Fracture and Vuggy Porosity

SPE Society of Petroleum Engineer'S of AIME SPE 10332 Fracture and Vuggy Porosity Bakker, Kon/Shell Exploration & Production Laboratories 'Member SPE-AIME «!Copyright 1981, Society o

SPE Society of Petroleum Engineer'S of AIME

SPE 10332

Fracture and Vuggy Porosity

Bakker, Kon/Shell Exploration & Production Laboratories

'Member SPE-AIME

«(!Copyright 1981, Society of Petroleum Engineers of AIME

This paper was presented at the 56th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AIME, held in

San Antonio, Texas, October 5-7,1981 The material is subject to correction by the author Permission to copy is restricted to an abstract of

not more than 300 words Write: 6200 N Central Expressway, Dallas, Texas 75206

ABSTRACT

The discovery of hydrocarbons in fractured

rock always brings up the controversial subject of

leaching and infilling often complicate the issue

The high recovery efficiency of oil from fractures

combined with the frequent absence of appreciable

matrix porosity, in many cases enhance the economic

importance of the estimated figures

Sometimes one can base ones predictions on

local production experience from similar fields,

but more often one has to rely on the limited

information from production tests, cores, logs and

occ<Jsionally neilrby outcrops At an early stage

this approach can only lead to the location of the

most fractured or vuggy zones and a very rough

estirnnte of the associated porosities

To reduce the uncertainty one can make use of

a statistical approach based on a classification

of fractured reservoir types with corresponding

porosity ranges collected from literature An

atten~t has been made to gather a comprehensive

worldwide data suite large enough to establish

reI iable porosity ranges for a series of reservoir

types

A simple practical classification into seven

types of fractured and leached reservoirs has been

adopted based on tectonic style and leaching

pnJCC'sses Any figures quoted in literature were

critically examined and only used when there was

clear evidence that they were based on thorough

fracture/vug spacing and fracture/vug width and

size studies or on reliable material balance

calculations This reduced the data set to a

fraction of its original size b~t the resulting

table is thought to stand up to scrutiny

INTRODUCTION Several years ago the authors recognized the need for a I;UlllPLefLt!U; overview of the subj ec t

of fracture porosity An increasing number of hydrocarbon accumulations were being found in fractured reservoirs every time bringing up the question of fracture porosity Also for fields which had produced for many years, the effective fracture porosity often remained in doubt rhe introduction of more sophisticated petrophysical

insight in the fracture porosity distribution and occasionally leads to realistic fracture void volumes Usually, however, that is only possible with the aid of a good geological understanding

of the reservoir

It WaS decided to carry out a literature review including any article of which the title indicated a relationship with fracture or vuggy porosity A total of some five hundred articles were studied and in addition data were gathered from Shell fields The first conclusion from this study was that it is remarkable how few reliable fracture or vuggy porosity figures are quoted in literature To augment the scarce data, the authors were able to arrive at reasonable estimates

in filling in the missing parameters from other similar occurrences

Although there is considerable variation between any pair of cases studied, a simple class-ification has been introduced to group the data in

a small number of groups of genetically related types, viz.:

3 Fracture porosity enhanced by leaching

4 Karst aquifers, surface to shallow

2 FRACTURE AND VUGGY POROSITY SPE 10332

Matrix type vugs not directly connected with

a continuous fracture system have been excluded

because their contribution to the reservoir porosity

can usually be determined quite well with logs and

cores

Basically, this paper is a literature study

and the emphasis is on providing references covering

the entire subject of fracture porosity together

with a realistic assessment of porosity ranges for

the various types

GENESIS OF FRAr.'l'TJRF AND KARSTIC POROSITY

This paper deals with porosity formed by the

following processes

2

Regional extensive strain, overpressuring of

pore fluids, decompaction by erosion, karstic

leaching, collapse brecciation because of

cave collapse or the solution of underlying

evaporates, shrinkage cracks and basement

erosion

Tectonic processes related to folding and

faulting

It is not the aim of the paper to describe

these processes in depth but instead an

annotated lite~ature survey is presented

A useful paper discussing the classification

of natural fracture systems has been written

by Nelson (1979) He emphasizes the need to

unravel the superimposed components of different

and warns against the inclusion of

surface related cracks resulting from release

of load in quarries and road cuts Stearns

and Friedman (1972) describe the various

common fracture systems and demonstrate the

influence of rock ductility/lithology and the

bedding thickness on the fracture density

Currie and Nwachukwu (1974) demonstrate that

incipient fracture porosity at depth can

develop gradually into a network of open

fractures under conditions of continued

uplift and erosional unloading

The mechanics of the development of faulting

and fracturing are treated by Price (1966)

The development of folds is discussed in some

detai 1 by Laubscher (1977) while the important

bedding plane slip occurring in folded anticlines

is described by Chapple and Spang (1974)

With respect to karst development, there is a

vast literature dealing with the formation of

cave systems Bogli (1976) proposes theories

on karst development in which he puts the

emphasis on the aggressive mixture of water

with different composition which is formed in

the phreatic zone and the effect of the CO 2

in the air in the caves of the vadose zone

The lithologic controls on the development

of solution porosity in carbonate rocks are

well documented by Rauch and White (1970)

Deike (1969) shows how the cave systems in a karst are often related to preexisting joints

or fracture systems An interesting re-construction of the development of a largely collapsed karst development associated with uplift and faulting and subsequently covered

by a marine erosion, was made by Poty (1980) There is clear evidence that the larger caves

in a karst tend to collapse with deeper burial An actual case causing an earthquake

in Libya has been described by Campbell (1968) Collapse caused by the dissolution

of evaporites underneath carbonates and other rocks is discussed by Herrmann (1968) In the oil field area of the North Sea similar phenomena have been recognized by Lohmann (1972)

Finally, we have the possibility of the creation of fracture pore space by fracturing

of the rock as a result of very high pore fluid pressures This process is often mentioned but little evidence is us'ually presented to substantiate this theory

Recently, however, Lapre and Pulga (1978) writing on the Emilio heavy oil accumulation

in the Adriatic, give fairly conclusive proof

of the intrusion of the heavy immature crude into the rock during a tensional phase of the comprehensive folding of the formation An unusual phenomenon associated with this process is the occurrence of horizontal oil-filled fractures

DETERMINATION OF FRACTURE AND VUGGY POROSITY The classic method of estimating fracture porosity is to examine cores, count the number of natural fractures and measure their width These methods are only realistic if one has a good handle on the fracture systems involved and the influence of bed thickness and lithology Outcrop studies are usually needed to provide this insight although great care has to be taken to recognise surface related decompaction fissures from these tectonic fractures Studies of this type have been carried out in Iran (fig 1), Turkey, and the U.S.A • Sangree (1969) who performed detailed studies of this kind in Iran, wrote a paper on these methods In Russia, much attention has been paid to the careful analysis of fracture porosity

on the basis of core studies, e.g Smekhov (1969) The bulk of the figures quoted in this paper have been derived in this fashion

In the course of such studies several factors have been recognised as essential in order to arrive at a realistic estimate of fracture porosity; viz :

1 Recognition of the various fracture systems, their relative age in relation to burial depth and oil migration In many cases the older fractures are cemented up by calcite and anhydrite and only the younger system is partly open Elimination of non-natural fractures

SFE 10332 K.J WEBER AND M BAKKER

2

3

4

Analysis of the influence of structural shape

(see Harris et aI, 1969), bedding thickness

stratigraphy (marly or shaly intercalations'

promote bedding plane slippage), lithology/

duc tili ty

Careful analysis of average open fracture

width for each fracture system Thin sections

are oIten used to study the micro fractures

Fracture widths of 0.01-0 I mm are common for

small joints Extension fractures of I to 25

feet in length can have open width of

0.1-I mm with an average in the order of 0.2 mm

Major extension fractures may be from 0.2 to

2 mm wide, usually with at least partial

infilling

Infilling of fracture pore space by various

in unraveling the age relationships of cements

in multiple fracture systems Karst features

often fill up with surface derived sediments,

ores or guano

Usually it is necessary to use a rock mechanical/

statistical approach to arrive at a realistic

fracture density distribution A good example

of this technique is presented by Kiraly

(1969) An excellent analysis of the relationship

of bedding thickness and fracture spacing is

given by Bock (1971) An example of a complete

analysis of cores and outcrops is the estimate

of the fracture porosity distribution of the

Gach Saran field in Iran (fig 2) The

fracture system was derived from observing

various outcrops of similar anticlines in the

Zagros mountains foothills (fig I) while the

fracture widths were derived from cores

Next in importance in determining realistic

fracture and vuggy porosities is the material

balance method Basically this method will

work very well if there is no effective

fluid interfaces accurately In aquifers in

karstic formations, this method can be quite

(~ffective A problem is the sometimes irregular

distribution of porosity in the vertical

sense In karst systems, near the surface

there are often one or more levels of cave

development which contain the bulk of the

cavernous porosity In collapsed karsts the

porosity is usually more evenly distributed

An excellent example of a reliable material

balance calculation of near surface karst

porosity could be made on the basis of the

account of the accidental flooding of the

West Driefontein mine in South Africa (Cousens

and Garrett, 1969) All the water from a

dolomite karst zone overlying the

gold-bearing formation drained into the mine via

a fissure In this area, the karst zone is

subdivided into separate compartments by

vertical igneous dykes Thus the bulk volume

of the drained karst could be accurately

established as well as the volume of water

~hich had to be pumped up The karst porosity

about 1% of the total drained bulk volume

However, most of the caves and larger cavities are restricted to the upper half of the interval and a porosity of some 2% for the main karstic zone is very likely

Besides the more basic material balance calculations, there exists a range of more sophisticated reservoir engineering methods

to determine fracture and connected vuggy porosity The basic thoughts were presented

by Warren and Root (1963) and in ideal circum-stances these methods work quite well There are however, several complicating factors

Firstly the pressure build-up curves associated with double porosity reservoirs are similar

to those observed for a stratified reservoir Secondly, there is often too little time to obtain a truly representative build-up curve Other complicating factors are baffles

to horizontal flow in the reservoir, skin effects, heterogeneous distribution of the fracture porosity, and variations in aquifer size and permeability

The basic problem is the assessment of the retarded matrix influx relative to the supposedly instantaneous fracture porosity influx Long shut-in periods are very helpful in solving this problem and both in Masjid-i-Suleiman (Gibson 1948) and in Amposta Marino fields such periods provided the key to a realistic estimate of the fracture porosity Mavor et

al (1979) discuss the analysis of one well pressure build-up curves of the type used for the Amposta Marino field The basic principle

is the recognition of two parallel, semi-log straight lines in the build-up graphs

Unfortunately the first line is usually obscured and we have to take recourse to more complicated methods

A type-curve approach is described by Bourdet and Gringarten (1980) Accepting the fact that the theoretically ideal dual porosity pressure build-up curve will usuailly be obscured by well storage, skin, and other disturbing factors, they derived type curves based on the behaviour of heterogeneous models which can be compared to measured build-up curves From this analysis, it is possible to estimate the ratio of the fissure storativity to the total storativity and the interporosity flow coefficient, which depends

on the shape the size and the permeability

of the matrix blocks

The numerous petrophysical methods to detect fractures and to measure fracture and vuggy pore space are excluded from this paper because they are the subject of several papers to be presented at the SPE annual conference of 1981

4 FRACTURE AND VUGGY POROSITY SPE 10332

This porosity development is characterized by

low density regional fracture systems with very

small open width (0.01-0.1 rnm) often formed by

de-loading during uplift Locally the fracture

density may increase near major fault zones (fig

3) or as a result of warping or differential

compaction Fracture porosity is usually very low

with a range of 0.01 to 0.1% of rock bulk volume

However, the fracture permeability is often crucial

in obtaining economic well productivities

Many very large oil fields fall in this

category, such as the large anticlinal structures

in Iran, Iraq and southern Russia For this

reason much effort has been given to the analysis

of the fracture systems and the related fracture

I width Outcrop and core studies, together with

reser~oir engineering calculations and some logging

exper1ments have gradually given a fairly good

ov~rview of the type of fracturing providing the

major fracture porosity

The individual beds are jointed in a mainly

orthogonal extensional pattern related to the

anticlinal axis direction Fracture density is

controlled by degree of bending, bedding thickness,

and bed ductility The degree of bedding plane

slippage along shaly or marly intercalations is

also important with respect to fracture spacing

and fold shape Huch bedding plane slippage often

lea~s to box fold type structures (Laubscher,

1977) On the anticlinal noses and flanks some

~hear fractures are formed but their contribution

to fracture porosity is minimal

Strong bending produces through going fractures

across series of beds, and partly depending on the

compe~ence of the core of the structure, keystone

fault1ng can develop parallel to the crest of the

structllres These major fractures and faults can

provide excellent vertical and lateral conununication

over large distances (fig J) In the Agha Jari

fi~ld, wells 7! miles apart undergo mutual pressure

adjustments to within a few psi (Drununond, 1964)

Fracture width ranges from values of the

order of 0.1 nun for the joints restricted to a

single bed to 0.2 - 0.5 nun for the larger fractures

intersecting several beds Partial or complete

infilling of the fractures by calcite or anhydrite

is conunon but in many cases a partially cemented

fracture may well retain a larger void volume than

an uncemented one Major faults are often associated

with brecciated zones and sometimes even tectonic

caves In these fault breccias internal fracture

porosities of some 5 per cent have been observed

in iran

Along the crest of sinusoidal anticlines and along the hinges of box fold type anticlines, fracture porosities of up to 0.4 per cent are possible The limbs of the anticlines show a quick increase of the fracture spacing downwards away from the crest or the hinge zones Therefore, the overall fracture porosity over a vertical interval of 1000 feet or more is unlikely to be larger than 0.2 per cent

FRACTURE POROSITY ENHANCED BY LEACHING, TABLE 3 Many fields in this category have undergone some near surface leaching and therefore corne close to the brecciated karst group However, the quoted examples probably never reached the stage where they could be called cavernous except perhaps the upper zone in Kirkuk

The bulk of the extra fracture porosity relative

to the foregoing group is formed by the enlargement

of the fracture width by leaching to up to O.S

cm In some cases, preferential leaching of certain fossils can also increase the apparent fracture porosity by creating vuggy zones connected

to open fractures or by causing brecciation

The well-known Mara and La Paz fields in Venezuela were long thought to have a fracture porosity of about 1 per cent However, subsequent studies showed that much of this porosity was actually situated in the intensively fractured basement rocks which consist of granodiorites mica schists, gneisses, and metamorphic quart;ites (Dikkers, 1964)

The fracture porosity range for this group is from 0.2 to about I per cent Values of about 0.5 per cent appear to be rather common

KARST AQUIFER, SURFACE TO SHALLOW, TABLE 4 Although cave systems sometimes appear to be very widespread and voluminous, the material balance calculations in karst aquifers indicate that the actual void volume is only of the order

of at most 3 per cent Moreover, figures of 3 per cent are probably only possible for the relatively

sm~ll vertical intervals of major cave development (f1g 4) For larger bodies of karst and vertical intervals of more than 300 feet values of about ) per cent are more realistic Early deep erosion and collapse may already cause pore space reduction and infilling by surface sediments

The range of the total porosity in caves and associated vugs and fissures is from 0.2 to 3 per cent

DEEPLY Not too many reliable figures could be found for this category although it is of increasing importance In the Mediterranean several such oil fields have been found such as Amposta Marino (fig 5) Castellon B5 and Nilde In the North Sea the Zechstein interval in Auk and Argyll should probably be classed in this group In the U.S., there is the large number of Ellenburger fields

It can be expected that in China many oil fields are of this type

SPE 10332 K.J WEBER AND M BAKKER

[n Auk and Argyll, it is difficult to

separate the contribution from the fractured

Zechstein from that of the Rotliegendes reservoir

and no reliable figures are as yet available

From the data from the other fields a range of

0.5 to about 2 per cent appears likely There

is frequently some evidence of matrix contribution

even though the matrix porosity may be very low

It is probable that the actual maximum fracture

and cavernous porosity is of the order of 1.5

per cent

FRACTURED CHERT, TABLE 6

The fractured chert of the Monterey formation

in California is famous for its extraordinarily

large fracture porosities The main reason for

this phenomenon is the occurrence of the chert

in thin beds separated by much more ductile

intercalations of shale, mudstone and dolomite

The softer rock keeps the brecciated chert

together and fracture widths of several millimeters

combined with fracture spacings of about 5 cm

can be observed Draping over a fault scarp as

in the Santa Maria field enhances the fracturing

and maximum fracture porosities of some 8 per

cent are possible

Few reliable data could be found for formations

in this category although there are several

important and also oil fields of this

t yp e Ii u b be r t eta I (I 955) des c rib e 0 i 1 fi e 1 d s

in serpentine rock in Southeast-central Texas

The oil is situated in fractures ranging from

hairlines to nearly 4 em width and which may be

nminly caused by the diagenetica11y alteration

of the rock The occurrence of oil in the

basement of the Mara and La Paz fields has

already been mentioned above

In the U.S.S.R., Bortnitskaya et a1 (1974)

describe oil fields in igneous rock in the

Dni.eper-Donets basin Cooling joints and intersected

ves iCllles in the b,lsal t can form fracture porosi ty

of up to 6 per cent

For chis heterogeneous group it is difficult

to estimate a likely range but values from 2 to

8 per cent have been observed

The fracture and vuggy porosity data presented

in this paper may not be very accurate individually

but the similarity between the figures in a

given group and the rather limited ranges give

some confidence in the overall picture It is

concluded that the tables can be used to slot in

new cases provided sufficient geological information

is available Although the mcidern logging and

reservoir engineering methods are an important

step forward, it is indispensable to prepare a

realistic geological model before deciding on a

fracture porosity figure

Care was taken to include a representative series of references in this paper in order that others interested in the subject can continue this type of work The authors do not doubt that there is much more published data while oil companies and universities probably have much additional information relative to the subject

Thus it is worthwhile to periodically review the available data to increase the understanding of fracture porosity development and consequentially

to improve the art of estimating and measuring this elusive parameter

ACKNOWLEDGEMENT The authors wish to thank the Shell International Petroleum Company and the Koninklijke/8hell

Exploratie en Produktie Laboratorium for permission

to publish this paper

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Sl'E 10332 K.J WEBER AND M BAKKER

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8 Fracture and Vuggy Porosity SPE 10332

'L\1l1,E I

t·lonoc 1 ines and low-dip anticlines : 0.0) 0.1% TABLE 3

Fracture porosity enhanced by leaching : 0.2 - 1.0%

I t on Country Lithology Fract Method Ref

porosity

Location Country Lithology Fract Method Ref Alsace- France Limestone 0.01-0.02 cores 23

& Vuggy

porosity

face

and

mite field

and Dolo- Zone bal

and

Zone Dukhan Qatar Limestone 0.02-0.04 cores 13

0.9 field

D :t U.S.S.R Limestone 0.2-0.33

f)eld~

Ukraine

Corbii Mad Roumania Limestone 0.9 cores 10

La Paz and Venez- Limestone 0.5 Mat 16

Diyarbakir Turkey Limestone 0.01-0.4 Outer Shell Pool Utah Dolomite

West Edmond U.S.A Limestone 0.08-0.56

Gach Saran Iran Limestone 0.17crest Outer Shell Hunton Pool

Oklahoma field and Dolo- 0.03flank & cores data

mite

•

Agha Jari Iran Limestone 0.22 Mat 25 The upper zone contains zones with very large vugs

Haft Kel Iran Limestone 0.21 Mat 25 be grouped with the karst cases

mite

field

Well Pakistan Siltstone 0.11-0.21 cores 51

Potwar

nni pnp,'--'" U.S.S.R Sandstone Up to 0.3 cores 5

Basin

D tan U.S.S.R Limestone 0.16-0.35 cores 32

Ural-Volga U.S.S.R Limestone 0.25-0.3 Mat 56

Stavrc~)ol U.S.S.R Limestone 0.1-0.3 cores 22

S.W Lacey U.S.A Silicious 0.17 R'es 40

Creek field,

Texas

SPE ~0332 K.J Weber and M Bakker

9

TABLE l~

TABLE 6

,

Fractured Chert : 5 - 8%

porosity

& 47

bal

Cetina

" " 0.2 Mat 39 S 1 md Germany Keratophyr 2.3-8 cores 27

-Ebinger Alb Germany

Schwabi-" 0.5-2.0 bal 58

zone,

±I for total interv

11<') Ik U.S.A Limestone 0.5 Hat 60

Kentucky

TABLE 5

Deeply burried brecciated karst,

collapse br~ccias : 0.5 - 2.0%

& Large size Vuggy porosity

Castel10n

Ellenburger Tex<ls

Pegasus

" " 2.8 cores 8

Ellenburger

Fullerton

" Limestone 0.33-1.04 cores 30

Lower flank with

conjugate shear fractures

" Axial fractures predominate in hinge zones

Fig 1 - Sketch of anticlinal Asmari formation outcrop in the Zagros range foothills, Iran

3-6 PER KM

2000 FTSS

4000 FTSS

6000 FTSS

. -' -'

Fig 2 - Distribution of fracture porosity in Gachsaran field, Iran

- - - - ' r ' - - - , - - - - - - - - ;<. -

EOCENE

-725 m - .f.-# ·-IH-Al-lf¥ +"""_-=-· -··· - J - - - - -

750m

5

2000 FTSS

4000 FTSS

6000 FTSS

8000 FTSS

tOOOO FTSS

- 775m .-+-11+1 - - - - -

.-.-. =-<-= . -I~. +-f . -_f_ 800m -+->~ -. . -~~- -

.' -' -'

Fig 3 - East-West cross-section fracture profile of Eschau field, France (After Ghez and