carbonate rocktype

8/10/2019 Carbonate Rocktype

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Carbonate Rock Type

FTKE , USAKTI

2014


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DUNHAM, 1962


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The goal of reservoir characterization is

to describe the spatial distribution of

petrophysical parameters such as

porosity, permeability, and saturation.

Petrophysical properties in themselves

have no spatial information and must be

linked with geologic models before they

can be displayed in 3D. This is best done

by developing relationships between rockfabrics and petrophysical properties

because permeability and saturation are

controlled by pore-size distribution,

which a product of depositional and

diagenetic history. Numerical engineering

data and interpretive geologic data arelinked at the rock fabric level because

pore structure is fundamental to

petrophysical properties and pore

structure is the result of spatially

distributed depositional and diagenetic

processes

ROCK FABRIC


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Porosity, permeability, and capillary pressure

data (used to estimate saturation) are obtained

from core analysis. Even in the most heavily

cored reservoirs, however, cores constitute a

fraction of their available wellbore data. The

most common source of wellbore data are

wireline logs. Porosity values are readily

obtained from wireline logs and saturation can

be obtained from a combination of porosity and

resistivity logs using the Archie equation andother methods. Permeability values, however,

cannot be obtained directly from wireline logs in

carbonate reservoirs because of the complexity

and large variability of carbonate fabrics.

The purpose of this module is to illustrate the

large variety of carbonate fabrics and present

the rock fabric method of relating these fabricsto permeability. Capillary properties and

saturation can also be related to the rock

fabrics but those relationships will be presented

in a future module. Methods for integrating this

information into wireline-log analysis will also

be presented in a future module.

Rock-Fabric Classification


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The classification used here has

been presented by Lucia (1983,1995, 1999). The foundation of

the Lucia classification is the

concept that pore-size distribution

controls permeability and

saturation and that pore-size

distribution is related to rock

fabric.Lucia (1983) showed that

the most useful method of

characterizing rock fabrics

petrophysically is to divide pore

types into pore space located

between grains and crystals,

called interparticle porosity, and

all other pore space, called vuggy

porosity

Carbonate Pore Type -1


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Vuggy porosity is pore space that is

within grains or crystals or that is

significantly larger thangrains or

crystals; that is pore space that is not

interparticle. Vuggy pore space is

further subdivided into two groups

based on how the vugs are

interconnected; (1) vugs that are

interconnected only through theinterparticle pore network are termed

separate vugs and (2) vugs that form

an interconnected pore system are

termed touching vugs. Vugs are

commonly present as dissolved

grains, fossil chambers, fractures,and large irregular cavities. Although

fractures may not be formed by

depositional or diagenetic processes,

fracture porosity is included because

it defines a unique type of porosity in

carbonate reservoir rocks.

Carbonate Pore Type -2


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Classification of Interparticle Pore Space

Interparticle porosity is defined as pore space located between grains or crystals that is not

significantly larger than the particles (generally


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In order to relate carbonate rock fabrics to pore-size

distribution, it is important to

1) determine which of the three major pore-type

classes are present, interparticle, separate-vug, ortouching-vug,

2) measure the volume (porosity) of interparticle and

separate-vug pore space, and

3) characterize the rock fabric of each pore type

petrophysically.

Rock-Fabric Classification


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Lucia (1983) showed that particle

size can be related to mercury

capillary displacement pressurein nonvuggy carbonates with more

than 0.1 md permeability,

suggesting that particle size

describes the size of the largest

pores. Whereas the displacement

pressure characterizes the largest

pore sizes and is largely

independent of porosity, the shape

of the capillary pressure curve

characterizes the smaller pore

sizes and is dependent oninterparticle porosity The

relationship between displacement

pressure and particle size is

hyperbolic and suggests important

particle-size boundaries at 100

and 20 m.


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. Lucia (1983) demonstrated that three permeability fields can be

defined using particle-size boundaries of 100 and 20, a relationship

that appears to be limited to particle sizes of less than 500 m


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Lucia (1995) named these three fields as petrophysical class 1 (>100 ) class

2 (100-20 ), and class 3 (< 20 ). These class designations will be used

throughout this presentation


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The addition of

vuggy pore spaceto interparticle

pore space alters

the petrophysical

characteristics by

altering the

manner in which

the pore space is

connected, all

pore space being

connected in

some fashion

Separate vugs are defined as pore space that is interconnected only through the

interparticle porosity. Touching vugs are defined as pore space that forms an

interconnected pore system independent of the interparticle porosity


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Intrafossil pore space, such as the living chambers of a gastropod

shell; moldic pore space, such as dissolved grains (oomolds) ordolomite crystals (dolomolds); and intragrain microporosity are

examples of intraparticle, fabric-selective separate vugs. Molds of

evaporite crystals and fossil molds found in mud-dominate fabrics

are examples of fabric-selective separate vugs that are significantly

larger than the particle size.

In mud-dominated fabrics, shelter pore space is typically much

larger than the particle size and is classified as separate-vug

porosity whereas in grain-dominated fabrics, shelter pore space is

related to particle size and is considered intergrain porosity.

Separate-vug pore space is defined as pore space that is

1) either within particles or is significantly larger than the

particle size (generally >2 x particle size), and2) 2) is interconnected only through the interparticle

porosity. Separate vugs are typically fabric-selective in

their origin.

Separate-Vug Pore Space


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In grain-dominated fabrics, extensive selective dissolution of

grains may cause grain boundaries to dissolve, producingcomposite molds. These composite molds may have the

petrophysical characteristics of separate vugs. However, if

dissolution of the grain boundaries is extensive, the pore space

may be interconnected well enough to be classified as solution-

enlarged interparticle or touching-vug porosity.

Grain-dominated fabrics may contain grains with intragrain

microporosity (Pittman 1971). Even though the pore size is

small, intragrain microporosity is classified as a type of separate

vug because it is located within the particles of the rock. Mud-

dominated fabrics may also contain grains with microporosity, but

they present no unique petrophysical condition because of the

similar pore sizes between the microporosity in the mud matrix

and in the grains.


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Touching-vug pore systems are defined as pore space that is(1) significantly larger than the particle size and

(2) forms an interconnected pore system of significant extent

Touching vugs are typically nonfabric selective in origin.

Cavernous, breccia, fracture, and solution-enlarged fracture

pore types commonly form an interconnected pore system on a

reservoir scale and are typical touching-vug pore types.

Fenestral pore space is commonly connected on a reservoirscale and is grouped with touching vugs because the pores are

normally much larger than the grain size (Major et al. 1990).

Touching-Vug Pore Space


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Fracture porosity is included as a touching-vug pore

type because fracture porosity is an important

contributor to permeability in many carbonate reservoirsand, therefore, must be included in any petrophysical

classification of pore space.

Although fracturing is often considered to be of tectonicorigin and thus not a part of carbonate geology,

diagenetic processes common to carbonate reservoirs,

such as karsting (Kerans 1989), can produce extensive

fracture porosity.

The focus of this classification is on petrophysical

properties rather than genesis, and must include

fracture porosity as a pore type irrespective of its origin.


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Rock-Fabric/Petrophysical Relationships

In grainstone fabrics, the pore-size

distribution is controlled by grain

size; in mud-dominated fabrics, the

size of the micrite particles controls

the pore-size distribution. In grain-

dominated packstones, however,

the pore size distribution is

controlled by grain size and by the

size of micrite particles between

grains.

Examples of nonvuggy limestone

petrophysical rock fabrics

Limestone Rock Fabrics

Petrophysics of Interparticle

Pore Space


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Grainstone and mud-

dominated fabrics arereasonably well-constrained

permeability fields. Although

grain-dominated packstone

fabrics plot at an intermediate

location between grainstones

and mud-dominatedlimestones, they show more

variability because of the large

grain size difference.

Grain size of grain-dominated

packstone ranges from 400

microns for oncoid fabrics to

80-150 for fine peloid fabrids

limestones compared with the permeability fields


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Dolomite Rock Fabrics

Dolomitization can change the rock

fabric significantly. In limestones,

fabrics can usually be distinguished

with little difficulty. If the rock hasbeen dolomitized, however, the

overprint of dolomite crystals often

obscures the limestone fabric

precursor. Precursor fabrics in fine-

crystalline dolostones are easily

recognizable. However, as the crystalsize increases, the precursor fabrics

become progressively more difficult

to determine. Dolomite crystals

(defined as particles in this

classification) commonly range in

size from several m to >200 . Lime

mud particles are usually


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The plot of interparticle porosity

against permeability

in mud-dominated fabrics,permeability increases as

dolomite crystal size

increases. Finely crystalline

(average 15 m) mud-

dominated dolostones plot

within the class 3 permeabilityfield. Medium crystalline

(average 50) mud-dominated

dolostones plot within the

class 2 permeability field.

Large crystalline (average

150 ) plot in the class 1

permeability field.


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Comp osi te porosi ty-ai r permeabi l ity cros s

p lo t for nonvugg y dolos tone fabr ics

compared with the three permeabi l i ty f ields.

Dolograinstones and large crystal

dolostones constitute the class 1

permeability field. Grains are very

difficult to recognize in dolostones

with a > 100 crystal size. However,

because all large crystalline

dolostones and all dolograinstones

are petrophysically similar, whether

the crystal size or the grain size is

described makes little differencepetrophysically. Fine and medium

crystalline grain-dominated

dolopackstones and medium

crystalline mud-dominated

dolostones constitute the class 2

permeability field. Fine crystalline

mud-dominated dolostones

constitute the class 3 field.


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Limestone and Dolomite Combined

The fabrics that make up the

class 1 field are (1) limestoneand dolomitized grainstones

and (2) large crystalline grain-

dominated dolopackstones

and mud-dominated

dolostones. An upper particle

size limit of 500 is imposedbut not well defined. The

upper limit is imposed

because as the grain size

increases the slope of the

porosity-permeability

transform approaches infinityand permeability becomes

independent of porosity.

Composite porosi ty-air permeabi l i ty cro ss plot

fo r nonvug gy l imestones and dolos tones

showing t ransforms for each c lass


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The first step is to classify the rock fabric and determine the

petrophysical class. The lithology is given because it cannot be determined

from the photo. In this example the lithology is limestone. The rock fabric isgrainstone because the fabric is grain supported and no lime mud is

present. Grainstones are petrophysical class 1. Both the ROCK FABRIC

CLASSIFICATION and the PETROPHYSICAL CLASS will be entered into

the approriate columns of your worksheet.

The second step is to identify the grains because pore space is classified

according to its location relative to the grains. In this image only the grains

are shown. They are approximately 500 microns in diameter and are

relatively well sorted. Grains with concentric rings are ooids. Some grains

are composite grains. You do not need to identify the grains.

The exercise is to estimate permeability from a thin section and measured

total porosity values. We will use photomicrographs of thin sections for thisexercise. The measured porosity of the core plug for this sample is 20.1%.

The value fpr TOTAL POROSITY will be entered in your worksheet.

Example classification procedure for limestones:


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The third step is to look within the grains for pore space. Pore space

within the grains is classified as separate-vug porosity and an accurate

visible estimate of separate-vug porosity is needed to estimate the

amount of interparticle porosity. In this example the separate-vug pore

space is colored green and is estimated to be 5%. The value forSEPARATE VUG (Svug) POROSITY will be entered in your worksheet.

The fourth step is to look between the grains. In this example pore

space (in red) and calcite cement (whitish color) is located between the

grains. The red is interparticle porosity space. Although visible

interparticle porosity can be estimated it will always be different than the

total interparticle porosity because some of the pore space is too small to

be seen in thin section. Therefore, the amount of interparticle porosity is

determined by subtracting the visual estimate of separate-vug porosity

from the measured total porosity. This value is automatically calculated in

this exercise.

The fifth and last step is to estimate permeability. This is done using

the porosity permeability chart. Enter the interparticle porosity value and

go up to the appropriate petrophysical-class transform and read the

permeability value. In this case, the interparticle porosity is 15 percent

and the petrophysical class is 1.


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Note that the interparticle porosity is given as a fraction rather

than as a percent because fractional porosity is used in all

engineering equations. Start on the x axis at 0.15, read up to the

class 1 transform, over to the permeability on the y axis, and read

the permeability. This is the estimated permeability; 350 md. Thisvalue would then be entered in your worksheet.

After you have completed the exercise we will provide you with

the measured permeability values to compare with your answers. In

this example, the measured permeability is 121 md.


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Example classification procedure for dolostones:

The exercise is to estimate permeability from a thin section and measured

total porosity values. We will use photomicrographs of thin sections for this

exercise. The measured porosity of the core plug for this sample is 11.4%.The value for TOTAL POROSITY will then be entered in your worksheet

The first step is to classify the rock fabric and determine the petrophysical

class. The lithology is given because it cannot be determined from the photo. In

this example the lithology is dolostone. The precursor limestone fabric is difficultto determine so we will call the fabric simply dolostone, or dolomudstone since

not grains are visible. If a grainstone and grain-dominated packstone precursor

fabric can be observed the classification procedure is the same as outlined in

the limestone example. The petrophysical class is determined by the dolomite

crystal size. The crystals are outlined in red is a small area. Using the scale,

measure the crystal size. Larger crystals should be used for this measurement

because the smaller crystals may be only the tip of the crystal. The crystal size

is about 120 microns and the rock fabric is large crystalline dolostone. Large

crystalline dolostones are class 1. You will then enter both the ROCK FABRIC

CLASSIFICATION and the PETROPHYSICAL CLASS into your worksheet.


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The second step is to identify separate-vug porosity. In this example the

dolomite crystals are the particles and there is no pore space within the

crystals. However, there are several pores that are larger than the crystals

and could be classified as separate vugs. The rule is that only pores that are

larger than 2.5 times the dolomite crystal size can be classified as separatevugs. A circle with a diameter of 2.5 times the crystal size is shown in yellow.

Three pores are tested but only one is more than 2.5 times crystal size, and

that pore is outlined in red and classified as a separate-vug. This vug by

have formed by dissolution of an allochem, but there is too much alteration

to tell. The volume of this vug, however, is less than 1 percent of the thin

section and separate-vug porosity is considered to be zero. You will enterthis value in your worksheet.

The third step is to look between the crystals for intercrystal pore space. All

the blue is intercrystal pore space except of the one separate vug outlined in

red. Although visible intercrystal porosity can be estimated it will always be

different than the total interparticle porosity because some of the pore space

is too small to be seen in thin section. Therefore, the amount of interparticle

porosity is determined by subtracting the visual estimate of separate-vug

porosity from the measured total porosity. This result of this calculation will

be entered in your worksheet.


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The fourth and last step is to estimate permeability. This is done using the

porosity permeability chart. Enter the interparticle porosity value and go upto the appropriate petrophysical-class transform and read the permeability

value. In this case, the interparticle porosity is 11 percent and the

petrophysical class is 1. Note that the interparticle porosity is given as a

fraction rather than as a percent because fractional porosity is used in all

engineering equations. Start on the x axis at 0.11, read up to the class 1

transform, over to the permeability on the y axis, and read the permeability.This is the estimated permeability; 29 md. This value will be entered in your

worksheet.

After you have completed the exercise we will provide you with the

measured permeability values to compare with your answers. In this

example, the measured permeability is 25.2 md.


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The rock fabric approach presented here is based on the premise that pore-size

distribution controls the engineering parameters of permeability and saturation

and that pore-size distribution is related to rock fabric, a product of geologic

processes. Thus, rock fabric integrates geologic interpretation with engineeringnumerical measurements.

To determine the relationships between rock fabric and petrophysical parameters

it is necessary to define and classify pore space as it exists today in terms of

petrophysical properties. This is best accomplished by dividing pore space into

pore space into three types.

The petrophysical properties of interparticle porosity are related to particle size,

sorting and interparticle porosity. Grain size and sorting is based on Dunham's

classification, modified to describe current conditions. Instead of dividing fabrics

into grain support and mud support, fabrics are divided into grain-dominated and

mud-dominated

The important fabric elements to recognize for petrophysical classification of

dolomites are precursor grain size and sorting, dolomite crystal size, and inter-

crystal porosity. Important dolomite crystal size boundaries are 20 and 100 .

Dolomite crystal size has little effect on the petrophysical properties of

dolograinstone. The petrophysical properties of mud-dominated dolostones,

however, are significantly improved when the dolomite crystal size is >20 .

Summary of Key Concepts


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Class 1 is composed of (1)

limestone and dolomitized

grainstones and (2) large


dolopackstones and mud-

dominated dolostones.

Fabrics that make up the class

2 field are (1) grain-dominated

packstones, (2) fine to medium


dolopackstones, and (3)

medium crystalline mud-dominated dolostones. The

class 3 field is characterized by

(1)mud-dominated fabrics

(mud-dominated packstone,

wackestone, and mudstone)

and (2) fine crystalline mud-

dominated dolostones

Petrophy sical /rock- fabr ic classes b ased on

simi lar capi l lary pro pert ies and interpart ic le-

poro si ty /permeabi l i ty transforms .


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Cont inuum of rock fabr ics and associated porosi ty-

permeabil i ty transform s. (A) Rock -fabric num bers

ranging from 0.5 - 4 defined b y class -average and

class-boundary porosi ty-permeabi l i ty transforms.

(B) Fabric continuum in non vug gy limesto ne. (C)

Fabr ic cont inuum in nonvug gy dolostone.

Although fabrics are divided into three

petrophysical classes, in nature there

is no boundary between the classes.

Instead, there is a continuum from

mudstone to grainstone and from 5 to

over 500 mud-dominated dolostones(Fig. 14b,c). Therefore, there is also a

complete continuum of rock-fabric

specific porosity-permeability

transforms To model such a continuum

the boundaries of each petrophysical

class are assigned a value (0.5, 1.5,

2.5, and 4) (Fig. 14a) and porosity-

permeability transforms generated.

These transforms were added to the

class 1, 2, and 3, transforms and an

equation relating permeability to a

continuum of class values and

interparticle porosity is developedusing multiple linear regressions.
http://www.beg.utexas.edu/lmod/_IOL-CM07/old-4.29.03/cm07-step04.htmhttp://www.beg.utexas.edu/lmod/_IOL-CM07/old-4.29.03/cm07-step04.htm


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To avoid confusion, the class values

generated by this equation are

termed rock-fabric numbers. The

resulting global transform is given

below. This equation is useful incalculating permeability from

wireline logs but is too detailed to be

useful for routine classification of

visual descriptions.

Mud-dominated limestones and fine

crystalline mud-dominated dolostones occupy

rock fabric numbers from 4 to 2.5 (Fig. 14b,c).

The class number decreases with increasing

dolomite crystal size from 5 to 20 in mud-dominated dolostones, and with increasing

grain volume in mud-dominated limestones.

Grain-dominated packstones, fine-to-medium

crystalline grain-dominated dolopackstones,

and medium crystalline mud-dominated

dolostones occupy the rock fabric numbers

from 2.5 to 1.5 (Fig. 14b,c). The class value

decreases with increasing dolomite crystal

size from 20 to 100 in mud-dominated

dolostones and with decreasing amounts of

intergrain micrite as well as increasing grain

size in grain-dominated pack-stones and fine

to medium crystalline grain-dominateddolopackstones. Grain-stones,

dolograinstones, and large crystalline

dolomites occupy rock fabric numbers 1.5 to

0.5 (Fig. 14b,c). The class value decreases

with increasing grain size and dolomite crystal

size from 100 to 500.


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Examples of separate-vug po rosity

Unusual Types of Interparticle Porosity

Diagenesis can produce unique types of

interparticle porosity. Collapse of separate-

vug fabrics due to overburden pressurecan produce fragments that are properly

considered "diagenetic particles". Large

dolomite crystals with their centers

dissolved can collapse to form pockets of

dolomite rims. Leached grain-stones can

collapse to form intergrain fabricscomposed of fragments of dissolved

grains. These unusual pore types typically

do not cover an extensive area. However,

the collapse of extensive cavern systems

can produce bodies of collapse breccia

that are extensive. Interbreccia-block poresproduced by cavern collapse are included

in the touching-vug category because they

result from the karsting process (Kerans

1989).


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Separate vugs are not

connected to each other. They

are connected only through

the interparticle pore space

and, although the addition of

separate vugs increases total

porosity, it does not

significantly increase

permeability (Lucia 1983)


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Cross plot i l lustrating the eff ect of separate-vug porosity on

air permeabil ity. (A) Grainstones with separate-vug

porosity in the form of grain molds plot to the right of the

grainstone field in pr oportion to the volume of separate-vug

porosity. (B) Ooid grainstone with separate vugs in the

form of in tragranul ar microporosity plot to the r ight of the

grainstone fi eld.

Permeability of a moldic

grainstone is less than would be

expected if all the total porosity

were interparticle and, atconstant porosity, permeability

increases with decreasing

separate-vug porosity (Lucia

and Conti 1987). The same is

true for a large crystalline

dolowackestone in that the data

are plotted to the left of the class

1 field in proportion to the

separate-vug porosity (Lucia

1983).


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Theoretical fracture air permeability-porosity

relationship compared to the rock-

fabric/petrophysical porosity, permeability fields

(Lucia, 1983). W = fracture width, Z = fracture

spacing.

Lucia (1983)

illustrated this fact by

comparing a plot of

fracture permeability

versus fracture

porosity to the three

permeability fields for

interparticle porosity


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Touching vugs can increase

permeability well above what

would be expected from the

interparticle pore system and

are usually considered to be

filled with oil in reservoirs.


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Petrophysics of Touching-Vug Pore Space

This graph shows that

permeability in touching-

vug pore systems has

little relationship to

porosity. Typical porosity-permeability cross plots

from touching vugs have

low (less than 6 percent)

porosity and display

unorganized scatter


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carbonate rocktype

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