choquette & pray 1970

7/27/2019 Choquette & Pray 1970

1/44

The American Association of Petroleum Geologists Bulletin

V. 54, No. 2 (February, 1970), P. 207-250,

13 Figs.

3 Tables

Geologic Nomenclature and Classification of Porosity in

Sedimentary Carbonates

PHILIP W CHOQUET1E2 and LLOYD C. PRAY3

Littleton, Colorado 80121, and Madison, Wisconsin

53706

TABLE OF CONTENTS

ABSTRACT

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

207 APPENDIX . GLOSSARY

F

POROSITYERMS ...

244

PART

1.

PERSPECTIVES

N

POROSITY

N

SEDIMEN-

TARY CARBONATES

. . . . . . . . . . . . . . . . . . . . . .

Complexity of Carbonate Pore Systems . . . . . .

Comparison of Porosity in Sandstones and Sedi-

mentary Carbonates . . . . . . . . . . . . . . . . . . . . .

Concept of Fabric Selectivity

. . . . . . . . . . . . . .

PART

2 .

GENERAL ONSIDERATIONS

P

POROSITY

NOMENCLATURE

Definitions of General Porosity Terms

Porosity Terms of Time Significance . . . . . . . . .

PART

.

CLASSIFICATION

F

CARBONATEO R O S I ~

Basic Porosity Types

. . . . . . . . . . . . . . . . . . . . . .

Genetic Modifiers

. . . . . . . . . . . . . . . . . . . . . . . . .

Pore Size and Pore-Size Modifiers . . . . . . . . . . .

Porosity Abundance

. . . . . . . . . . . . . . . . . . . . . . .

Porosity Descriptions and Code . . . . . . . . . . . . .

Abstract

Pore systems in sedimentary carbonates are

generally complex in their geometry and genesis, and

commonly differ markedly from those of sandstones.

Current nomenclature and cbssifications appear in-

adequate for concise description or for interpretation

of porosity in sedimentary carbonates. In this article

we review current nomenclature, propose several new

terms, and present o classitlcation of porosity which

stresses interrelations between porosity and other geo-

logic features.

The time and place in which porosity is created or

modified are important elements of a genetically

oriented classification. Three major geologic events in

FIGURES

1. Time-porosity terms and zones

..........

2. Classification of porosity ...............

3. Format for porosity name and code

.....

4.

Common stages in evolution of a pore . . .

5.

Interparticle porosity . . . . . . . . . . . . . . . . . .

6.

Intraparticle boring and shelter porosity

7.

Intercrystal porosity in dolomites

. . . . . . . .

8.

Moldic porosity

. . . . . . . . . . . . . . . . . . . . . . .

9. Fenestral porosity . . . . . . . . . . . . . . . . . . . . .

10. Vug and channel porosity

. . . . . . . . . . . . . .

11.

Fracture and breccia porosity

. . . . . . . . . . .

12.

Compound porosity types

. . . . . . . . . . . . . .

13.

Forms of moldic porosity

. . . . . . . . . . . . . .

TABLES

1.

Comparison of porosity in sandstone and

carbonate rocks

......................

2. Attributes used to define basic porosity

types

................................

3. Times and modes of origin of basic porosity

types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

can be differentiated. On the basis of the three major

events heretofore distinguished, we propose to term

the early burial stage "eogenetic," the late stage

"telogenetic," and the normally very long intermediate

s age "mesogenetic." These new terms are also ap-

plicoble to process, zones of burial, or porosity formed

in these times or zones (e.g., eogenetic cementation,

mesogenetic zone, telogenetic porosity).

The proposed classification is designed to a id in

geologic description and interpretation of pore systems

Manuscript received December 31 1968; revised

July

16 1969;

accepted July

31 1969.

Published with

permission of Marathon Oil Com~anv.

the history of a sedimentary carbonate form a practical

basis for dating orig in and modification of porosity,

Denver Research Center Marathon Oil Company.

independent of the stage of lithification. These events lDepartment and Univer-

are (1) creation of the sedimentary fra'mework by

sity

clastic accumulation or accretionary precipitation (final

This article was largely formulated and written

deposition),

2)

passage of a deposit below the zone of

while both writers were part of a continuing research

major influence by processes related to and operating

Program on carbonate facies at the Denver Research

from the deposition surface, and

(3)

passage of the Center of Marathon Oil Company. We are pleased to

sedimentary rock into the zone of influence by processes

acknowledge the very appreciable help received from

operating from an erosion surface (unconformity). he

our colleagues both within and outside Marathon in

first event, final deposition, permits recognition of evolving concepts expressed in this article. We extend

predepositional, depositional, and postdepositional special thanks to D. H Craig D B. MacKenzie P. N

stages of porosity evolution. Cessation of final deposi-

McDaniel and

R

D. Russell of Marathon R. G. C.

tion is the most practical basis for distinguishing pri-

Bathurst of the University of Liverpool and P.

0

mary and secondary (postdepositional) porosity. Many Roehl of Union Oil Company for critical reviews of

of the key postdepositional changes in sedimentary

drafts of the manuscript and to A. S. Campbell of the

carbonates and their pore systems occur near the

Oasis Oil Company of Libya Inc.3 fo r stimulating dis

surface, either very early in burial history or at a

cussions.

penultimate stage associated with upl ift and erosion. 1970. The

American Association of

Petroleum Geolo-

Porosity created or modified at these times commonly gists.

ll

rights reserved.

2 7


2/44

1 8 Philip W. Choquetfe and

Lloyd

C. Pray

ond their carbonale host rocks. It i s a descriphve ond

genelic system in

which 15

basic

porosity lypes

are

recognized: seven abundant types (inlerparlicle, inlro-

purticle, intercrystal, moldic, fenestral, Iraclure, and

uug . a n d eight

more

specialized

lypes.

Modifying

terms

are used

to characterize genesis, size and

shape

and abundance of pomrity, The

genelic

modifiers in-

volve

(1)

process

of

modificat ion (solution, cemenlo-

tion, and inte rna l sedimentation),

(2)

direction or stage

of

modificat ion {enlarged, reduced, or filled), an d

(31

time

of

p m s i t y formation (primary, secondary, pre-

deposit ional, deposit ional, -genetic, mesogenetic, an d

telogenetic).

Used

with the basic porosity type, these

genetic modifiers permit explicit designation

of

porosity

origin

nnd

evolution. Pore shapes are classed as irragu-

lar or regular, ond

the lattar

are subdivided inlo equant,

tubular, and platy

shapes.

A grnde scale for size

of

regular-shaped

pares,

uli l izing

the

average diamefer

of

equanl or tubular pores

and

the width of platy

pores

has

three main clssser:

micmpore~


3/44

G eo l ag i c

Nomenclature

a1

Home

nd

Classification of

Porosity

209

This

article is in

Ihruc

~ n a i n

ptri\

c.tch

s o n ~ e w h a t ependent on thc

ntherr

I ' i ~ r i

prcr-

vides

perspectives

on

the

natilrc ol

p l r n ~ i t :

In

sedimentary

carbonate?.

4

strcrses Ihc senctir

and geometric complexity o c a r h l n i ~ r cpure

systems, the distinctiveness ol nlust carbonate

porosity

in

comparison

to

that

o f porous

s ~ n t l -

stones, and the importance or ascertaining rc1:1-

tions of porosity to fabric elements of carbon-

ate rock-*them concept

of

fabr ic selcctivit ~.

Part

2

presents

the

more general aspects of po-

rosity nomenclature. The general

terms

poros-

ity, pore

pore sysfern and pore i r z ~ e ~ ~ c o n r r ~ c -

rions

are

reviewed. Terminology relating to the

time of porosity

origin and

modification

is

dis-

cussed, and new terms and related

concepts

useful in designating

time

(and place) of

po-

rosity origin

are

presented. Part

3

presents

the

classification we propose.

he

major

clements

of the system

arc

summarized

in Figure

2

and

illustrations of its use

are given in

Figures 3-

12. Following the t ex t is a glossary ant1 discus-

sion

of

most

geologic terms that have becn ap-

plied

to

thc porosity of sedimentary carbonatcs

i n the

past several decadcs (Appendix

A .

Most

terms are defincd briefly as befits a glossary,

but more extended discussions are provided for

important and much-uscd

terms

such

as

vug ,

far which

we

believe that clarified definitions

and

more

consistent

usage

are

nceded.

Thc

glossary i s

intcndcd to serve

both

as

a general

reference

and

as the main source

for

definitions

and

usages of the terms employed

in our

pro-

posed classification.

Considerations of aom enc laturc and

classifi-

cation in

any

scientific field present a reviewer

with

two

end-member alternatives:

either adapt

preexisting terminology to th present state

of

knowledge of the ficId under review, o r create

a

new system with a new nomenclature. De-

spite

som

distinct advantages

in

t he second

al-

ternative for the description, classification, and

interpretation

of

pore

systems in sedimentary

carbonates,

we

do not believe that wholesale

changes in the current body

of

terms are justi-

fied by the present state of the art." Fo r the

present, it seems more practical to use

current

terms

as much as pos ble, sharpening or re-

stricting

usage where

current concepts

suggest

that this

will improve the

precision o r clarity of

the

term.

ilrnily.

hcir

poros i~y likewise cotnplcx and

i l istinctivc wareness

of

thc

many

possihlc

\ t a p in p ~ r o ~ i t yvolution

i s

essentiill

Cor

geol-

crgists concerned with \t i~rlic\

of carbonate

fa-

cics.

whethcr porous or not.

Althongh

t h e

origins of porosity

arc

reason-

itbly

well

understood,

many

modifications of

p ~ r o s i t y

r i

carbonates are still

inadequsltely

k m w n . ITclr

example.

t

long

has

been

recog

nized

that

much Fore

space

in sedimentary car-

bonates

i s

created alter deposition, and atten-

tion has been given to the processes

of

solu-

tion and

dolomitization believed

to have

created

most

of this porosity. B u t much less at-

tention seems to have been

given to

the domi-

nant proFess in porosity ev olutien, which is

the

wholesale obliteration of both primary and sec-

ondary porosity

that

h a s

occurred in

most

an-

cient

carbonates.

Newly

deposited carbonate

sediments

commonly

have

porosity of 40-70

percent; ancient carbonates

with

more than

a

few percent

porosity are

unusual.

Thc

volume

af

pore-filling

cement

in ancient carbonates

commonly may approach o r

exceed

the volume

of the initial sediment (Pray and Choquettc,

1966 . Most

porous

ancient carbonates are re-

garded more correctly

as

representing arrested

stages in

thc

normal trend toward obliteration

of

porosity t h a n ns examples of enhanccd po-

rosity

in

Cormerly

less

porous

facies.

Even

the

creation

ol

molds by solution of aragonite par-

ticlcs,

widely regarded

as

increasing

rack po-

rosity, mtly not involve much net change in

porc volume,

and

the

change

may e a slight

diminution rather than

an

increase

in

the pore

volume

(Harris

and

Matthews, 1967; Land

i

al. 1967). The

long-claimed increase of poros-

ity that occurs

during

dolomitization

is

quanti-

tatively minor compared to the overalI

porosity

decrease which must have occurred in nearly

all ancient dolomitcs. Processes

causing

this

large decrease, however, have

been

largely ne-

glected.

Clearly, the evolution

of

porosity both

its

genesis

and modification) in sedimentary

carbonates

not

only is commonly comp~cx, ut

also records

a

very important

part

of the for-

ma tio n o

ancient

ca rh na te facies.

The

discussion that follows stresses features

of

porosity in sedimentary carbonates

that

pro-

vide useful perspective for the consideration of

nornencIature and the classification

presented

in

Parts 2

and 3.

Complexity of Carbonate

Pore

Systems

Sedimentary carbonates are being recognized

The

pores and pore

syst ms

of sedimentary

increasingly

as

a

complex

and

distinctive

rock

carbonates are normally complex both physi-


4/44

210

Philip

W .

Choquette

Home

and L loyd C. Pray

cally and ge~lelically. I

hc

p l r u

\ p i ~~c .

st1111c

sedimentary carbonates conrists :~lm ostcntirclr,

of interparticle (in ter gr an ul: ~~ ) lpeilings bc-

tween nonporous sediment grains ul rclntivcly

uniform

size

and shapc. Porosity ul t h i b

k ind

may be relatively simple in gcornetr . [ I

t

formed at

the

time

of

deposition

ilnJ

was littlc

modified by latcr diugcnesis,

the

resulting

pow

system may claseIy resemble that of many

well-sorted sandstones. I t represents

a

physical

and genetic simplicity that is unusual in sedi-

mentary carbonates; much greater complerily

is the

rule.

I t i s not surprising that geologists

generaICy

have not atFeempted to describe quantitatively

the

geometry of pore openings; their sizes,

shapes

and

the

nature of their boundaries com-

monIy show extreme variability.

The

three-di-

mcnsional physical complexity can be visual-

ized readily in

some carbonates, but i n many it

is

appmiated best

by injecting

plastic into the

pore

system,

dissolving the rock with

acid

after

the

plastic

has hardened, and directly observing

the pore system. Illustrations of thc

results of

this

technique

(Nuss

and Whiting, 1947) are

provided in

articlcs by lmbt and Ellison

1946) and Etienne

(19631,

The size and

shape

complexity of pores

in

carbonate rocks

is

caused by many factors. It

relates partly

to

thc

wide

range

in

size

and

shape of sedimentary carbonate particles,

which

create

pores either

by

their

packing or

by thcir solution. It also relates partly

to

the

size

and s h a p variation of

pores

6e ate h within

sedimentary particles by skeletal secretion. Ex-

tensive size and shape variation reIatev in par t

to

the

filling of former openings by carbonate

cement

or internaI

sediment

The physical complexity of porosity in cir-

bonate

rocks is increased greatly by solution

procxsses,

which

may

creak

p6re

space

that

precisely mimics the siz and shape of deposi-

tional particles or

form

pores that

are

indepen-

dent

of

both depositional particles

and

diage-

netic crystal textures . ~ r a c t u r e penings dso

are common in

carbonate

rocks and can

strongly

influence solution. Pores range in size

from

openings

1p

or

less

in diameter (if a sin-

gle linear measure is applicable) to openings

hundreds of

meters

across like

the

"Big

Room

at

Carlsbad

Caverns New Mexico, termed a

macropre by Adams and Frenzel

1950,

p.

305).

Size

complexity,

in

addition

to a

wide

range in possible pore sizes, may involve juxta-

position

of

large and minute openings in the

same

rock

unit or single sample.

Size

and shape

complexity

applies eq ually

we11

to all openings,

whclht '~

11

5 cs I I ~ 3utc

r l l t f t

~ ~ 1 l n e c t 1 0 1 1 s

I orcs in

serIimcnt;try

c : ~

honates

arc iully

a

complex

y ~ n c i i c ~ ~ l l ys they are geometrically.

Carbonarc porosii) i s polygenetic in the sense

r f

both rime and modes

of

origin. Although

~ntc rpar tivle orobity

cmatecl

a t thc time of

final

~e d i m e n t

~leposit

on or

accretion

s

important

in many carbonate rocks, porosity creatcd in

sedirnenk~l.y particles either beforc their final

deposition or after deposition commonly

rankr i_~ inrc

involved either secondarily or

not i ~ t ~ l t

n

thc

definitions. One

basic

type,

cavern

porosity. u

detined solely on the basis oi size. Othcrs

such

as

moldic, boring, and shrinkape arc defined

solely o n the basis of origin. Still others

w c h us

vug,

channel,

and

variaus minor

types are

morc

compIcxly defined on the

basis

of sevcral attrjh-

utes.

Determining which of the basic porosity

types are present in

a

sediment o r rock s not

only

a matter of identification and interpreta-

tion;

it aIso involves judgments as to which of

the

types

best

serve

the

classifier s

needs.

v-

cral oi the porosity types are not mutually ex-

clusive. Thus, he porosity within a pellet com-

posed of aragonite crystals

may

be interparticle

parosily with

reference

to the component crys-

tals, but

on

a Iarger scaIe it is intraparticle po-

rosity. In F i p r c 8F the porosity ot thc szdi-

mc nt within the gastropod shell is prim ary inter-

particle porosity, but is also part of thc intra-

par tick porosity in the gastropod. l ikewise , do-

lomitized sedimcnt in a

burrow

m y contain in-

turcrystal porosity tha t could be

designated

as

burrow porosity,

as

intercrystal porosity, or as

both.

Some

of the basic types of porosity

arc

little more

t h an

physical or genetic varieties of

other

basic types; for example, Zenesrral, shel-

ter. and breccia are

all

varieties of interoarticlc

porosity. Clearly, classification cannot be sep-

arated from one s objectives. Deciding

which

basic porosity

types

are t o be used relates

to

one s purpose.

The interrelations of the basic porosity types

with the

time

of their origin

relative

to final

de-

position and with

their

mode of

origin are sum-

marized

in

Table 3.

It

is important to recognize

that many of the types can be created

a t

differ-

ent

t imcs and

by

different processes. For exam-

ple, although interparticle

porosity

commonly

forms during the process of final deposition

of

the carbonate sediment, some can form pr ior to

final deposition.

Of

more significance, interpar-

ticle porosity can aIso form after deposition by

selective solution of

matrix

particles from

be

tween

larger particIes

Thus,

the practice of

equating

the interparticle

(intergranular) po-

rosity

of

carbonates with primary

or

deposi-

tional porosity (e.g.. Levorsen, 1967, 113 is

an unfor tunate simplification, t h o u ~ ht m y be

satisfactory for most sandstones.

7

cornpiexi-

ty introdiced i n l o tho intcrpre~ationof car-

bonate puroity hy multiple modes and times

ol

origins

i a

pri111c rc:~son (or wing gcnctic

modifier\ with h,~sicporo.;ity

types.

( ~ ~ r r p o t ~ ~ ~ r l

nd

grirriarionul oric pnrmity

rypt,s.--% any c;lrhonatc facies contain two o r

mom ba vc types or porosity that arc easily dif-

Ferentiatcil. C ~ m p o z r n i l ore systems are thme

c o m p s e t l o f two or more bnsic types of porcq,

each

typc

physically somewhat discrete and

easily di\linguishable. Common examples are

those composed

o

hoth interparticle ;ind intra-

particle

pr

~ro sity , l rnoldic and intercry stalline

porosity, or of

any

fabric-selective

type

of po-

rosity combined with fraotiire porosity (Fig.

12) .

Gradar~onal ores or pore systems cannot

be

clearly differentiated physically and/or geneti-

cally. Thcy m y he intermediate

in

characteris-

tics betwecn two

basic

types; or they m ay inter-

grade in very

short

distances within

a thin

sec-

tion, hand specimen, or small part

of

an expo-

sure; or

thcy

may interconnect in

a

manner

that makes separate recognition difficult. As an

cxamplc,

fracture poros j ty commonly gadcs

both spatially

and

genetically into breccia po-

rosity,

a

situation approximated in Figure 11A

and B. n somc sucrose dolomitcs wherc dcpo-

sitional tcxturc s poorly prcscrvcd, t h e larger

intercryst,~lporosity

may

grade into, and be

in-

distinguishable

from, t h a t

of small molds

or

vugs. As another exarnplc, carbonates may

have some porosity that is both interparticle

and moldic,

but

much

of

the porosity may

not

bc

resolvible into one or the other

type;

their

porosity is gradational.

Grad;~tionalporosity

also

may he designated

in the

many instances in which fabric-selective

porosity hecomes nonselective within very short

distances. For example, a pore of channel

shape m;tr

have

margins that

are

Iocally fabric

selective; the channel

may

have

begun

to f o r m

by solution enlarg em ent of in terparticle voids,

some

o f whose edges i tre preserved along its

margins.

Ano ther type of gradation of basic

types

oc-

curs between interparticle and sheIter porosity

or

interparticle and fenestral porosity.

Shelter

and fenestral are varieties of interparticle po-

rosity, and distinguished from

i t

partly by the

larger size of the pore in relation to the asso-

ciated pwticles. As pore size diminishes in

reIa-

tion

to

these fabric elements, the distinctions

also diminish and pores can be classified

as

of

either

type

o r as gradational between them.

Fabric selectivity 0) basic

porosify

types.


18/44

Home

hilip

W . Choquette ond Lloyd

C. Pray

ROSl

T Y

TYPES

.

INTERPARTICLE

BP

IUTR&PARTICLE W P

IMKRCRYSThL

E

IUTERMAL SEOIYENT

T

M OF

FORMATION

p m - d t ~ i t l o w l

smo11

nmurmob

rn cmiMarprllck

SECOND RY

Fm tubula pores

ura

avsmqe

c r v n - w r r e

Fw

p l y w a s uwrbirti und mk amkopr

Fro

2. GwIogic

~Iassification

f pores

and

pore systems

in

carbonate rocks


19/44

Home

Geologic Nomenclature

and

Classification

of

Porosity

225

CONSTRUCTION

OF POROSITY DESIGNATION

ANY MDDlFYllYG TERMS ARE

M I M I N E 0

WITh THE BASIC

POROSITY

TYP

IN SEOUENCE GIVEN

RFLOW :

m + m + m + F I

ODIFIER MWlFlER

EXAMPLES.

inimport~dewroslfy, la percent

WP(KI )

p i m a y

mesointmprlicle

pamsity P-msWP

solution-enlorged primary

htroporficle prosity

sxP- WP

mionmoldr

wrosity K)

percent

rnc &

K 10 )

telogenetic r n

porosily S1 -CV

FIG. 3,Fomat for

construction of

porosity

name

and code

designations.

Additional examples are shown

in

captions

of

Figures 5-12.

Fmmm s/YE S O U J m

lMKl

AL

MOW

SOWTION

-

ENLARGED WG

STATE 1MO) MOLD a -MO )

( VU G)

PORE

MOLD

SOLUTKIN

-

ENLARGED VUG

I r - M O )

MOLD

trsx-

MO) tr-VUG)

FILLED FILLED

FILED

MOW 90UlflOM-

ENLARGED

VUG

I f - M O I M O L D I f s x - M O ) ( f -YUG)

FIG.4 . 4 o m m o n

stagcs

in

evolution

o

one

basic

typ of pore, a moId, showing applications of genetic

mdiilers and classification code. Starting

material

b

crinoid

mlurnnal

(top left).

It, and ma Ax adjoining it

then may e

dissolved

in

varying degm s .

Depending

on extent

of

solution (top row), resulting pare s classed

as

rnoId, solution+nlarged mold

or

rug

if

precursor s identiiy is Iost. Filling by cement could occur after each

solution stage.


20/44

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226

Philip W

hoquette and loyd C Pray


21/44

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Geologic Nomenclature ond Classification of Porosity 227

T h e

bi~Ju ~w~ros i t y

pc.5

car I)c ct i , i r i ic lcr i~e l l

usefully on

the

basis of f a h r ~ c e lcc t r k~ ty qcc

Fig.

2

and

discussion

in Part

). I propert

which stresses relation s bct ween pore 3p:tce a n d

other constituents. The two fabric-sclcctivi~y

criteria o

pore position

and porc-houndar)

configuration help both

in identifying

hasic

pel-

rosity

types

and

in interpreting their timcs 1

origin. Jnterparticle, intercrystal, moldic. find

fenestral

pores have both their pol;itions and

their boundaries determined by thc fshric ele-

ments,

hence

are

fabric selective. Most intra-

particle porosity also i s

fabric

selective and is

so classed

in

Figure 2. Some nonselective iutra-

particle porosity may be present, howcver, such

as a vug within a

clast.

By

definition,

vugs and

channels are not

fabric

selective.

Fracture

po-

rosity

is

generally insensitive

to

the

smaller

scale

features

of

the rock, and hence

is

consid-

ered not fabric selmtive.

B u t where

pare sys-

tems that would

be

classified as vugs, channels,

or

fractures

o n

the basis

of

their indiscriminate

position relative to fabric elements show fabric

selectivity

along part or a11 oE their

boundaries,

they may have

formed

before

complete ccrnen-

tation of

the

rock. Thus,

the

abnormal lslbric

selectivity

of

somc basic porc types has genetic

significance.

Genetic

Modifiers

Genetic information

is

implicit

in

some of

the designations

of

basic porosity types, but

other basic-type t e rms supply little or n o gc-

netic information Table

2 ) . Many

types

of

porosity

can

originate

at

different times in rela-

tion to

sc t l~men l

c p o s ~ t i o ~ ~r

hutial, and

by

reveral prtbcesses. And, once created, porosity

can

be

modified

by

various processes,

These

proccsscs, and

thc

direction and extent:

of po-

rosity evolution,

are significant descriptive and

interpretive elements.

Thc

eenetic modifers in

lhis

class~tication

provide

a

way

to

designate

such elcn~znts .They c : ~ t ~

e

used either with

the basic porosity types or independently.

We recognize 13 genetic

modifiers

of three

types which denote l the t ime of

origin

of

thc peroslty, 2 ) the

prore rs

involved in ts

subsequent

modification.

ant1

3 ) the

direction

end extent

of

such modificationjs),

herein re-

ferred to simply as the "direction." The three

types are listed in the summary diagram

of

Fig-

ure

2

with examples

showing

how individual

senetic modifiers arc used,

singly

o r

in

combi-

nations.

l hr

complete genetic-modifier term

coupled ~ i t hhe

basic

porosity type provides

a definitivc porosity characterization

Pig.

3 .

Titne rrrodifiers.-The

seven

modifers reIat-

ing to time o f origin consist of the

two

most

general t ime terms. printnry and secondary

and

five rnorc detailed t ime

terms

that are sub-

divisions of

the two gencral terms. Specifically

these

are

pre~leposirionaland

deposifionul

both

subdivisions

of

primary; ant1 eogenefic

mesoge-

aerie and

irlogeneiic

all

subdivisions of =con-

dary.

Thcsc

turns

are

defined in

the

gIossary

and discussed in

Part

2

The five

more cxpIicit

time term\ c a n

be

combined directly with the

basic

porosity

type

c.g., depositional

interparti-

cle porosity, eogenetic moldic porosity,

or

tclo-

genctic w g , but

the full designation using pri-

FIG.

.-Examples

of interparticle

porosity

A. Interparticle porosity in o6litic grainstone Grainstone is we

sorted

and

free

of interparticle matrix.

Little

of

its

deposltiond

porosity black)

has

been

filled.

Classification: primary depositional

interparticle

porosity

( ~ d - B P ) . te. Genevieve Limestone (Mississippian),Bridgeport

field,

Illinois. Thin section, crw-polarized light.

Reduced primary interparticle pormity (black) in ooliric grainstone. Calcite cement

some

as synraxial

rims

on

crinoid wlurnnals. has filled most porc space. Classification:

cement-reduced

primary interparticle

pornsit

crP

BP). Remaining

voids

arc classified as small

mesopom sms)

in contrast to large

mesopores Ims)

of A. Jte ~inevisueLimestone {Missisaippinn), Bridgeport

field,

Illinois. Thin section, crass-polarized

light.

C

Solution

interparlicle porosity in foraminifera

packstone.

Pores are white. Note

irregular, erratically

distributed pores and

finely particurate

matrlx

(dark

gray)

within

and between

forarns.

Porosity appears

to have

resulted

from

solution of

matrix

Tertiary.

Libya. Thin

sct ion, plane Iight.

D. Solution

interpartscle

porosity in

crinoid-fusulinid

packstone.

Pores [black)

were created largely

by

solution of matrix, in places with partial corrosion of large particles (arrow). Classi~ccation ode: s-BP. Penn-

sylvanian, Hulldale field,

Texas.

Polished

core

surface.

E

Primary

and reduced

primary interparticle

and

intraparticle porosity

in

phylloid algal grainstone. In

places,

as on

right side

of photograph, some pores may have been

soIution

enlarged. Some

alga1

plates have

trapped

fine

sediment. CIasslfication code:

rP-WP/PBP.

Paradox Formation

(PennsyIvanian).

Honaker Trail,

San

Juan

River Canyon. Utah. hin section, plant light.


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Philip W Choquette and Lloyd

C

Pray

* , + *


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Geologic Nomenclature and Classif icat ion of Porosity

229

mary or secondary

together with thc Inme e l -

plicit

terms may

be desirable e . ~ . ,

rimary de-

positional or secondary eogenetic)

Modification

process and direr~ior~.--Six of

the 13

genetic modifiers relate to the

proccsws

by

which

pores

are

modified,

and

to

thc

direc-

tibn(s) taken by modifications.

An example

of

some of the possible modification effects s

shown diagrammatically

in

Figure

4

J'or

a

cri-

noid

mold.

Of a variety

of

modification processes, sol~i-

tion

cemenration

and internal

sedimentation

are recopized in

this classification.

Solution

processes both create and modify pores. The

main

use of 'kolution

as

a

genetic

modifier s

to note solution enlargement of basic types of

porosity and to designate a solution origin for

those

basic

types

that designate position

in a

fabric, namely interparticle

and

in~ercrystal.

l h e

designation of soIution is not required for

moldic porosity,

a basic

type defined

as

solu-

tion

created.

As

solution normalIy

is

assumed

to have been the

genetic

mechanism in vug,

channel

an d

cavern porosity, with these term%

it need

not be specified,

Cementation, used

here

in

the broad

sense for thc filling

of

voids by

precipitation of mineraI

matter from

solutions,

probably accounts

for

most of the

wholesale re-

duction

of

porosity

from newly

deposited

sedi-

ments to

ancient

rocks.

The reduction

process

can

be noted specifically

by

thc

mo ifier term

cementation. The quantitative importance

of

internal

sedimentation as

a

porosity-reducing

process

in

carbonates

is

still

being

debated, but

the

process

is being

recognized

increasingly as

a

useful indicator of special postdcpositional

events,

nuch

as vadove circulation (Dunham,

1963).

hormally

i t

occurs

as

particle-by-parti-

cle deposition

within the

interstices

of a porous

sediment

or

rock.

Processes of mass

sediment

injection :issociatedwith limestone clastic dikes

Pray, 1964) also may eliminate some poros-

ity.

The porosity of some a r b o n a te may be re-

duced

by changes in packing consequent

upon

physical compaction.

This s not

provided

for

in our

sptern, because

i t

is believed

to

be un-

cornman and is difficult to rscognize or

record

on the basis of pore

characteristics. Other

pro-

cesses

of

porosity modification, such

as gas dis-

tension or mineral-volume change, likewise are

not considered

feasible

to

note for the purpose

of this cl;lssification.

The

direction or extent of

the

porosity modi-

fication

i

noted by three

modiftcrs,

enhrged

and redui-ed

as

the

main direction terns,

and

filled for the commonly encountered end stage

of

porosity reduction.

These

arr

usod

best

with

the notation of

process,

but can

be

used

independentIy. The modifier enlarged nor-

mally is used

to denote enlargement by solu-

tion. It

is

applicd only to modifications that do

not obliterate the

identity

of the original pore.

Reducedn' is

used

lor stages

of porosity re-

duction

between the

initial

state

and the end

stage

of

filled.

Tn

view of

the

almost

universaI

reduction of pores by somc cementation or

other

form of filling, the modifier reduced

is

used, as in reduced primary

interparticle

poros-

ity, normally

only

if

the volumetric

reduction

is appreciable, perhaps 30-50 percent or more.

Examples

of

reduced porosity

are shown

in

Fro.

6.-Examples

of

intraparticle, boring,

and

shelter

porosity.

A. Shelter

rosily SH), ty of inhrpanicle

r m s i t y ,

in ,algal

packstone.

J a g s

pores (black) mr sheltered from

benaa~Lbmbrel la l ike . yllotd algae Para ox Purrnabon (Pennsylvanian), Ratherford field.

Utah.

Polished con

B. Prima

(depositional)

shelter

porosity below reef framework megabreccia

dast)

virtually

filled

by white

sparry

calcite

Claui%ation: cement-filled depi t iona l shelter porosity (cfPd-SH).

Upper

Bone

Spring Limestone (Permian), Fuaddup

Texas.

Polished surface.

C

Shelter

porosity etween

coarse

s t m a t o p r o i d fragments in coarse-textured part of fine-grained,

nearly

nonporous

pack-

Loosely packed. relatively coarse debris

prevented infilling

by

finer

contemporaneous sediment

(white). W u c

Formation

Redwater

field, Alberta, Canada. Polished core

surfacc.

D.

lntraparticlc

porosity

within

fusulin~ds.Classification: primary

mesointraparticle porosity

P-msWP). ansing Group

Pennsylvanian), Kansas. Polished wre surface.

Intraparticle porosity in

horn

wral . Pennsylvanian, Hulldale field,

Tern.

Polished

core

surface.

F.

Boring porosity

BO)

f largemesopore size which truncates growth laminations {accented

by

retouchin

o

photo)

in

rtromato mid. Matrix

at

left

is

B n a g a h e d

packsfone.

Ledue

Formation

Devonian),

Redwater

field,

&*a,

Canada.

core

s u g e .

C Boring

in thick-shelled pelecypod. Note

partial

filling by

internal

sediment,

which suggc that shell was

bored

before

final deposition. Clasficatioa sediment-reduced redepositional boring porosity (irPpRO).

Matrix

surrounding shell and

sediment arc porous, dolomitic, bioclsstis pacfrtonc. Tertiary Libya. Polished core surface.


24/44

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Philip W Choquette

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C

Pray

AU


25/44

Geologic Nomenclature

n

Home

d

Classi f ica* ion of Porosity 2 3 1

rucks rclaini~lgbomc

poluslly.

1:illc~l

rncr

voids may involve mnrc lhan h;~lC hc \n lumc

of

the

rock1

The direction tern1

k o t n p l t e

cm he ilsel ul

for designating incompletely formed molds,

h u ~

for

simplicity and

hecause the

tern1

is not in

widely applicable

as

thc

others

it

is

not

n f o r -

mal part of the classificatiun.

Pore Size

and

Poresize Modifiers

The size of pores in sedimentary carbonates

is an important

descriptive

parameter, but

onc

difficult to treat. The distinction has

been

made

k t w c c n

pores

an d

pore

throats or intcrconnec-

tions.

Some quantitative visual characicrization

of

pore size generally

can

be

made

without

undue difficulty, but determination of pore-

throat

size

by direct observation is generally

difficult or impossible. Pore-throat

size

can be

determined inbirectly by observation

in

those

unusual

carbonates that consist of grains of

uniform size,

shape,

and

packing;

but unce-

mcnted, well-sorted oiilitea

and

their textural

analogs

are

rare?

The vast preponderance

of

ancient carbon ates and most modern carbonate

scdimcnts

require capilIary-pressure measure-

mcnts combined with other mnss-response pe-

trophysical data to characterize pore-throat s i x

quantitatively.

Such

characterizations, though

important for

an

understanding of reservoir

he-

havior, are outside thc

scope of t i s

classifica-

tion; here we are concerned primariIy with

pore-system attributes that can

be

direcity and

readily observed. Haw

c n

thc sizc of

the

pores as differentiated from pore throats, best

be characterized? What level of precision is

feasible for most geologic description

and in

terpretation? O u r system is summarized in

Figure

2.

Several factors make it dificult or impossible

to be

precise

in

a

visual

characterization

of

pore

size. One

is

that the physical boundaries

of an individual pore are arbitrary

if,

as is nor-

mal,

the pore is

part

of a continuous pore

sys-

tem. A second limiting factor

is

the

difficulty

(and even impossibility) of observing

t h e

three-dimensional

shape

of pares.

Shapes

gen-

erally must be visualized from a two-dimen-

sionaI surface

in an opaque rock. A third

fac-

tor is t h e irregular

shape of

pores.

But

the

main control on useful precision relates,

not

to

these difficulties in determining the

size of

an

individual pore, but to the normal range in

sizes and shapes of all the pores

in

a reservoir,

outcrop, hand specimen, or even thin section.

The common

need

for geoIogic description is

I I ~ cxpruswm rbl

averasc

h t ~ c

t. ; l v C ~ i i ~ t :

17c

r.tnge

aas

dctern~inedhy

quick

visual inspection.

I'rncticalit thus

dictates

n need fo r broad size

[ ~ i l dhilpc

classes.

If size is

expressed

by a

J~ a me tc rmcasure, thc question oE which diam-

t ie r i s solnewhat academic i f many pores are

cclnsidercd

A

pore-size pad e scalc

as

detailed

3 ; the Wenrwo~.th cale

fur

grain sites. utilizing

a

class

interval

ratio of two or even

o

four

( Todd, 19h6 , is i~siially

much too refined tor

carbonate porosity.

Some nccd

for

uxprcssing

pore

size in

carbo-

nates can I elirnin:ltcd by careful IithoIogic

description. coupled with a notation

of

the

basic porosity rype. rhus, the interparticle po-

rosity in a xlightly cemented, well-sorted, medi-

um-grained

oBlite rarely nee s a direct

pore-

size

description. Likewise, the description,

fus-

ulinid moldic porosity," may convey ade-

quateIy both

size

and shape of pore.

Another

way of simplifying

pore-size

expressions is to

describe thc porc size of each porosity type in-

dividually, rather than the whole pore-sizc

spectrum collectivefy.

To designate pore

size

quantitatively, pore

shape first must

be

considered,

and

in carho-

natcs the hhapes can

be

extremely diversc. We

divide porc shapes into t w o hroad categories:

regular

with

shapcs

that

can bc

characterized

hy

one-,

~uo -

r-three-diameter

measurcs

and

irregular

with shapcs so coniplcx they

cannot

be dcscribc~l dequately

by a

few measurements.

I t

i s

impractical

to

subdivide

the

many possible

shapes

of irregular pores. Regular

pores how

ever

can hc classified on the

basis of

their di-

ameters

and

pore shapcs: equant,

tubular,

and

platy.

Tubular and

platy

pores are notably elon-

gate i n

onc

or two directions or diameters, in

comparison to the shor t diameter. The eq u an t

class includes pores whose

three

diameters

are

about

the

same

and pores

that

are

not

so

dis-

tinctly elongate

in

one

or

two dimensions

as

to

be

cal lzd t r~ bul ar r platy. The

range

in

shape

of dosely associated pores

makes

unnecessary

much

conccrn with precise boundaries between

these three regular-shape categories,

For size classification. equant

pores

can be

characterized adequately

by

a single measure,

an average diameter.

Sizes of tubular

and platy

pores c n be

characterized adequately by an

average crtlss-section dia me ter o r width. In this

pore-size classification,

i

shape is not specified

i t

can

be

assumed

t h a t

essentially equant-

shaped pores are referred to Shape should

be

specified explicitly where pores are tubular or

platy unless shape is implicit in the porosity-


26/44


27/44


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Philip W

hoquette

and Lloyd

C Pray


29/44

Geologic Nomenclature and lassification o Porosity 35

in the mesopore rangc. h,lost mcsopores itre

fabric-selective types of pores, whereas man\

of the niegapores represent types that are not

fabric selective (channels, vugs, ancl caverns)

and were formed by solution bencnth erosion

surfaces in the telo,qcnetic zonc.

Subclass boundaries within the mcsopore

range also correspond in part to natural group-

ings. The interparticle porosity of many pisoli-

tic limestones, much moldic porosity due to so-

lution of bioclastic debris, much interparticl-

porosity of coarse bioclastic limestones, and

most fenestral porosity generally are prcdomi-

nantly in the large-mesopore size range

( $-3

mm). Pore sizes in most oolitic limestones and

significant amounts of the intercrystal pore

space in sucrose dolomites that is coarser than

microporosity


30/44

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Philip W hoquette and

Lloyd

C Pray

B i

i

r n

-

Y A

>


31/44

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Geologic Nomenclature and Classif icar~on

of

Porosity

237

Porosity Devcriptin~ls u~d

Cudr

A

complete porosity

description

using the

elements of this classification includes

I

Jesig-

nation

o f basic

porosity type(?)

and one or

more accessory modifying

terms

relating to

genesis,

size

( a nd

shape) ,

and

abundancc.

The

sequence in which these terms may

be com-

bined is shown

n

Figure 3, with

exampIes

illus-

trating

various

levels of descriptive detail.

I t

is

further illustrated

in

the

captions

of Figures

5

through 12.

Mnemonic letter symbols for

the basic

po-

rosity

trpes

and most porosity modifiers, cou-

pled with percentages and/or ratios for abun-

dance, adapt this

classification for brief poros-

ity

notation

or

coding for field

or

wellsite

de-

scriptions

(Fig.

2 ) .

Ihe

symbols suggested

in

Figure

2 have

proved

easy

to learn

and

useful.

The code symbols use upper

case

letters for

the

basic

porosity

types

and

thc

modifying

terms

primary

P)

and

secondary IS).

Symbols of most modifying terms use

lower

cast.

Ietters. The

derivation

of

the

symbols

are

apparent,

except

for

the three

porosity types,

interparticle,

intraparticle,

and intercrystal,

whose

lettcr

conitruction

makes

a direct

rnncmonic notation difficult.

Code

letters for

these three basic types are the initial letters

W

for

within

(intra-)

and

B

for

bctwcen '

(Inter-);

thus

the letter symbols are

BP,

WP,

and BC,

respectiveIy.

Vug

is not

abbreviated.

J t

can be

useful to record

pore-shape infor-

mation directly, in place of

or

in addition to

pore-size informa tion, though in our experience

sh pe modifiers are commonly unncedcd. Pore

shape

can be

expressed by the following sym-

bols enclosed in parentheses: (Eq) or

equant

(Tb) or

tubular,

(PI)

for platy,

and

( Ir)

for

ir-

regular.

I n

:I cclde rmtatiiin the shape

modifier

~ pIaced

ju\t

to the left ut' the pore-size

modi-

fier

or

basic porosity type symbol. Eramples

are (Eq)

M O and ( T b ) m g C H .

For some purposes it s desirable simply fo

record

the sizc.

abundance,

or

some genetic in-

formation

irhout

porosity without designating

the basic porosity type. The symboI

PO

is used

for

porosity

or pore, as in

mcPO

for micro-

pores.

S-PO

or wcondary pores. o r PO 1

)

fo r 15 percent porosity.

Compound

and

gradational porosity name

and code designations involve the

same

basic

construction as in

Figure

3. T o

designate

com-

poun pore systems

Fig.

12),

the individual

porosity t y p a n d its modifying terms should

be

separated

by

the

word

and.

T h e

most

abundant pomsity type should be listed last

and followed by the porosity-abundance param-

eters; for cxamplc, ' r e d u i d moldic and

re-

duced primary interparticIe porosity

15%

(1

:4)

.

For

the porosity

code, separate

the

in-

dividuaI

porosity-type

terms

(and

their modifier

tcrrns,

if used) by

a

slash mark,

so

that thc

ex-

ample just given

would be

repregenled as r-MO/

rp-BP 5 b

) :

) .

rdarional

porosity

types (Fig.

11B)

are separated by thc word

to i described in words,

as

in thc description

solution-enlarged interparticle

to

channel

po-

rosity, and are separated by

a

long dash in code

form,

as in

FR-CH.

Study

of the illustrated cxamples of porosity

and their code desipations (Fig. 4-12), and

some

practicc

with

actual rock specimens suf-

ficc t o show the descriptive and interpretive

Icvcrage of the system, and

the

case of learn-

ing it. For

very

detaiIed porosity characteriza-

tions, additional parameters

can bc addcd

t o

FIO.

10.-Examples

of vug and

channel

porosity.

A. Channel pare system [CH in dolomite. Large opening at lef t and right

are

inkrconnected

in

three

dimensions. In places

(see

arrows)

intercryst l porwi

connects and is

gradational

with

channel

porosity.

Leduc

Formation (Devonian), Big

Valley

6.16, Alberta. Po?shed core surface.

lk Vug porosiq in rn~crocrystalline dolomite.

A

few v u g have been

filled

rfly

to coolpleteIy

by

internal

sediment (small

arrows)

prior ra dolarnitization.

Y u p

are mostly rnesovvg ( G). Leduc

Formation

TY-

vonian), Big Valley field,

Albem,

Canada. Polished core surface.

C

Rerluced channel porosity in dolomite. Channels of elongate to platy shapes that parallel lamination have

been

reduced

ar fitled

by

cementation. Cement

is

coarsely crystalline dolomite.

Classificat~on

ode: cr-CH.

Tren-

ton

Formation (Ordovician), Scipio field,

Michigan.

Polished core

surface.

D.

Irregular

surface

of

non-fabric-selective me

apore

in

dolomite.

Dis t indon between Iarge-scale channels

and vugs in reletivriy small rock samples

may

not parible, huoni nn , Alberta. Two polished sore surfaces.

E. Solution-developed megapores (cavern, solutionenlarged fractures, channels,

and vugs) in

bioclastic lime

grainstone.

Cavern s about

3V

m

across.

SoIution

development of cavern was selective

fo very

permeable

zon

of primaty

interparticle

porosity (interval shown by vertical bars) where fracnures intersected this zone. Salem

Limestone

(Miss~ssippian),

quarry

near Oolitic, Indiana


32/44

Home

Philip

W hoquetfe and Lloyd

C

Pray

FIG.

1.-Examples of fracture and brcccia

porosity.

A.

Fracture porosity in

stylolitic

lime

mudstone.

Dark gray patches

are

caused

hy ail

stain.

MarIison

Group

(Mississippian),

Oregon

Basin

fierd,

Wyoming.

Pol

i ~ h e d

ore

surface.

B. Fracture porocity

gradins

to breccia poroqity

FR-RK) in microcrystalline dolomite.

Porosity

along

rnicroIractures

is

shown

by oil

stain darker

gray). Leduc

Formation Devonian),

Big VaIley

field, Alberta.

Polished

core surface.

C. Solution-enlargcd breccia porosity

qx-BR) in

microcry~tnlline dolomite. Leduc Formation Devonian),

Big Valley fieId,

Alberta.

Polished

core

surface.

suit one's purpose.

But

for many uses, we

find

that sirnpfe cornhinations o only two or three

of the parameters are satisfactory. The major

advantage which the classification system pro-

vides is not, however, that

of

providing an easy

method of characterizing porosity in sedimen-

tary carbonate?,

h u t

that

o

forcing

more criti-

cal observations of thc pores in relation to the

enclosing rock. Use of t he ~ I a s~ i f i c a t i o nys-

tem should resldt in morc accurate gcnetic

interpretations.

1.

The origin and modification of porosity

are important for understanding scdimentary

carbonates,

and

in exploring for and exploiting

their fluids. A genetically oriented system ot

porosity nomenclature and classification help?

to devclop

th

requisite

understanding.

2. Modificntions in poroqity are

a

major

and

commonly thc predominant diagnetic process

in

mo5t scdimentary carbonates. 1-he vast re-

duction

in porosity from the initial sediment

to th negligible porosity

of

most ancient

car-

bonates is accomplished largely by cementa-

tion.

The

volume of cerncnt filling form er pores

appmachcs or exceedq the volume of the frame-

work in

many

carbonate rocks.

3. Even though most porosity

in

limestones

and dolomites can

be

relatcd

to

primary

fea-

tures, m a n y pores form after deposition (sec-

ondary) .

4 Porosity in carbonate rocks is normally

physically complex, geneticalIy diverse,

and

dis-

tinct Born that of othcr sedimentary rocks.

Carbonate porosity generally diffcrs signifi-

cant ly f rom tha t of

sandstone

(Table I ) , with

which

i t

commonly s compared, in that the

amount

of

pore space is ordinarily smaller;

in-

terparticle porosity is css important and intra-

particle, intercrystal, moldic, and other types

much

more important; pore size a n d shape

can

he

much

more varied; and both the

pre-

and

postdcpositiona1 periods are more im portant in

forming and modifying porosity.

5

Pore

space which reflects by its position

and boundaries the depositional or diagenetic

fabric elements of

a

sediment or rock is termed

fabric selective. Porosity formed early in di-

agenesis is commonly fabric selective,

in

con

trast to much of the porosity formed later

when unstable carbonate minerals

and

ost or

a11 former porc

space has

been eliminated.

Much carbonate porosity is fabric selective.

6.

The

t ime of final deposition and burial

provides a practical basis for subdividing the

porosity history o l sedimentary carbon ates into

three main

stages: prrdeposi t iond deposi

tional and postdeposiiional. Primary porosity

forms d uring th e first two stages, and secondary

porosity forms during

the

last one. T he use of

all

these

terms

is

independent

of

lithification.

7. Much postdepositional creation and modi-

fication

o porority o c c u r either very

early

o r


33/44

Home

Frc 12.-Examples of compound porosity types.

A. Mesopores

(black)

in skeletaI packstonc.

Larger

interparlicIe

voids

show evidence of solution enlarge-

ment. Somc nummulitid forarns contain

smalr

intraparlicle mesoporcs (arrows). Notice

partid

pore fiilings of

calcite overgrowths on echinoderm

debris,

seen

best

in centml part o phuto Classificalion

code smsW1 Jsx-

ImsBP (1: lO). Tertiary, Libya. Thin section, cross-polarbed light.

B Moldic and intercrystal porosity in sucrost: dolamitc. Molds (large black areas have

been

filled in part

by do omite

rhambs

(arrows)

and

Iarge

anhydrite crystak,

A . Several

undi~solvedcaIcitic echinoderm fragments,

C,

are

visible. Simple

~Iassification

code

representation would

be

cr-MOJl3C;

more

completc designation wouId

be cr-ImsMO/srn~BC(l

I2 .

Madisan

Group (Mississippian), Oregon

Rasin

field, Wyoming. hin section, cross-

polarized light.


34/44

Philip

W

Choquette

and

Lloyd

C

Pray

Table 2. lttributes ljsed to Define I3as1cPorosity Types

M a l l 1

ar l r ihutec

are inciicateci by X and

a t t r ibute \

o f lesser importance

h j

x.

[letailed definitions are given

In

glossarj)

Basic Porosity T jp e Si:r

i l q ~ c

I'osrrioi~

.Mode r , j lubrir

Exampb

Or

I irbrir Oriji,,r Selectiar

(Fig.

No .

~ .. . . . . . . ~

Boring x X VnriLlble 6F, G

Burrow

A X

X Yes

Breccia X Variable 1IC

Cavern X X Uncotnmonly IOE

Channel

X

No IOA,

C

Fenestral

X? x X x

Yes 9A-F

Fracture

\ Y

Uncol~l~iionly ?)

I

IA,

C

Growth framework

X X

Y s

Intercrystal

X X a

Yes

7 A 4

128

Interparticle X Y s 5A-E, 12A

Intraparticle X Yes 6D, E, 12A

Moldic X Ycs 8A--H, 12 8

Shelter xz X

X

Yes 6A, B

Shrinkage X Variable

vug

X

X No IOB, D, E

.

. .

. . .

Solution is the dominant process, but interpretation of process is not required for the definition.

2 The size implication is that pore size is large in relation to the normal size of interparticle fabric elements.

3 lntercrystal porosity applies largely to carbonate rocks composed of dolomite.

very late in burial history, when the sediment

or rock is influenced significantly by surface-re-

lated processes. Therefore, it is useful to subdi-

vide the postdepositional period into three main

burial stages (Fig. 1 )

: ( a ) t h e eogenetic stage,

when newly deposited and/or recently buried

deposits are subjected to processes operating

from or related to a deposition surface or a

surface of intraformational erosion; (b ) th e tel-

ogenetic

stage, when long-buried rocks are af-

fected by processes at or just below an erosion

surface; and c)

the

mesogenetic stage, or in-

termediate time of burial at depths below sig-

nificant influence by surficial processes. These

three term s also can be used t o designate the po-

rosity formed in each stage, the processes act-

ing during each stage, or the corresponding

burial zones.

8. Current porosity nomenclature can be im-

proved by adding a few new terms and by

sharpening or restricting the definitions of cur-

rent terms. Key elements of the nomenclature

we suggest are: (a) definition of primary and

secondary and predepositional, depositional,

and postdepositional as major porosity time

terms; (b) recognition of the eogenetic, meso-

Table 3. Times and Modes of Origin of Basic Porosity Types

(Letter symbols denote dom inant, D; subordinate, s; and rare, r)

Mode of Orrgln

T ~ m e f Oripin Relatrue to T ~ m e

i n a l eposition

Basic Porosity Type

Frame,,ork

Sor ril ,g, Organic or So ulion.

Before During After

Physical Decompositior~

Accre t'r 'n Packing Disruption or Replacement'

Boring

12

Breccia r2

Burrow r2

Cavern

Channel

r2

Fenestral r2

Frac ture rz

Growth framework

12

Intercrystal r2

Interparticle

12

Intraparticle

D

Moldic

3

Shelter

1 ?

Shrinkage r2

vug r2

Exclusive of porosity of recycled extraformational rock fragments.

2 This relates to porosity of individual particles, including intraformational clasts, that subsequently were moved to the site of final

deposition.

Intercrystal porosity of dolomites is of chief interest for purposes of this table.


35/44

Home

Geologic Nomenclature and Classification of

Porosity

241

senetic

and telogenetic ~ i m c tages , m J c i x r c -

sponding burial zones; ( c } restriction uf v try

and channel to ports of contrasting shapc

that

are not

fabric selective

(sce

glosrary); and

d ) proposal of a size

grade =ale l o r porosily,

the term8 for which can be used as prcfixes ci-

ther

to

a

porosity-type

term

e.g.,

micromold,

mesovug)

o r

t o pore

(e.g., micropore,

meso-

pore).

A glossary with

discussion

of most po-

rosity terms is appended

ta this

article.

9.

The

geologic classification

of

porosity

we

propose

incorporates

most

current

nornencIa-

ture

and the modifications cited, and is summa-

rized

i n

Figure

2.

Its main elements are

15

basic

porosity

types defined by

physical and / o r

genetic features.

Of these

types

seven (inierpar-

ticle, intraparticle, intercrystal, rnoldic, fenes-

tral,

vug,

and

fracture)

arc

thc dominant

forms

in

sedimentary carbonates.

Fach

basic type can

be

used

independently

or

combined with modi-

fying

terms that give

inlormation about genesis,

size, and abundance of porosity.

Genetic modi-

fiers

pertain to the

time

of porosity origin, the

process

of porosity modification (solution, ce-

mcntation, or internal

sedimentation), and the

direction

or

stage of porosity modification (en-

larged,

reduced,

or filled). These genetic

modi-

fiers give

the

classification much o I its interprc-

tive

value.

As

a

better

understanding of porosity

in sedi-

mentary

carbonates is

developed,

it undoubt-

edly wil1 prove d e s i r a b l e t o b h d more

e l a b

rate

or different classifications

of porosity,

and

an

entirely

ncw nomenclature

may prove Eeasi-

blc fo r general a i ~ d pecialized purposes.

Per-

haps

t h c

system advocated here

will

speed these

develonments. I n the interim w e

how

this

4

article will help to

focus

more attention on t h e

useful geoIogic

information available

from

scrutinizing pores

in

relation

to their carbonate

host.

Adam J. E. 1953, Non-reef limestonemrvoin: Am.

Assoc.

Petroleum Geologists Bull., v.

37,

p. 2566-

2569

and

H. N.

FrenzeI. 1950. C a~ i tan

arrier

reef,

Texas and

New

~ c x i c o : O&. deology,

v. 58,

p:

289-312.

and

M.

L.Rhodw, 1960, Dolomitization by

seepage refluxion: Am. Assoc.

Petroleum

Geologists

Bull.,

V.

44,

p.

1912-1920.

American Geologicrml Jnstirute, 1960, Glossary of ml

oey

md

related

sciences

with

r u p p k m c a :

Warfin;

ton, D.C. Am. Ged.

Inst.

Pub. 501,

397

p.

Archie, G E.,

1952,

Classification of carbonate

reser-

voir rocks

and

petrophysicd considerations:

Am.

Assoc.

Petroleum Geologists Bull.,

v. 36, p. 278-298.

.4


36/44

242

Philip

W . Choquette

Home

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Lloyd C.

Pray

(reef) in New Mcxico (abs.): Ain. Asmc. Pctroleun~

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965,

Vadose p~sulite in thc Capital Reef

(abs.)

:

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p. 338.

Engelhardt, W. von,

1960,

Der Porenraum rlcr Sedi.

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Etienne,

J.

1963.

Rock impregnation

by

colored resins

fo r studying porosily in thin section: Inst. Fran~a15

Petrole Rev.,

v.

18 no.

4. p.

611-623.

Evamv.

B. D..1961.

DedoIomitization and

the

develon-

mei t of rhornbohedral

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Sed. Petrology, v. 37 p.

1204-121

5.

Evans, H.

B.,

1965 GRAPE-A device

for conlinuous

determination of material density and porosity: 6th

Ann. Soc. Prof. W ell t o g Analysts

Symposium

Trans., DaIlas,

v. 2 p.

B1-B25.

Fairbridge, R W. 1967,

Phases

of diagcoesis snd au-

thignesis,

in

G.

Larsen

and G .

V.

Chilingar, eds.,

Diagenesis

in

sediments: nevelopments in Sedimen-

tology, Amsterdam EIsevier Pub. Co. v 8 p.

179-

--

JLL

Pay,

A,

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1962 Modern wnoepts and

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J. W.,

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The

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R.

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1966

Pore geometr

of carbonate racks

(abs.1:

Am.

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50

p. 619

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P.D.,

1948

The

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L. S.. 1967

Diagenesis

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Leighton

M W. and G. Pendextcr, 1962 Carbonate


37/44

Geologic Nomencloture an

Home

d Classificer~ion

o f

Porosity 243

rock types,

in W.

E. Hxn

ed.

Classlfic:~~ion f ca l-

bonate

rocks--a

sympocrurn: Am.

AFWIS.

I a t t r ~ ~ l r ~ r n

Geologists

M e m .

1 p. 33-hl.

Levorsen A. I. 1967

Geology

of

pc~rolcum

2d

etl.

San Francisco,

W. H.

Freeman

Ca. 714 p.

Lindstrom, M., 1963, Sedimentary folds ant7 the ttevtl-

opment of limestone in an Early

Orctovician


38/44

Home

Phi l ip W .

Choquette and

Lloyd C.

Pray

Todd, T. W., 1966, Petrogeneuc classilicat~r~nf

cilr-

quantitalibu irnportnnce. C:irbonate brcccii~%rc vf d i

bonate rocks: Jour. Sed. Petrology. v. 76. p. 117-

verse

originr (Howard,

1967).

Some Form by deposition

349.

of

angular clasts,

These

depositional brecc~as

m y re-

Waldxhmidt,

W

A .

P.

E. Fitzgerald, and

C,

L. I.un*-

tain some

primary

porosity in the ancient

geologic rec-

ford, 1956. Classification of porosity and fractures

in

ord if they were well sorted initiatl and were com-

reservoir rocks:

Am.

Assoc Petroleum Geologists

posed of rehtirdy large particles; b u t typically.

the

Bull., v. 40, p.

953-974.

more paorly sorted, matrix-rich depositional brec-

Waring,

W.

W.,

and

D.

B.

Layer,

1950

Devonian

do-

c i a ~uch

as

carbonate debris

flows,

retain negligible

lomitized reef,

D-3

reservoir,

Leduc

field, Alberta,

porosity. P(~stdepositiona1breccias form by fracturing

Canada: Am. Assoc. Petroleum Geologists Bull., v.

of prcvioully depoaiied sediment or rock.

These

can be

34,

p.

295-31 2.

termed "fracture breccia" and

any

associated porosity

"frac tur~breccia porosity." If the process responsible

for fracturing is known, he fracture breccias

can

be

identified more specifically

as

collapse

breccias (Stan-

PPENDIX A

ton,

1966),

fault breccias,

iectonic

breccia$, etc Any

Glossary

oE Porosity erms

In this

Glossary most

of the

terms

t ha t

have

been

used in the past

f w

decades

to

characterize porosity

insediments carbonates are d d ne d and/or discussed.

The listing orterms i r alphabetic. A usage i s suggested

for

each

term

which either reilects prevailing

usage

as

we understand it, or seems desirable in view of p r e n t

knowledge

about

carbonate rocks. For

somc

of

the

terms, thc

glossary gives

the

original definition and rc-

views significant subsequent usap. But for

most

tcrms,

particularly

the

oIder

oncs

and those which havc

evolved gradually

and

sumewhat haphazardly from

a

nontechnical usage into more precise

usage,

details

of

the evolution of

the

term have little relecancc.

The discussions of many

terms

not

only

conrider

definitions

and

usagc, but briefly treat the geologic nc-

currencG and/or the

origin

of the porosity features.

Birdmye, birdstye fabric, bfrdmye pordty.-Zn

sedi-

mentary carbonates, the term "birdseye" commonly ia

used fo r oonspicuous, somewhat lensshapcd or globular

masses

of

sparry

carbonate cement

a

few

millimeters to

I

cm

or more

in size. Although

the

term normally refers

to either

the

sparry

carbonate

features thernselvcs or to

the

carbonate

r wk

containing

thcrn {Folk,

1959; Ham,

1954; ][ling, 19591, it has afso been applied tu voids

of

like sizes and shapes; hence, the expression "birdseye

porosity." Mwt birdseye features appear to

be

identi-

cal

to what

has

k n ermed more recently "fenestral"

(Tebbutt ef a ., 1965). We recommend adoption of

"fenestral"

q.v.) for the

individual

features

whether

open or infilted, and for the fabric of the rocks cnn-

taining such fkatures.

The

use of "fenestral" achieves

more precision than

birdseye

and

avoids

possible

coofusion arising from the use of "birdseye" for lens-

like

or

"augen"

features

of

varied origins in

nonsedi-

mentary rocks.

Boh& s ,

b r h g

pm&y . T n i n g s created in rela-

tively

rj id

constihJents or roc

by boring

organisms.

A

ngid

\mt

s

the feature

which

distingui.hes boringr

from burrows: the latter form

in

unconsolidated sedi-

ment.

Porosity created by boring

organisms

is not

abundmt

in most

ancient carbonate

rocks, but

b o h g s

constitute a distinctive and wmmonly genetically

im-

portant minor type of porosity (Fig.

6F,

G . Borinp

can he

formed

by by

variety

of

organisms in

n wide

array of depositional or eogenetic environments and

also can

be

formed

in

the telogenetic zone [Fig. 1 ) .

Recognition of borings (whether as porosity or as in-

filled openings)

can,bc

important

ia

environmental

and

stratigraphic anal

Discussions

on

borings in

car-

bonate r o c k a n ~ & t i s l e r ave been

given

by Gins-

burg

I956), Behrens

(19651,

Bathurst

1964,

1966).

Matthews

1

9664, and Boekschoten (1966).

Bmccla prosity.-The

type

of interparticle porosity

in breccia Breccia3

are

rather

common

in many

carbonate

facies, but breccia porosity is only localty of

assmiatcd hreccia porosity can be designated simitorly.

Fracture-breccia porosity commonly intergrades with

iracturc porosity. We ilifferentiare

the

two on the basn

of the amount of diqptncement or chaos created by

choquette & pray 1970

Documents