PROCEEDING SIMPOSIUM NASIONAL IATMI 2001
Yogyakarta, 3-5 Oktober 2001
APPLICATION OF MICROSCOPIC ANALYSIS IN RESERVOIR
CHARACTERIZATION: SECONDARY MINERALS AND THEIR INFLUENCED ON
CLASTIC RESERVOIR ROCK IN SOME AREAS IN WESTERN INDONESIA
Junita Musu & Hadi Prasetyo
Sedimentology Lab., Exploration Division, PPPTMGB “LEMIGAS
Key words: Petrographic analysis, reservoir characterization, authigenic minerals, reservoir quality, reservoir damage,
migration of fines, acid sensitivity, clay swelling.
ABSTRACT
An integrated petrographic analysis i.e. thin section, x-ray diffraction (XRD) and scanning electron microscopy (SEM) /
energy dispersive x-ray (EDAX) is one of various analyses available, which is relatively cheap and gives quick and accurate results
in supporting reservoir characterization study. This analysis has documented studied sandstones (mostly taken from North / Central
Sumatra, northwest / northeast Java, Barito and Kutai Basins) contained micro porosity and various authigenic minerals such as
cements as well as grain alteration or grain replacement which significantly effect on well log characters, reservoir quality, well
completion and production. Authigenic phases include authigenic clays i.e. kaolinite, illite, chlorite, and mixed-layer illite /
smectite, and carbonate minerals i.e. calcite, siderite, Fe-dolomite and pyrite.
Accordingly, the sandstones evaluated may have three main engineering problems i.e.: migration of fines (kaolinite), acid
sensitivity (calcite, Fe-dolomite, pyrite, chlorite and illite) and clay swelling (mixed-layer illite /smectite).
INTRODUCTION
Many studies of petrographic analysis have produced a
detailed understanding of sandstone diagenesis. The main
objective of sandstone diagenesis study is not only for
predicting reservoir quality but also for better understanding
of certain diagenetic (secondary/authigenic) minerals which
have greatly effect on the formation evaluation and reservoir
damage if improper type of fluid was used during well
completion and or production (Pittman, 1982; Schaible et.al,
1986; Davies, 1988)
The following analytical techniques i.e. thin section
petrography, Scanning Electron Microscopy, equipped with
EDAX and X-Ray Diffraction are of particular importance for
quantifying and determining the development of secondary
minerals within sandstones pore system.
The objective of this paper is to illustrate some diagenetic
processes resulting in a variety of secondary minerals that is
susceptible to formation evaluation and formation damage.
The samples that are being used for case studies have been
taken from sandstone reservoirs in some areas containing
prolific hydrocarbon in western Indonesia.
SANDSTONE
MINERALS
DIAGENESIS
AND
SECONDARY
Diagenesis is an all-encompassing term for every physical
and chemical change that may affect sediment from the
moment it is deposited until the onset of metamorphism. This
includes processes of compaction, cementation, dissolution,
recrystallisation and replacement. Many workers invoked that
the main factors controlling the diagenesis are generally
temperature, pressure, detritus composition, pore water,
sedimentary facies, tectonic and time. Consequently, various
distinctive diagenetic processes and their products would be
expected from one area to another.
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Detailed discussion will be documented by considering some
diagenesis studies done by authors and their colleagues on
Miocene and Pleistocene sandstone reservoirs in some areas
in Western Indonesia (Figure- 01).
In a case of Central Sumatra Basin, sixty-four (64) samples
from selected Miocene sandstone intervals in Central
Sumatra Basin have been prepared for petrographic, XRD
and SEM analyses. These sandstones may represent fluvial
deltaic to marginal marine deposits. All sandstones are
predominantly composed of monocrystalline quartz with
subordinate feldspars, rock fragments, mica and glauconite.
According to quartz, feldspar and rock fragments ratio, most
sandstones can be classified as sublitharenites though
subarkoses and litharenites is also contributed.
The diagenetic processes affected the sandstones are
compaction, cementation and dissolution of feldspar and the
formation of authigenic clays. The most common cement in
the sandstones analyzed is quartz overgrowths ranging from
2% to 7%. Other cement such as calcite and siderite / Fedolomite can be locally found significant in amount, reaching
up to 35% and 22% respectively. The dominant authigenic
clays are kaolinite (2-23%) and illite (6-11%) with occasional
smectite (trace - 8%) that is locally identified in several
locations.
Study of diagenesis in Northwest Java Basin using integrated
petrographic, XRD and SEM examinations were carried out
on fifty-four (54) selected sandstones and presented as
follows. The sandstones, based on core description results
may indicate as shallow marine deposits.
In general, the framework grains are dominantly
monocrystalline quartz (more than 50%), glauconite (5-13%),
rock fragments (8-13%) and feldspar (9-12%). Minor grains
include mica (2%) and fossil fragments (less than 1%). Many
of feldspar grains have been dissolved creating secondary
porosity and kaolinite. Detrital matrix contents are relatively
Application of Microscopic Analysis in reservoir Characterization : Secondary minerals
and their influenced on clastic reservoir rock in some areas in western Indonesia
low, approximately less than 10% of bulk rock volume. All
sandstones can be classified as lithic-arkoses and feldspathic
litharenites.
The most significant diagenetic cements observed in the
sandstones analyzed include (in order of significance) porefilling calcite, pore-lining chlorite and illite, pore-filling
kaolinite and quartz overgrowths. Pyrite, siderite, dolomite,
mixed-layer illite/smectite are only found as minor cements.
In some samples, XRD data indicate high amounts of calcite
(20-40% by weight percent). This calcite is resulted from
principally neomorphism process of many fossil fragments.
Chlorite is found in all sandstones (3-14%), coating most of
detrital grains. Formation of illite and mixed-layer
illite/smectite is a product of early stage diagenesis, in- situ
alteration of glauconite shortly after deposition or a direct
precipitation of formation water. Kaolinite formation ranging
from 1% to 8%, formed as pseudo hexagonal plates and is
interpreted as originating from ions released into solution by
feldspar dissolution.
In Kutei Basin, sixty-six (66) Miocene sandstone samples are
prepared for integrated Petrographic study. On the basis of
framework grains composition, chiefly quartz, feldspar and
rock fragments the sandstones can be mostly classified as
sublitharenites.
The sandstones are dominantly composed of quartz (60-70%)
with small amounts of rock fragments (3-6%), feldspar (23%) and organic matter (1-7%) and other accessories
minerals such as mica, glauconite, heavy minerals and fossil
fragments.
The framework grains are lying within matrix, considered as
tiny dispersed grains less than 20 microns in size, ranging
from 4% to 12%.
The cementing agents are clays, consisting of grain-coating
chlorite and illite, pore-filling kaolinite, quartz overgrowths,
pyrite and carbonates identified as calcite and siderite.
Among clays, based on XRD and SEM analyses, kaolinite is
the most significant cement in sandstones compared to
chlorite and illite. Pyrite is one of the most common cement
precipitated in the sandstones with the proportion ranging
from 3% to 6%. Siderite and calcite cements occur
significantly in several sandstone intervals as pore-filling
cements, while quartz overgrowths are of lesser importance.
Fifty-five (55) core chip samples from the Barito Basin, are
evaluated using integrated thin-section petrographic, SEM
and XRD examinations. According to core description
results, the sandstones envisaged as fluvial deposits.
In general, the sandstones are mainly composed of 24%-57%
monocrystalline quartz and volcanic rock fragment (13%37%) with additional feldspar (2%-12%) and small
proportion of detrital clay matrix. At the upper part of
interval, volcanic rock fragment content is rare. All
sandstones can be classified as feldspathic litharenite,
sublitharenite and subarkose. Many of feldspar grains have
been dissolved producing both secondary porosity and
kaolinite.
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Junita Musu, Hadi Prasetyo
The main diagenetic cements that have documented from the
sandstones analyzed (in order of significance) i.e.: pore
bridging / lining smectite, pore filling kaolinite / calcite /
zeolite and pore lining chlorite. XRD analysis reveals that the
sandstones contain 4%-16% smectite, 2%-19% kaolinite,
2%-15% zeolite and 1%-6% chlorite.
POTENTIAL RESERVOIR PROBLEMS AND
TENTATIVELY SOLUTION OR MINIMIZING
THE PROBLEMS
Diagenetic minerals, mainly authigenic clays and carbonate
minerals are particularly sensitive to reservoir quality.
Poupon et.al. (1970), Pittman (1977) and Munson (1988)
explained that mode of occurrences of authigenic pore clay
significantly controlled permeability rather than porosity
values (Figure-02). Introduction of drilling, stimulation and
recovery fluids can have a profound effect on these diagenetic
minerals and resulting either enhanced reservoir or reduced
reservoir quality.
For typical sandstones analyzed in this paper, it appears that
diagenetic modification on sandstones results in variable
amounts, mode of occurrence and types of authigenic clays
and carbonate minerals that lined and or filled the pores. Due
to the presence of both authigenic minerals (with in order of
significance summarized in Table-01) the sandstones may
have three main engineering problems i.e.: migration of fines
(kaolinite), acid sensitivity (calcite, Fe-dolomite, pyrite,
chlorite and illite) and clay swelling (mixed-layer
illite/smectite and smectite).
All of the sandstones presented in this paper contain
significant amounts of kaolinite. Kaolinite occurs as
hexagonal plates that are generally loosely bound to pore
walls (Figure-03). Kaolinite clay crystals are easily
dislodged and migrated under conditions of high fluid
turbulence and if contact with fresh water based fluids or
high pH fluids (higher than 10). The migration of fines
problem related to high fluid turbulence results from high
well bore/formation pressure differentials during perforating
or production may be minimized using low under balanced
differential pressures (no exceed than 2000 psi). The use of
KCL, CaCL2 and NaCL as completion fluid candidates and
the pH of drilling mud below 10 are recommended in
inhibiting the migration of fines due to contact with fresh
water based fluids and with high pH fluids respectively.
The sandstone reservoirs containing calcite supplemented
with iron carbonate (siderite, Figure-04; and Fe-dolomite),
iron clay (chlorite, Figure-05; and illite, Figure-06) and
pyrite (Figure-07) are susceptible to damage caused by
incorrect acidization, in case of iron-hydroxide precipitation.
A routine use of mud clean-up acid (15% HCL or 12% HCL
plus 3%HF) should probably be avoided in the reservoirs.
The use of 15% HCL will liberate iron from the common
siderite, Fe-dolomite, chlorite, illite and pyrite and this
results in reprecipitation of ferric hydroxide occurring as a
brown gelatinous mass which will occlude both pore spaces
and throats. Hydrofluoric acid should not be used in the
formation unless care has been taken to remove some of the
calcium prior acidization with HCL. Weak HCL (less than
Application of Microscopic Analysis in reservoir Characterization : Secondary minerals
and their influenced on clastic reservoir rock in some areas in western Indonesia
Junita Musu, Hadi Prasetyo
7%), together with appropriate iron and calcium chelating
agents, will prevent formation damage results from incorrect
acidization.
1)
Migration of fines due to the presence of kaolinite. The
fines migration problem appears to have seriously
affected the sandstones in all basins studied.
The sandstones from the Central Sumatra and the Northwest
Java Basins comprise small amounts of mixed-layer
illite/smectite and smectite, except for Pleistocene sandstone
in Northeast Java and Barito Basins containing smectite and
zeolite in significant quantities (Figure-09). They occur as a
thin, pore lining material and as structural shale fragments
that replaced feldspar. If the reservoirs containing mixedlayer illite/smectite requires fracing or flushing, it will be
recommended to use of a 3% KCL solution (or oil-based
solutions) to avoid any potential swelling problems caused by
the presence of smectite clay.
2)
Acid sensitivity due to the commonly presence of calcite,
Fe-dolomite, pyrite, chlorite and illite.
The result of an integrated thin section petrographic,
XRD and SEM analysis illustrate that acid sensitivity
problem of: The sandstones in North Sumatra basin will
be affected by a combination of chlorite and Fedolomite; the sandstones in Central Sumatra Basin is
caused by a combination of illite, calcite and siderite;
the sandstones in Northwest / northeast Java Basin is
due to the presence of illite, chlorite, illite/smectite,
heavy minerals and calcite, and the sandstones in Kutei
Basin is greatly affected by calcite, siderite and pyrite.
SECONDARY MINERALS INFLUENCE ON LOG
RESPONSE
3)
The combined thin section petrographic, SEM and XRD
analysis exhibit mineralogical compositional variation in the
vertical sequence of the studied sandstones. These vertical
changes in mineralogy have certain influence on log response.
1) Gamma Ray Log
The radioactive elements that have been detected by these
tools are mainly K-40 in both K-feldspar and illitic clay. The
analyses show that the studied sandstones containing of
significant proportion of K-feldspar and illitic clay.
Therefore, the gamma-ray reading will be increased.
Consequently, determination of shale volume for the studied
sandstones directly from GR. log is difficult or will be overestimated.
2) Density and Resistivity Logs
The presence of iron bearing and conductive minerals (such
as siderite, pyrite and chlorite) will be influenced on both
density and resistivity logs (Figure-10). The density value
will be increased, consequently, calculated porosity from log
analysis is under-estimated. On the other hand, the resistivity
log will be decreased, therefore, determined water-saturation
will be too large.
CONCLUSIONS
An integrated thin-section petrographic, SEM and XRD
analysis provides useful tool for supporting reservoir
characterization study.
Study of sandstones diagenesis in some areas in Western
Indonesia with references sandstone samples from the North /
Central Sumatra, Northwest / Northeast Java and Kutei
Basins indicates various distinctive diagenetic processes and
their products. The products of cementation process, chiefly
authigenic clays, carbonate minerals and also pyrite play
main role not only to the reduction of porosity and
permeability of sandstone reservoirs but also to give effect on
the log response, reservoir sensitivity and reservoir damage.
According to authigenic minerals present the sandstones
may have three main engineering problems as follows:
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Clay swelling caused by the presence of mixed-layer
illite/smectite. This clay-swelling problem, though it is
of lesser importance, would be present in the sandstones
in local area in the Central Sumatra, and Northwest Java
Basins, and especially in Barito and Northeast Java.
ACKNOWLEDGEMENTS
We would like to thank our colleagues for supporting this
study.
SELECTED REFERENCES
Almon, W.R. & Davies, D.K., 1981. Formation damage and
the crystal chemistry of clays, Clays and the Resource
Geologist, Mineralogical Association of Canada, Short
Course, F.J.Longstaffe ed., Calgary, p.81-103.
Crowe, C.W., 1985. Evaluation of agents for preventing
precipitation of Ferric hydroxide from Spent Treating Acids,
JPT (April), p.691-695.
Krueger, R.F., 1986. An Overview of Formation damage and
well productivity in oil-field operations, JPT (February),
p.131-152.
Davies, D.K., 1988. Sandstone Reservoirs, with emphasis on
Clays, Stimulation and Formation evaluation, a three days
course.
Pittman, E.D., 1982. Problems Related to Clay Minerals in
Reservoir Sandstones. In Oil Field Development Techniques,
AAPG Memoir No.28, p. 237 - 244.
Prasetyo, H., and Wicaksono, B., 1996. The Relationship
Between Diagenetic Minerals and Formation Damage of
Clastic Reservoirs in Some Areas in Indonesia. One day
Seminar and Workshop on Formation Damage Paper,
LEMIGAS, Jakarta.
Prasetyo, H., 1997. Applied Microscopic Analysis for
Petroleum Industry. Makalah Ceramah Ilmiah dan Diskusi,
Jurusan Geologi, Fakultas Teknologi Mineral, UPN
“Veteran” Yogyakarta.
Application of Microscopic Analysis in reservoir Characterization : Secondary minerals
and their influenced on clastic reservoir rock in some areas in western Indonesia
Junita Musu, Hadi Prasetyo
PPPTMGB “LEMIGAS” & THE BRITISH GEOLOGICAL
SURVEY, 1993. The North Sumatra Basin: Hydrocarbon
potential of the Pertamina UEP-I area, Vol.1 & 2,
unpublished
report,Lemigas.
Schaible, D.F., Akpan, B., and Ayoub, J.A., 1986.
Identification, Evaluation and Treatment of Formation
Damage, Offshore Louisiana. SPE Paper No.14820.
Poupon, A., et.al., 1970. Log Analysis of Sand-Shale
Sequences – A Systematic Approach, JPT, p.867-881.
Table-1
Secondary Mineral Types in Studied Area
SECONDARY MINERAL TYPES IN THE STUDIED AREAS
SAMPLE
LOCATIONS
C L A YS
CARBONATES
OTHERS
Kaolinite Illite Illite / Smectite Chlorite Calcite
Fe Siderite Pyrite Zeolites
Smectite
Dolomite
N.Sumatra Basin
XX
C.Sumatra Basin
XX
XX
XX
N.E Java Basin
XX
N.W Java Basin
XX
X
Kutei Basin
XX
X
Barito Basin
X
Note:
X
XX = significant
XX
XX
XX
XX
X
X
XX
XX
XX
X
X
XX
XX
X
X
X = less-significant
Figure-1
Studied Area Location Map Related To Indonesia Basins
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X
X
XX
XX
X
XX
Application of Microscopic Analysis in reservoir Characterization : Secondary minerals
and their influenced on clastic reservoir rock in some areas in western Indonesia
Junita Musu, Hadi Prasetyo
EFFECT OF AUTHIGENIC
PORE CLAYS ON
POROSITY AND
PERMEABILITY
DISCRETE PARTICLE KAOLINITE
GRAIN
GRAIN
PORE LINING
CHLORITE
GRAIN
GRAIN
PORE BRIDGING ILLITE
GRAIN
GRAIN
Figure-2
Thin section photomicrograph, // n, x 33
SEM photomicrograph, x 260
Figure-03
Pore filling kaolinite (red arrows) reduce porosity and can be create fines migration problem, blue is porosity.
IATMI 2001-06
Application of Microscopic Analysis in reservoir Characterization : Secondary minerals
and their influenced on clastic reservoir rock in some areas in western Indonesia
Thin section photomicrograph, // n, x 33
Junita Musu, Hadi Prasetyo
SEM photomicrograph, x 200
Figure-4
Pore lining siderite (patchy brown) which significantly effect on reservoir quality development,
well-log character and reservoir sensitivity / damage, blue is porosity .
Thin section photomicrograph, // n, x 126
SEM photomicrograph, x 220
Figure-5 :
Pore lining chlorite (red arrows) which significantly effect on reducing permeability,
well-log character and reservoir sensitivity / damage .
Thin section photomicrograph, // n, x 33
Figure-7
The presence of pyrite (black) that significantly influences
on both resistivity and density logs reading.
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SEM photomicrograph, x 600
Figure-6
Illite pore bridging (red arrows) which is significantly
reduce permeability and influences on gamma-ray log
reading.
Application of Microscopic Analysis in reservoir Characterization : Secondary minerals
and their influenced on clastic reservoir rock in some areas in western Indonesia
Thin section photomicrograph, X n, x 128
Junita Musu, Hadi Prasetyo
SEM photomicrograph, x 360
Figure-9 A
Extensively precipitation of both pore lining (high birefringence rims on grains – left photo)
and pore bridging (right photo) smectite that is significantly reduced permeability.
Thin section photomicrograph, X n, x 128
Thin section photomicrograph, X n, x 128
Figure-09 B
Grains coated by authigenic smectite (left) and zeolite cemented (white – yellowish light brown – right photo)
pore-space followed by smectite rims.
IATMI 2001-06
Application of Microscopic Analysis in reservoir Characterization : Secondary minerals
and their influenced on clastic reservoir rock in some areas in western Indonesia
Junita Musu, Hadi Prasetyo
Komposisi
Litologi
Saturasi dari
tekanan
kapiler
Saturasi
dari
log
resistivitas
Water Volume
HC Volume
Figure-10
Low resistivity pay zone due to the presence of iron bearing / conductive minerals
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