F-CoreW-18-PEP-AkasiaBagus
PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION
Core Workshop, Jakarta 16-17 October 2018
EARLY MIOCENE CARBONATE DIAGENETIC OF NORTHWEST JAVA BASIN,
IMPLICATION TO RESERVOIR PROPERTIES; CASE STUDY AKASIA BAGUS STRUCTURE
W. L. Ambarwati*
D. M. Sulistyo*
M. Fajar*
ABSTRACT
Akasia Bagus structure is a discoveries in Cemara –
Tugu Area, located in North West Java Basin.
Exploration activity within this structure had begun
since 1994 with 2D seismic acquisition, followed by
exploration well on 2009, 3D seismic acquisition on
2011, and continued with delineation wells on 2014
and 2016. In general, Akasia Bagus structure possess
multiple vertical reservoir layer especially in Upper
Cibulakan Formation, among them is carbonate
reservoir known as Z-16.
The Z-16 layer is a limestone reservoir with 111 m
gross thickness, this contains porous zone as thick as
54 m, the accoustic impedance inversion of this layer
shows a facies barrier between the Akasia Bagus
Structure and the other structures to the north, which
also confirm the difference of fluid contact between
those two structures.
For this paper writer used one seismic composite and
inverted seismic section, three Wells data and
laboratory analysis results from A-1, A-2, and A-3
wells. Two conventional core and rotary sidewall
core samples from A-2 and A-3 wells were used for
routine core analysis, while the special core analysis
only perform on selected conventional core plug
from A-2 Well.
From FMI analysis it is known that the lateral facies
development within the Z-16 layer in A-3 well is
more complex compare to A-1 and A-2 wells,
characterized by the thickening of limestones
sediments. The lateral facies distribution in A-2 well
dominated by the red algae, and large foram as
fragments embedded in grain supported matrix
(packstone-grainstone) (Dunham., 1962), while the
Z-16 layer in A-3 well showed that the original
matrix is replaced by dolomite. Dissolution,
dolomitization, and the formation of stylolite become
diagenetic stage markers that indicate the
mesogenesis stage.
*
Vuggy and mouldic are the main porosity in A-2
well, it ranges from 1.20 % to 26 %, the permeability
ranges from 0.0001 md to 481 md, and the density
ranges from 2.70 to 2.74 g/cc. The porosity system
in A-3 well is mainly vuggy with intercrystalline
pores, cementation and compaction during
diagenetic stage were the determining factor that
reduce the porosity (3.7 % to 9 %), permeability
ranges from 0.025 md to 5.28 md, and the density
ranges from 2.70 to 2.79 g/cc.
Keywords : Carbonate, North West Java Basin,
diagenesis, Z-16, Upper Cibulakan Formation.
INTRODUCTION
The Akasia Bagus Structure is located in the southern
part of Jatibarang Sub-Basin and directly adjacent to
the Cipunegara Sub-Basin in the west and south
(Figure 1). Jatibarang and Cipunegara Sub-Basins
are interpreted formed during the Eocene - Oligocene
Period through a dextral strike slip mechanism. This
displacement formed a pull-apart basin and local
high in several places. Cipunegara Sub-Basin is also
interpreted to be the kitchen that generates
hydrocarbon to all Cipunegara areas including the
Akasia Bagus Structure.
The tectonostratigraphy of sediment filling in
Northwest Java Basin can be divided into three (3)
units (Figure 2) ; synrift deposits, postrift deposits,
and back arc deposits. Synrift deposits phase are
marked by the development of strain tectonic
(extension) represent by graben or half graben with
North – South direction. Conformable with the basin
development processes, successively the Jatibarang
Formation/Pre Talang Eq are deposited on Late
Eocene – Early Oligocene, followed by lower
Talangakar deposits on Late Oligocene.
Post Rift deposits marked by the dominance of sea
level rise wherein the tectonic processes are less.
East Area Exploration, PT Pertamina EP
© IPA, 2018 – 42nd Annual Convention Proceedings, 2018
Upper Talangakar Formation were deposited on
Early Miocene, marked the beginning of these cycle.
Baturaja Formation were sedimented above the
Talangakar Formation characterized by limestones
that grow
in high area with intercalated claystone. On top of
Baturaja Formation, Cibulakan Formation were
deposited, charaterized by the dominance presence
of
claystone
intercalated
with
sandstone and limestone.
Back arc phase is the results of emplacement from
Java Volcanic Arc, it is shown by the deposition of
volcanic breccia and lava of Jampang Formation
observed in the Southern Mountain of West Java
area, during this period the basin changes from the
rift basin to the back arc basin. Sedimentation in the
southern part of the basin is dominated by volcanic
materials, while in the North West Java Basin
sedimentation begins with fine clastic sediments and
carbonates (Parigi Formation and lower section of
Cisubuh Formation), ended with fluvial deposits
(Upper Cisubuh Formation), it is a manifestation of
the uplifting in the south.
DATA AND METHODOLOGY
Three Pertamina EP wells from Akasia Bagus
structure in North West Java Basin are being used;
A-1, A-2 and A-3. All of the wells are covered the
Early Miocene age of Carbonate (Z-16) interval. The
three exploration wells were sampled for
petrography, XRD, SEM, biostratigraphy, Routine
Core Analysis and Special Core Analysis.
Rotary sidewall cores and cuttings from A-1 Well
were taken for biostratigraphy and petrography
analysis in Z-16 interval, meanwhile the
conventional core and rotary sidewall cores from A2 and A-3 wells were sampled for Routine and
Special Core, Petrography, XRD and SEM analysis
in Z-16 interval (Table 1)
Quantitative biostratigraphy analysis of small &
larger foraminifera, calcareous nannoplankton and
palynology, performed in sediment samples of A-1
well, are used to determine the age and depositional
environment of the Early Miocene Carbonate (Z-16)
interval.
Rotary sidewall core and conventional core plug
samples from A-2 and A-3 wells were used to
perform a routine core analysis to measure the
porosity, permeability, fluids saturation and grain
density properties. The pore volumes (porosity) were
measured with Boyle’s Law method, by using gas
helium that passed through the samples and the
pressure were monitored until stable. The
permeability measurements were made using
unsteady state pulse decay technique, the methods
being used to determine are; air and Klinkenberg
method. The porosity and permeability are measured
at ambient condition (500 psig) and Net Overburden
(NOB) condition (3000 psig).
Special core analysis only performed on selected
conventional core plug samples of A-2 Well at
carbonate Z-16 interval to measure the reservoir
properties (Table 2).
RESULT AND DISCUSSION
Initially the Z-16 layer in Akasia Bagus Structure
was interpreted as a large reeefal carbonate structure,
however after proven drilling of A-3 well, it is shown
that there are compartments differences, especially
the oil water contact (OWC) limit is far from the oil
zone boundary in Gantar (Figure 3). Judging from the
seismic section there is no fracture separating the
compartment, indicating that the carbonate reefal
development which likely to be form at different
stages of deposition. Between Z-16 and Middle
Cibulakan can be distinguished from changes in
strong reflector seismic characters, Based on
petrophysical evaluation of the Middle Cibulakan
level, limestone lithology has undergone a
dolomitization process into dolostone, and it has the
characteristics of tight porosity.
In addition the Z-16 also developed a localized multi
layers reefal carbonate. The presence of these layers
can’t be seen with normal seismic section, therefore
the accoustic impedance attributes are used to see
the reservoir porosity within the Z-16. Results from
Accoustic Impedances from A-3 and A-1 wells,
shows that the lower zone of Z-16 tends to be a
tighter carbonate in A-1 well compared to A-3 well
(Figure 4). These results also inline with the
petrography results, which shows that the Carbonate
Facies that develop in the Akasia Bagus are Red
Algae, Large Foram and in some places Coral and
Mollusks. The presence of those biota above are
indicating a Limestone reef facies, whilst the rare
presence of Miliolid Fossils and Plangton Forams
indicating that the position of the reef are closer to
the back reef
The geological model of Akasia Bagus Structure is
based on seismic characters, biostratigraphic data,
© IPA, 2018 – 42nd Annual Convention Proceedings, 2018
and petrographic data. At the time of Z-16
deposition, the position of Akasia Bagus Structure is
on the shelf margin while the coastline is in the
northern part of this structure (Figure 5). In the
northern part there is Cemara Field which is also
deposited a limestone, only the carbonate facies are
worse because too many mudstone content. Open
marine are in the west and south in accordance with
the formation of the Cipunegara basin in the area.
During the regression period, part of the open marine
area in the south was uplifted, so that it had almost
the same level as the northern region, this is
characterized by the relative thickness of the rock
layer, eventhough in the northern region which was
close to the land had been affected by clastic
sedimentary rocks. This is characterized by the
development of sandstone reservoirs in the Cemara
Area while in the Akasia Bagus Area still developing
carbonate build up alternate with shale (Figure 5).
Carbonate facies are determined from thin section
petrographic. The results shows a facies differences
from A-2 and A-3 wells in Z-16 interval. In A-2 well,
the pore types mainly consists of vuggy and moldic,
the other types are intraparticle, interparticle,
intercrystalline and microfractures. The porosity is
controlled by significant proportion of matrix
content,
recrystallization,
cementation
and
compaction during diagenetic stage. The late stage
dissolution of calcite cement and matrix produce
secondary vuggy and moldic porosity (Figure 6).
Based on Routine Core Analysis results, there are
two types of Limestone Facies in Z-16 interval of
Akasia Bagus Structure. The physical properties
from A-2 well Core showing good properties with
porosity ranges from 1.20 % to 26 %, permeability
ranges from 0.001 md to 481 md and the density
ranges from 2.70 to 2.74 g/cc. Meanwhile, the
physical properties from A-3 well Core showing
poor properties with porosity ranges from 3.7% to
9%, permeability ranges from 0.025 md to 5.28 md
and the density ranges from 2.70 to 2.79 g/cc
(Graphic 1 & 2).
In the A-3 well, the porosity system mainly consists
of vuggy, accompanied with rare intercrystalline
pores. The recrystallization, cementation compaction
and dolomitization during diagenetic stage were the
controlling factor that reduced porosity &
permeability in A-3 well samples.
Special Core Analysis results of A-2 well showed
that the individual cementation of porosity exponent
“m” ranged from 1.97 to 2.01 with an average value
of 1.99 (Table. 3) and the individual saturation
exponent “n” ranged from 1.99 to 2.05 with average
value of 2.02 (table. 4)
CONCLUSION
In Early Miocene age there are two carbonate facies
that developed in Akasia Bagus Structure; Core reef
which characterized by the presence of Red Algae,
large Foram, Coral and Mollusks. The other one is
reef facies that characterized by the presence of
milliolids Fossils and Plangton Forams.
In terms of Z-16 layer, it has different
characacteristics depending on the wells position, In
A-1 well the lower zone of Z-16 tends to be tight,
while the same layer in A-3 well is not. The thin
section petrographic analysis from core data also
shows that Z-16 have several phases and undergoes
early to advanced diagenesis processes that affect
reservoir quality and diagenesis processes including
the mineral dissolution in form of aragonite and
recrystallization in form of dolomite.
ACKNOWLEDGEMENTS
Writer would like to say thank you to the
Management team of PT. Pertamina EP for the
support and permission to publish this paper,
especially to the Vice President of East Area
Exploration; Mr. Muharram Panguringseng, Vice
President West Area Exploration; Mr. Tri Widyo
Kunto, Exploration Regional & Study Senior
Manager; Mr. Agung Prasetyo, Jawa Exploration
Senior Manager; Mohamad Wahyudin, and to all
colleagues in Regional & Study in general.
REFERENCES
Dunham, R.J. 1962. Classification of Carbonate
Rocks According to Depositin Textures. In:
Classification of Carbonate Rocks, W. E. Ham (Ed.).
AAPG Memoir No.1, Tulsa, OK, pp. 108-121.
PT. Pertamina EP., 2014. Laporan Penentuan Status
Eksplorasi struktur Akasia Bagus. PT. Pertamina
EP. Jakarta
Usman, T.K.,Yuliandri, I., Panguringseng, M.J.,
Sadirsan, W.S., Priambodo, D., 2011. New Concept
of Paleogene Basin Evolution of Northern West Java.
Proceeding JCM Makassar.
© IPA, 2018 – 42nd Annual Convention Proceedings, 2018
A-3
ID
111
115
117
128
131
Amott wettability
Seal capacity
1914,6
1914,6
1915,1
1915,1
1915,2
1915,2
1918,4
1918,4
1918,45
1918,45
1919,3
1919,3
1919,4
1919,4
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Sample ID
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Table.3 m values of A-2 Well Core samples
Sample Depth
Rock compressibility

Depth
(m)
Liquid permeability as function of
volume throughput

Well
Unsteady state water-gas relative
permeability [kw/kg] endpoint only

Special Core
Analysis
Unsteady state gas-water relative
permeability [kg/kw]

Routine Core
Analysis
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Unsteady state water-oil relative
permeability [kw/ko]
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Unsteady state gas-oil relative
permeability [kg/ko]
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Mercury Injection & pore size
distribution – Free Water Level
A-2
Petrography XRD SEM Biostratigraphy
Air-brine capillary pressure by porous
plate
A-1
Depth
1772
SWC
1787,5
SWC
1792
SWC
1835
SWC
1857,5
Cuttings
1794-1796
Cuttings
1816-1818
Cuttings
1820-1822
Cuttings
1824-1826
1910,7
1911,52
1911,82
1912,7
1913,6
1914,2
Conventional Core Plug
1915,5
(1910.35-1919.70)
1916
1916,45
1917,5
1917,9
1918,4
1919,3
1865,75
1866
1866,3
1866,65
1867,4
1867,85
1868,65
Coventional Core Plug
1868,9
(1865.62-1875.47)
1869,25
1869,65
1869,9
1871,75
1872,05
1872,42
1872,85
SWC
1769,00
SWC
1774,00
SWC
1805,80
SWC
1839,00
SWC
1856,00
SWC
1883,50
SWC
1891,00
SWC
1908,50
SWC
1922,00
SWC
1932,50
SWC
1973,80
SWC
2026,00
SWC
Resistivity index at overburden
Sample Type
Formation factor at overburden
Well
A-2
Table 1. List of laboratorium analysis in Early Table 2. List of Special Core analysis in Early
Miocene Z-16 samples
Miocene Z-16 samples of ABG-Y Well
Horizontal
Formation Porosity
Permeability Porosity Resistivity Exponent
to Air,
Factor
at NOB
at NOB
meter
md
fraction
F
m
1.97
1913.30
317
0.231
17.89
1.99
1914.50
18.8
0.162
37.84
1.98
1915.10
126
0.220
20.24
2.00
1918.40
12.1
0.173
33.62
2.01
1919.30
2.88
0.142
50.25
Table.4 n values of A-2 Well Core samples
Horizontal
Formation Porosity
Brine Resistivity Saturation
Permeability Porosity Resistivity Exponent Saturation
Index
Exponent
to Air,
Factor
Fraction
at NOB
at NOB
meter
md
fraction
F
m
Vp
I
n
1.000
2.01
0.459
4.778
1913.30
317
0.231
17.89
1.97
2.01
0.362
7.725
2.00
0.288
12.004
1.99
0.234
17.965
1.000
2.02
0.636
2.491
1914.50
18.8
0.162
37.84
1.99
2.02
0.522
3.727
2.03
0.400
6.442
2.02
0.288
12.312
1.000
2.01
0.542
3.421
126
0.22
20.24
1.98
1915.10
2.02
0.441
5.24
2.03
0.344
8.71
2.02
0.264
14.79
1.000
2.03
0.556
3.286
1918.40
12.1
0.173
33.62
2.00
2.02
0.462
4.769
2.03
0.419
5.851
2.04
0.377
7.347
1.000
2.04
0.579
3.055
1919.30
2.88
0.142
50.25
2.01
2.04
0.516
3.847
2.05
0.476
4.588
2.05
0.424
5.790
Depth
© IPA, 2018 – 42nd Annual Convention Proceedings, 2018
Graphic.1 Porosity values of A-2 and A-3 wells Graphic.2 Permeability values of A-2 and A-3
Core samples
wells Core samples
Figure 1 - Study Area Location
© IPA, 2018 – 42nd Annual Convention Proceedings, 2018
Figure 2 - Tectonostratigraphy of Northwest Java Basin
© IPA, 2018 – 42nd Annual Convention Proceedings, 2018
Figure 3 - Carbonate Reefal development at differet stages of development from Z-16 Layer
© IPA, 2018 – 42nd Annual Convention Proceedings, 2018
Figure 4 - Accoustic Impedance cross section in TWT showing Z-16 facies differences From Akasia Bagus
– G- W Structures.
Figure 5 - TWT seismic cross section showing the time during the Z-16 deposition.
© IPA, 2018 – 42nd Annual Convention Proceedings, 2018
Figure 6 - Carbonate Facies differences from G-13, A-3, A-1, and A-2 Wells in Z-16 Interval.
© IPA, 2018 – 42nd Annual Convention Proceedings, 2018