II. PETROGRAPHY
a.
Introduction
Petrographic analyses were carried out on fifty-five [55] samples. The
lithologies and reservoir characteristics of these samples are tabulated,
Figs. 3 & 4, and the petrographic descriptions together with
photomicrographs are provided Plates 1-55.
b.
Procedures
The sidewall cores were impregnated with blue-dyed epoxy resin to
facilitate porosity recognition. They were half-stained with a mixture of
Alizarin Red “S” and Potassium Ferricyanide reagents.
Two color photomicrographs were taken from each thin section to
illustrate representative features of the samples.
Carbonate rocks are classified according to the scheme introduced by
Dunham [1962], while porosity types were identified according to
Choquette and Pray [1970]. The clastic rocks are classified to the
scheme of Folk [1971]. The amount of various components of rock
samples was inferred with the help of the comparison chart for visual
percentage estimation according to Terry and Chilingar [1955].
Modal analysis by point counting was not requested by Gulf.
The following quantifiers
petrographic data sheets:
>15%
5-15%
1-5%
<1%
abbreviations
are
used
in
of the rock volume
of the rock volume
of the rock volume
of the rock volume
Lithost
ratigra
phy
[Gulf]
A
abundant
C
common
M
minor
Trace
and
Lithology
Classified
Lithology
Tuban
Limestone
Wackestone
2
Rancak
Limestone
Wackestone
3
Kujung
Limestone
Pack-grainstone
SWC
Depth
[feet]
Plate
No.
3738.0
1
4148.0
4760.0
the
c.
4772.0
4
4782.0
I
Limestone
Packstone
5
Limestone
Wackestone
4786.0
6
Limestone
Wackestone
4792.0
7
Limestone
Wacke-floatstone
Depositional Environment & Reservoir Quality
The following are some general comments, by formation, on depositional
environments and reservoir quality of the rocks studied.
The comments on depositional environment can only be regarded as
tentative, without additional evidence from biostratigraphy etc.
i.
Tuban Formation
Only one [1] sidewall core, 3738’, was analyzed from this formation
and it is described as a limestone, Plate 1, which is a coarse grained,
poorly sorted, skeletal wackestone, with a minor amount of lime mud
matrix. The matrix has generally been replaced by very fine dolomite.
Skeletal grains are predominantly benthic foraminifera with common
molluscan and coral debris, minor red algae, echinoderm debris, and
planktic foraminifera. Sand and silt-sized grains are sparsely
distributed. Calcite cement occurs as fine to medium, crystals, in the
chambers of skeletal components. It locally forms isopachous cement.
There is good visible porosity [20% visual estimate], comprising a
combination of primary intraparticle pores associated with coral and
foraminifera, secondary pores resulting from dissolution, and
intercrystalline pores between dolomite rhombs. Fractures are also
significant, connecting the pores.
The skeletal grain and depositional textures suggest that this limestone
was deposited in a low energy shallow carbonate shelf environment.
ii.
Rancak Formation
Only one [1] sidewall core, 4148’, was analyzed from the formation
and it is described as a limestone, Plate 2, which is a lower coarse
grained, poorly to moderately sorted, skeletal wackestone locally
grading to packstone, with abundant lime mud matrix. The matrix has
been selectively replaced by very finely, euhedral to subhedral
dolomite. Skeletal grains are predominantly larger benthic foraminifera,
with minor red algae, echinoderm debris, smaller benthic foraminifera,
corals and micritized skeletal grains. Calcite cement is minor, occurring
in finely to medium crystalline equant to blocky forms, in the chambers
of skeletal components.
This limestone is characterised by good porosity [12% visual estimate],
consisting of a combination of primary interparticle pores between
skeletal grains, secondary pores resulting from dissolution, and
intercrystalline pores between dolomite rhombs. These pores are often
well interconnected by fractures.
The skeletal grain assemblages and depositional textures suggest
deposition in a low energy, shallow carbonate shelf environment.
iii.
Kujung I Formation
Sixteen [16] sidewall core samples from Kujung I were analysed,
fourteen [14] of which are limestones and two [2] dolostones.
Depositional Environment
The Kujung I limestones from this well are dominantly wackestones,
with minor grainstone, packstone and floatstone textures. The
wackestones contain varying amounts of skeletal fragments, generally
set in an abundant lime mud matrix. This matrix has been locally
replaced by dolomite.
The assemblage of skeletal grains is very similar in most of the
Kujung I limestones. The sizes of skeletal grains range from fine to
pebble, but are generally between coarse and very coarse, with poor
sorting coefficients. The most common skeletal grains observed, in
whole and fragmented forms, are larger benthic foraminifera, Plates
4A & B, 12, 13B, 15B. including lepidocyclinids, however, coral
fragments are sometimes present in significant amounts and prevail
over larger benthic foraminifera at 4792’, 5161’ and 5500’, Plates 7A &
B, 14A. Other skeletal grains that are frequently present, but in
varying amounts are red algae, Plate 4B, smaller benthic foraminifera
including miliolids, echinoderms, Plates 6A, 7A, peloids, Plates 8B,
13B, molluscs and micritized skeletal grains. Green algae and planktic
foraminifera, Plate 16B are also present in trace amounts in some
samples.
The dolostone at 5349’ is composed of finely crystalline euhedral
dolomite rhombs, which forms a hypidiotopic fabric providing
numerous porous zones. The original texture of this dolostone could
not be determined with certainty, since no surviving depositional
fabrics remain, Plate 17, due to the intensive dolomitization. The
dolostone at 5500’ also resulted from the intensive dolomitization of a
precursor limestone. It contains minor skeletal grains including
molluscan and coral debris, and patches of lime mid matrix that are
suggestive of a wackestone depositional texture, Plate 18.
Based on the presence of miliolids, the low diversity of faunal
assemblage and the abundance of lime mud matrix, the predominant
palaeoenvironmental setting of Kujung I is interpreted as a low
energy, relatively restricted, shallow marine shelf. The abundance of
coral fragments in some of the limestones, such as 4792’, 5161’,
indicates reefal or “near-reef” conditions, although evidence for the
presence of in-situ reef facies is not conclusively seen in thin sections
analysis.
Visible Porosity and Reservoir Quality
The Kujung I carbonates have variable visible porosity from poor to
good, but most of samples [eleven of the sixteen] have fair to good
porosity, ranging from 6% to 22% by visual estimate. The best
reservoir zones identified by petrographic analysis are found at 4792’,
4896’, 5205’, 5349’ and 5500’ with visible porosity being greater than
15%.
Porosity, in the limestones, consists generally of a combination of
vuggy pores, Plates 11 A & B, 13B, resulting from dissolution, and
tectonically generated fractures, Plates 13A, 16A & B, 18 A & B.
Fractures, though volumetrically minor, are thought to provide
considerable permeability assistance to the pore network. Mouldic
pores and primary intraparticle [intraskeletal] pores associated with
the chambers of foraminifera and corals, Plates 7A & B, 15B and
intercrystalline pores are also present in some of the limestones.
In the dolostones, 5349’ and 5500’, porosity is dominated by
intercrystalline pores occurring between dolomite rhombs Plates 17 A
& B, with minor amounts of vuggy pores, which sometimes form
oversize pores ranging up to 2.30mm across. These oversize pores
were probably created by vadose groundwater dissolution.
The diagenetic processes that have affected the limestones and their
reservoir quality from Well Jenggolo-1 include cementation,
replacement and dissolution.
The diagenetic processes commenced syn-depositionally in the marine
phreatic environment, and were followed by major diagenetic changes
and modifications in vadose and meteoric phreatic environments.
Diagenesis continued with increasing overburden pressure and later
tectonism caused the development of fractures and stylolites during
deeper burial conditions.
All of the limestone samples are considered to display a similar
sequence of diagenetic fabrics. A generalized pattern of diagenesis of
the limestones as interpreted from thin sections is presented below
and they are listed in relative chronological order. However, not all of
the events and diagenetic fabrics can be seen and found in all of the
limestones.
i.
Early/Syn-depositional Lithification of Matrix and
Micritization of Skeletal Grains:
Most depositional lime mud matrix is thought to have lithified early,
neomorphically
stabilizing from lime mud to microspar Plates
34B, 40B, 41B. Matrix stabilization and micritization of skeletal
grains probably took place in the marine phreatic environment, and
is likely to have been accompanied by some minor early fringing
and isopachous cements [a few such cements are locally recognized
in thin section such as in samples 5848’, 6198’].
ii. Dissolution of Unstable Skeletal Grains:
This process is
considered to have occurred under freshwater vadose diagenetic
conditions, forming moulds and enlarged moulds or vugular pores.
Leaching was very extensive in places particularly in the Kujung I
and II carbonates, and probably occurred due to flushing of the
pore network by large quantities of meteoric water in a fresh water
vadose environment. The development of vadose conditions is
suggested from the existence of oversize pores in the limestones
Plates 15A, 17A, 18A & B. Under this diagenetic regime, inversion
of skeletal aragonite [e.g. coral and mollucan debris] has also
probably taken place.
iii. Calcite Cementation of Moulds and Vugs: Mouldic porosity
resulting from the early stage dissolution of unstable skeletal
grains, primary interparticle and primary intraparticle porosity was
completely filled by finely to medium to locally large, mosaic calcite
cement crystals, Plates 8B, 26B, 32B, 47A & B, 50A. This is thought
to have occurred in a fresh water phreatic environment. However,
not all porosity was cemented, and some mouldic and vuggy
porosity survives. Some calcite cement crystals are large, and
occasionally they are very slighty ferroan. This suggests that at
least some of the calcite cement formed slowly, possibly after
considerable burial.
iv. Compaction and Collapse of Sediment: The limestones appear
to have locally experienced compaction and possible collapse. This
is evidenced by disordered vugs and by plastically deformed and
broken skeletal particles, and the local presence of grain-to-grain
contacts, Plates 4A, 31B. These fabrics suggest that compaction of
the sediments took place before major calcite cementation.
v. Dolomite Precipitation:
Dolomite is present in significant
amounts in a number of Kujung and Ngimbang carbonates
occurring mainly as a very finely to finely crystalline replacive
mineral, Plates 37A & B. Replacive dolomite has mainly affected the
argillaceous lime mud matrix. Dolomite is also observed as porefilling material and tends to be minor in quantity and found in open
vuggy pores as cement. These two types of dolomite may have
formed relatively early during burial. Later stage dolomite formation
probably occurred along stylolites as stylocumulate, and it appears
to post-date calcite cementation in most cases, and may be related
to dewatering of the rock due to stylolitization.
vi.
Stylolitization and Fracturing:
Stylolites were observed,
particularly in some of the Ngimbang limestones, Plates 42B, 43A,
44A, indicating that reductions in rock volume have occurred since
deposition. They are generally low to moderately peaked, are
generally lined with dark insoluble material including pyrite, clay
and dolomite. Minor leaching associated with stylolites appear to
have occurred prior to hydrocarbon migration. Open, tectonically
generated extensional fractures are also present in most samples,
Plates 14A & B, 16A & B, 20A, 43A, but they are generally
volumetrically minor as estimated from thin sections. Locally laterstage calcite cement has selectively filled fractures, Plates 6B, 13A,
49B.
vii. Late Dissolution: Some of the observed secondary porosity of small vugs in
the limestones probably formed at a later diagenetic stage of burial due to
non-fabric selective dissolution, as suggested by the presence of small
vuggy pores that are completely free of any cement. The fact that the often
well developed matrix intercrystalline pores have not collapsed or been
compacted also suggests some leaching may have occurred during later
stages of burial
Lithology : Limestone
Classification
: Wackestone
FRAMEWORK GRAINS
Larger benthic foraminifera
Smaller benthic foraminfera
Molluscs
Corals
Red algae
Planktic foraminifera
Echinoderms
Quartz
MATRIX
Lime mud
40% CEMENTS /
REPLACEMENTS
C Dolomite
C Calcite
C
C
M
M
M
M
5% VISIBLE POROSITY
M Pore Type
%
35%
A
M
20%
TEXTURE
Grain Size
Minimum
Mode
Maximum
Grain
contacts
Sorting
Abrasion
:
:
:
Fine
Coarse
Pebble
:
Floating
:
Poor
:
Abraded>unabra
ded
Intraparticle
Fracture
Vuggy
Intercrystalline
Mouldic
7%
5%
4%
3%
1%
Summary:
Lithology, Texture and Composition: Coarse grained, poorly sorted,
skeletal [foraminiferal] wackestone, with a minor matrix of lime mud.
The matrix has generally been replaced by very finely dolomite [Plate
B]. Skeletal grains are predominantly benthic foraminifera [Plate A, DE7] with common molluscan and coral debris [Plate A: I14], minor red
algae, echinoderm debris [Plate A: G-H4-5], and planktic foraminifera.
Most skeletal grains are poorly sorted and abraded. Sand and silt-sized
grains are sparsely distributed [Plate B: E15, J-K12]. Calcite cement
occurs in finely to medium crystalline equant to blocky forms, in the
chambers of skeletal components, and locally forms isopachous cement.
Depositional Environment: Relatively low energy, unrestricted shallow
carbonate shelf environment, with input of terrestrial quartz grains and
redeposited coral debris.
Main Diagenetic Sequences: Dissolution of unstable skeletal grains to
form moulds, dolomitization of matrix, precipitation of calcite cement,
non-fabric selective dissolution of carbonate components at a relatively
late diagenetic stage, late-stage fracturing.
Visible Porosity: Good [areas of blue-dyed resin in Plates A & B],
comprising a combination of primary intraparticle pores in coral and
foraminiferal chambers, secondary pores resulting from dissolution,
and intercrystalline pores between dolomite rhombs [Plate B]. These
pores are often connected by fractures and filled with indeterminate
opaques resembling residual oil or bitumen.
Plate A:
Plane-polarized light, 1cm = 0.30 mm
Plate B:
Plane-polarized light, 1cm = 0.07 mm
Lithology : Limestone
Classification
FRAMEWORK GRAINS
Larger benthic foraminifera
Smaller benthic foraminifera
Red algae
Corals
Echinoderms
Micritized grains
MATRIX
Lime mud
: Wackestone
25% CEMENTS /
REPLACEMENTS
C Dolomite
C Calcite
M
M
M
M
28% VISIBLE POROSITY
A Pore Type
%
TEXTURE
Grain Size
Minimum
Mode
Maximum
Grain
contacts
:
:
:
Fine
Lower coarse
Very coarse
:
Floating
Interparticle
Intercrystalline
Vuggy
Mouldic
5%
4%
2%
1%
35%
C
C
12%
Sorting
Abrasion
:
Poor-moderate
:
Abraded>unabra
ded
Summary:
Lithology, Texture and Composition: Lower coarse grained, poorly to
moderately sorted, skeletal [foraminiferal] wackestone locally grading
to packstone, with an abundant matrix of lime mud. The matrix has
been selectively replaced by very finely, euhedral to subhedral dolomite
[Plate B]. Skeletal grains are predominantly larger benthic foraminifera
[Plate A: C6, F7], with minor red algae, echinoderm debris, smaller
benthic foraminifera, corals and micritized grains [Plate B: F10-11].
Calcite cement is minor, occurring in finely to medium crystalline equant
to blocky forms, in the chambers of skeletal components.
Depositional Environment: The skeletal grain assemblages and
depositional textures suggest that this limestone was deposited in a low
energy, unrestricted [?], shallow carbonate shelf environment.
Main Diagenetic Sequences: Dissolution of unstable skeletal grains to
form moulds, neomorphic stabilization of lime mud, dolomitization of
matrix, precipitation of calcite cement, non-fabric selective dissolution
of carbonate components at a relatively late diagenetic stage, latestage fracturing.
Visible Porosity: Good visible porosity [areas of blue-dyed resin in
Plates A and B], comprising a combination of primary interparticle
pores between skeletal grains, and secondary pores resulting from
dissolution, and intercrystalline pores between dolomite rhombs.
These pores are often well interconnected by fractures.
Plate A:
Plane-polarized light, 1cm = 0.15 mm
Plate B:
Plane-polarized light, 1cm = 0.07 mm
Lithology : Igneous Rock [extrusive]
Classification
: Andesite
PRIMARY MINERALS
Plagioclase feldspar
Pyroxene
Opaques [ores]
Hornblende
45% SECONDARY MINERALS
A Pyrite
C Chlorite
C
M
20%
M
C
Quartz
GROUNDMASS
Altered
Trace
30% VISIBLE POROSITY
A Pore Type
%
5%
TEXTURE
Porphyriti
Dissolution
5%
c
This rock sample is a porphyritic andesite composed principally of
phenocrysts of feldspar, with subordinate pyroxene [F-K3-7], opaque
minerals [ores, A-B11, B-C2, F-G8], minor hornblende and traces of
quartz, embedded in a widespread microlithic groundmass. The
groundmass is composed mainly of secondary chlorite [yellowish
brown interference color between plagioclase phenocrysts and
microliths, small plagioclase laths [generally ranging from 0.07 to 0.18
mm, E7.5, E4-5, E-F14, etc] and other unidentifiable microcrystalline
minerals. Feldspar is mainly plagioclase with minor potassium feldspar
[orthoclase?]. Plagioclase phenocrysts [E-K1-2] range up to 2.0 mm,
and are anhedral to subhedral and mostly unfresh but there are some
are quite fresh, zoned types. The degradation of plagioclase is due to
either the partial alteration to sericite or partial replacement by
chlorite. Secondary chorite is also observed, occurring as a
replacement of former unknown minerals [mafic minerals?].
Patches of oversize pores [up to 1.80.mm in diameter] are present
[C6.5, I-J14] and interpreted to be related to the dissolution of
unstable minerals such as mafic minerals. This andesite can be
regarded as being moderately altered.
Plate A:
Plane-polarized light, 1cm = 0.15 mm
Plate B:
Cross-polarized light, 1cm = 0.15 mm
Lithology : Siltstone
Classification
FRAMEWORK GRAINS
Quartz
: Quartzarenite
58.4 CEMENTS /
% REPLACEMENTS
46.4 Calcite
29.2
%
25.6
Muscovite mica
Plagioclase
Chlorite
Organic material
Indeterminate heavy minerals
Chert
MATRIX
Clay
% Pyrite
4.4%
2.4%
1.6%
1.6%
1.2%
0.8%
12.4 VISIBLE POROSITY
%
12.4 Pore Type
%
%
%
3.6%
0%
TEXTURE
Grain Size
Minimum
Mode
Maximum
Grain
contacts
Sorting
Roundness
:
:
:
N/A
0.06mm
0.18mm [fine]
:
Planar, point
:
Good
:
Subangular>ang
ular
Summary:
Lithology, Texture and Composition: This sample is a well-sorted,
massive, quartzarenitic silstone that has been well cemented by
slightly ferroan calcite. Framework grains are predominantly quartz,
with minor to trace amounts of muscovite mica, plagioclase, detrital
chlorite, chert, organic material and indeterminate heavy minerals.
Most grains are subangular to angular and have planar and point grain
contacts. Detrital clay forms patches of interstitial matrix and
laminated fabrics. Widespread ferroan calcite cement has filled
available intergranular pore space. Pyrite occurs both as a minor
secondary cement and replacement mineral.
Depositional Environment: Diagnostic indicators
environments of deposition were not observed.
Main Diagenetic
cementation.
Sequences:
Minor
pyrite,
relating
early
to
stage
the
calcite
Visible Porosity: The sandstone has no visible porosity, due to calcite
cementation.
Plate A:
Plane-polarized light, 1cm = 0.15 mm
Plate B:
Cross-polarized light, 1cm = 0.15 mm
Lithology : Claystone
Classification
FRAMEWORK GRAINS
Quartz
Mica flakes
MATRIX
Clay
: Quartz arenit
2.4 CEMENTS /
% REPLACEMENTS
1.2% Pyrite
1.2%
3.6%
93.2 VISIBLE POROSITY
%
93.2 Pore Type
%
%
0.8%
3.6%
TEXTURE
Grain Size
Minimum
Mode
Maximum
Grain
contacts
Sorting
Roundness
:
:
:
N/A
Clay-size
0.06mm [silt]
:
N/A
:
N/A
:
N/A
Fracture
0.8%
Summary:
Lithology, Texture and Composition: Claystone containing minor
amounts of silt-sized quartz grains and mica flakes. The groundmass
appears to be composed predominantly of illitic clays. Pyrite is the only
secondary mineral observed, occurring as a replacive mineral [Plates
A&B: E-F11-13].
Depositional Environment: Indeterminate [no diagnostic indicators of
depositional environment observed].
Main Diagenetic Sequences: Early stage formation of pyrite, late stage
fracturing.
Visible Porosity: Negligible, only limited to few tiny fractures.
Plate A:
Plane-polarized light, 1cm = 0.07 mm
Plate B:
Cross-polarized light, 1cm = 0.07 mm
Lithology : Claystone
Classification
:
FRAMEWORK GRAINS
Quartz
Organic material
Feldspar [plagioclase?]
MATRIX
Clay
2.8
%
1.6%
0.8%
0.4%
90.4
%
90.4
%
CEMENTS /
REPLACEMENTS
Pyrite
Calcite
5.2%
VISIBLE POROSITY
1.6%
Pore Type
4.4%
0.8%
%
TEXTURE
Grain Size
Minimum
Mode
Maximum
Grain
contacts
Sorting
Roundness
:
:
:
:
N/A
Clay size
0.30mm
[medium]
Fracture
1.6%
N/A
:
N/A
:
N/A
Summary:
Lithology, Texture and Composition: The claystone is composed
essentially of clay minerals, most of which are possibly of illitic
composition. Organic material in disseminated forms, and silt to sand-
sized quartz and feldspar grains are present in minor amounts. Pyrite
[I-J7-8] and calcite occurs sporadically as minor replacement minerals.
Depositional Environment: Indeterminate.
Main Diagenetic Sequences:
stage fracturing?
Minor pyrite and calcite formation, late
Visible Porosity: There is no visible porosity, except for few tiny
fractures.
Plate A:
Plane-polarized light, 1cm = 0.07 mm
Plate B:
Cross-polarized light, 1cm = 0.07 mm
Lithology : Siltstone
Classification
: Quartzarenite
FRAMEWORK GRAINS
Quartz
Muscovite mica
Plagioclase
Chlorite
Organic material
Indeterminate heavy minerals
Chert
MATRIX
Clay
58.4
%
46.4
%
4.4%
2.4%
1.6%
1.6%
1.2%
0.8%
12.4
%
12.4
%
TEXTURE
Grain Size
Minimum
Mode
Maximum
Grain
contacts
:
:
:
N/A
0.06mm
0.18mm [fine]
:
Planar, point
CEMENTS /
REPLACEMENTS
Calcite
Pyrite
29.2
%
25.6
%
3.6%
VISIBLE POROSITY
Pore Type
%
0%
Sorting
Roundness
:
Good
:
Subangular>ang
ular
Summary:
Lithology, Texture and Composition: This sample is a well-sorted,
massive, quartzarenitic silstone that has been well cemented by
slightly ferroan calcite. Framework grains are predominantly quartz,
with minor to trace amounts of muscovite mica, plagioclase, detrital
chlorite, chert, organic material and indeterminate heavy minerals.
Most grains are subangular to angular and have planar and point grain
contacts. Detrital clay forms patches of interstitial matrix and
laminated fabrics. Widespread ferroan calcite cement has filled
available intergranular pore space. Pyrite occurs both as a minor
secondary cement and replacement mineral.
Depositional Environment: Diagnostic indicators
environments of deposition were not observed.
Main Diagenetic
cementation.
Sequences:
Minor
pyrite,
relating
early
to
stage
the
calcite
Visible Porosity: The sandstone has no visible porosity, due to calcite
cementation.
Plate A:
Plane-polarized light, 1cm = 0.15 mm
Plate B:
Cross-polarized light, 1cm = 0.15 mm