a few less common others. The bond

between the metallic cation and the carbonate group is not as strong as the internal

bonds in the CO 3 structure, which in turn are not as strong as the covalent bond in

carbon dioxide (CO 2 ). In the presence of hydrogen ions, the carbonate group

breaks down to produce CO 2 and water. This breakdown reaction, commonly

experienced when acid is placed on limestone, is the chemical basis for the fizz test

that distinguishes carbonates from noncarbonates. It is also used to distinguish

dolostones, which fizz slowly, from limestones, which fizz rapidly. Carbonates occur

naturally as sediments and reefs in modern tropical and temperate oceans, as

ancient rocks, and as economically important mineral deposits. The common

carbonates are grouped into families on the basis of their crystal lattice structure, or

the internal arrangement of atoms. The families are known by the crystal systems in

which they form, namely, the hexagonal, orthorhombic, and monoclinic

crystallographic systems. The most common carbonate minerals are in the

hexagonal system, notably calcite (CaCO 3 ) and dolomite (Ca,Mg(CO 3 ) 2 )

(Figures 1.1 and 1.2). Aragonite has the same composition as calcite, CaCO 3 , but it

crystallizes in the orthorhombic system. The monoclinic system is characterized by

the beautiful blue and green copper carbonates—azurite and malachite,

respectively. Calcite and aragonite are polymorphs of calcium carbonate because

they share the same composition but have different crystal structures. Dolomite, like

calcite, crystallizes in the hexagonal system, but it is different from calcite. The

small size of Mg ions compared to calcium ions causes a change in the dolomite

lattice resulting in a loss of rotational symmetry. Aragonite is common in the

modern oceans but it is rare in the ancient rock record; therefore it is safe to say

that carbonate reservoirs and aquifers are composed of calcite and dolomite—

limestones and dolostones. Together, those rocks make up about 90% of all

naturally occurring carbonates (Reeder, 1983). Only a small fraction of the

remaining 10% of carbonate minerals includes azurite and malachite, which are

semiprecious stones and are commonly found in jewelry or other ornaments.

1.1.2 Reservoirs

Reservoirs are usually defined as storage receptacles. To a petroleum geoscientist,

reservoirs are porous and permeable rock bodies that contain commercial amounts of

hydrocarbons. Reservoirs owe their porosity and permeability to processes of deposition,

diagenesis, or fracturing—individually or in combination. Although we will focus on

hydrocarbon reservoirs in carbonate rocks, many porous and permeable carbonates are

groundwater aquifers. Reservoirs are three-dimensional bodies composed of rock matrix

and networks of interconnected pores. If the three dimensional geometry (size and shape)

of a connected pore system is known, it is possible to (1) determine drilling locations in

exploration or development prospects, (2) estimate the volume of the resource in the

reservoir or aquifer, (3) achieve optimum extraction of the resource, (4) determine the

practicality of drilling additional (infill) wells to achieve the optimum spacing between field

wells during development, and (5) predict the path that will be taken by injected fluids as

they “sweep” remaining hydrocarbons during secondary and enhanced recovery. In the

broad sense, reservoir studies include reservoir geology, reservoir characterization, and

reservoir engineering. To avoid confusion in terminology about carbonate reservoirs, some

common terms are discussed in the following paragraphs. Reservoir geology deals with the

origin, spatial distribution, and petrological characteristics of reservoirs. The reservoir

geologist utilizes information from sedimentology, stratigraphy, structural geology,

sedimentary petrology, petrography, and geochemistry to prepare reservoir descriptions .

Those descriptions are based on both the fundamental properties of the reservoir rocks and

the sequence of geological events that formed the pore network. Data for these descriptions

comes from direct examination of rock samples such as borehole cores and drill cuttings.

Borehole logs and other geophysical devices provide useful information, but they are

indirect measurements of derived and tertiary rock properties. They are not direct

observations. Direct observations of depositional textures, constituent composition,

principal and accessory minerals, sedimentary structures, diagenetic alterations, and pore

characteristics provide the foundation for reservoir descriptions. The geological history of

reservoir formation can be traced by interpreting depositional, diagenetic, and tectonic

attributes. The goal of such interpretations is to formulate geological concepts to guide in

predicting reservoir size, shape, and performance characteristics. In the absence of direct

lithological data from wells, as in the case of frontier exploration and wildcat drilling,

geologists commonly study nearby outcrops of the same age and geological formation as the

expected reservoir. A measure of care is given to interpreting reservoir geology from

distant outcrops because depositional and diagenetic characteristics may vary significantly

from place to place and from outcrops that have been altered by surface weathering to

subsurface reservoirs that have never been exposed to weathering. Reservoir

characterization, like reservoir geology, deals with physical characteristics of the reservoir.

It differs from geological description in that data on petrophysics and fluid properties are

included. In addition to data from direct examination of reservoir rocks, reservoir

characterization involves interpretation of borehole logs, porosity–permeability

measurements, capillary pressure measurements, reservoir fluid saturations, and reservoir

drive mechanisms. Reservoir engineering deals with field development after discovery. The

main goal of the reservoir engineer is to optimize hydrocarbon recovery as part of an

overall economic policy. Reservoirs are studied throughout their economic lives to derive

the information required for optimal production. In addition to geological data and

borehole log characteristics, reservoir engineering deals with reservoir pressures, oil–water

saturation, and gas–oil ratio in order to provide estimates of in-place hydrocarbon

volumes, recoverable reserves, and production potential for each well in a field (Cossé,

1993). Petroleum geoscientists not only study reservoirs, but they also study traps, seals ,

and source rocks that make up most of the petroleum system described by Magoon and

Dow (1994). Traps are bodies of rock where hydrocarbons accumulate after migrating

from their source and are restricted from further movement. It is convenient to think of

traps as large-scale geometrical features that form boundaries around porous and

permeable reservoir rocks. Traps are created by structural, stratigraphic, hydrodynamic,

or diagenetic processes. It is important to recognize that the geometry of the reservoir–trap

system may or may not correspond with present-day structural configurations. Subsurface

structures may form and then be deformed by later episodes of tectonism. Ancient or

paleo-highs may become present-day lows or saddles. Likewise, paleo-lows may be

tectonically elevated to exhibit present-day structural closure and be “high and dry.” This

is called structural inversion and it is especially characteristic of basins with mobile salt or

shale in the subsurface and in some structural settings where multiple episodes of tectonism

have changed older structures. Seals are the physical mechanisms that restrict fluids from

flow out of the trap and are usually described in terms of capillary pressures. Seals may

extend along the top, side, or bottom of the trap. Later we will define seals on the basis of

the high capillary pressure exhibited by the seal rock as compared to the reservoir rock.

These differences usually correspond to changes in rock type such as a change from

sandstone to siltstone or shale in the case of siliciclastics, or porous grainstone to mudstone

in carbonates. Most seals are not completely impermeable and will allow some leakage of

hydrocarbons. Less commonly, seals may consist of totally impermeable barriers to flow

such as evaporite deposits. Source rocks are rich in kerogen, the parent organic matter that

produces petroleum hydrocarbons when it reaches a threshold temperature during burial

and thermal maturation. Source rocks usually consist of shales or lime mudstones that

were deposited in oxygen-deficient environments where lipid-rich organic matter was

preserved and converted to kerogen on further burial.

1.2 UNIQUE ATTRIBUTES OF CARBONATES

Texture is defined as the size, shape, and arrangement of detrital grains in a sedimentary

rock. In siliciclastic rocks, it is strongly influenced by parent rock type, weathering, and

transportation history. Most sandstones are classified on the basis of how much quartz,

feldspar, rock fragments, and matrix they contain. Fossils are generally ignored. Sandstone

diagenesis is usually treated as a process that alters depositional texture, fabric, and

porosity only after extensive burial. Sandstone porosity and permeability are nearly always

described as facies-specific; that is, the rock properties of the depositional facies determine

reservoir characteristics. Fractured reservoirs in terrigenous clastics are less commonly

reported in the literature than are fractured reservoirs in carbonates, although fractures

are certainly not exclusive to one rock type or the other. In contrast, carbonates have

unique attributes that distinguish them from siliciclastics and that require different

methods of study. Some of these attributes were recognized decades ago by Ham and Pray

(1962). First, carbonates form within the basin of deposition by biological, chemical, and

detrital processes. They do not owe their mineralogical composition to weathered, parent

rocks and their textures do not result from transport down streams and rivers. Carbonates

are largely made up of skeletal remains and other biological constituents that include fecal

pellets, lime mud (skeletal), and microbially mediated cements and lime muds. Chemical

constituents, including coated grains such as ooids and pisoids, cements, and lime mud, are

common in carbonates but are absent in most siliciclastics. Clastic grains exist in

carbonates, as they do in siliciclastics. In carbonates, however, these grains are mainly

clasts of intraformational, lithified sediment (intraclasts) or of reworked, older rock

(lithoclasts). The second major difference between carbonates and siliciclastics is that

carbonates depend greatly on biological activity. They are composed mainly of biogenic

constituents, component grains may have undergone size and shape changes as they were

eaten by organisms, and the stratification of carbonate rock bodies is extensively modified

by burrowing and boring organisms. The third major difference is that carbonates are

susceptible to rapid and extensive diagenetic change. Carbonate minerals are susceptible to

rapid dissolution, cementation, recrystallization, and replacement at ambient conditions in

a variety of diagenetic environments. Finally, although not stressed by Ham and Pray

(1962), fractured reservoirs are probably more common in carbonate rocks than in

siliciclastics (as indicated in Table 1.1), but work by Laubach (1988, 1997) and Laubach et

al. (2002) suggests that fractures are more common in siliciclastic reservoirs than was

previously recognized. In short, porosity and permeability in carbonate reservoirs depend

on a broad array of rock properties, on diagenetic episodes that may continue from just

after deposition through deep burial, and on fracture patterns related more to the

geometry of stress fields than to rock type. Choquette and Pray (1970) highlighted some of

the differences between carbonate reservoirs and those in siliciclastics. A summary of their

findings is given in Table 1.1. Three other significant differences between carbonate and

terrigenous sandstone reservoirs are: (1) electrofacies maps from gamma ray and

resistivity log data do not indicate depositional facies in carbonates as they can do with

terrigenous sandstones; (2) Focke and Munn (1987) demonstrated that a strong

relationship exists between pore type and petrophysical characteristics in carbonate

reservoirs such that saturation calculations using the Archie equation will vary greatly

depending on the chosen “ m ” exponent and its dependence on the proportion of vuggy

and moldic pores compared to interparticle pores; and (3) carbonates form in temperate as

well as tropical environments. Because temperate carbonates have decidedly different

mineralogical and component grain type compositions from tropical carbonates, their

reservoir characteristics could also be different than expected. Differences between

sandstone and carbonate reservoirs influence the way we study them. Sandstone porosity is

mainly interparticle; therefore it is related geometrically to depositional texture and fabric.

Because permeability usually correlates rather well with interparticle porosity in

sandstones, it can be related to depositional texture and fabric, as illustrated in a study of

pore geometry in sphere packs and in terrigenous sandstones (Berg, 1970). Assuming that

porosity and permeability are closely related, laboratory measurements made on small core

plugs of terrigenous sandstones may be assumed to be representative of large rock volumes.

That is, small samples are representative of large populations if the populations are

homogeneous. Carbonates do not always exhibit interparticle porosity; they may have a

variety of pore sizes, shapes, and origins, and measured porosity values do not always

correspond closely with permeability. In short, carbonate pore systems are not usually

homogeneous. While a 1-inch perm-plug will provide reliable data on sandstone porosity

and permeability, entire core segments 4 inches in diameter and 1 foot long may be

required for reliable measurements on carbonates. Relatively simple porosity classification

schemes are useful for siliciclastics but a compound scheme of genetic classification

augmented by measurements of pore geometry is needed for carbonates. Carbonate

porosity classifications are discussed in Chapter 2.

http://www.eia.gov/beta/MER/?tbl=T01.01#/?f=M

EIA power consumption [1]

http://www.slb.com/services/technical_challenges/carbonates.aspx [2]

http://www.slb.com/~/media/Files/industry_challenges/carbonates/brochures/

cb_carbonate_reservoirs_07os003.pdf [3]

[4] Carbonate Facies in Geologic HistoryAuthors:

 

B. A., M. A., Ph.D. James L. Wilson

http://wiki.aapg.org/Early_carbonate_diagenesis [5]http://wiki.aapg.org/Porosity [6]

7 Ahr, W. M.. Geology of Carbonate Reservoirs : The Identification, Description and

Characterization of Hydrocarbon Reservoirs in Carbonate Rocks. Hoboken, NJ, USA: Wiley,

2008. ProQuest ebrary. Web. 22 August 2015.

8 http://wiki.aapg.org/Permeability