A Late Pliocene Shallowing Upward Carbonate Sequence And Its Reservoir Potensial, Nortesast Java Basin.pdf

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PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Twenty-Fifth Silver Anniversary Convention, October 1996

A LATE PLIOCENE SHALLOWING UPWARD CARBONATE SEQUENCE AND ITS RESERVOIR POTENTIAL, NORTHEAST JAVA BASIS Sabardi Musliki* suratman*

ABSTRACT ‘The Late Pliocene carbonate facies assemblage of the Kalitik-Ngepung-Selorejo or Kalinges Formation is widely distributed in the Northeast Java Basin. It outcrops mainly in the Kendeng and Kembang zones; the rest remains in the subsurface in the Randublatung zone and possibly extends under the alluvial plain of Ngawi-Baantas and Pati-Rembang areas. Based on extensive field and laboratory studies, the Late Pliocene carbonate facies assemblage of the Kalitik-Ngepung-Selorejo or Kalinges Formation is interpreted as a shallowing upward sequence deposited during a global drop in sea level that started in the Late Pliocene. about 2.9 MA. A shallowing upward trend within the Kalinges Formation is indicated by the deposition of different carbonate facies. This begins with the marls of the of the Kalibeng - Mundu or Kalimu interval and is followed successively by the Globigerina Marl facies, Globigerina Limestone facies, Reefal Limestone facies, Limestone Debris facies and Mollusc Limestone facies. This sequence was finally covered by the clays of the Lidah Formation. The best reservoir potential is found within the Globigerina Limestone facies. These high porosity and permeability limestones are composed almost entirely of sand size planktonic foraminifera that have retained their primary interparticle porosity. Significant quantities of gas (Balun gas field) and oil (Lidah, Kruka, Kuti, Metatu and Bogomiring oil fields) have been produced from this facies. *

PPT Migas - Cepu

Detailed palaeogeographic studies and subsurface mapping of the Late Pliocene carbonate facies are still needed to locate and map the distribution of the best reservoir facies and to aid in the continuing exploration and development of the area’s oil and gas potential .

INTRODUCTION Carbonate deposition is favoured by shallow water depths, moderate salinity, clarity: temperature and oxygen content, plus the solar energy needed for photosynthesis. Tropical areas. such as Indonesia, readily provide these requirements. The stratigraphic succession of the onshore area in the Northeast Java Basin includes several carbonate units. Typically, these develop at the beginning of depositional cycles (sequence boundaries) as exemplified by the Prupuh Formation (Late Oligocene), the Bulu Formation (Middle Miocene), the Ledok Formation (Mid-Pliocene) and the Kalitik-Ngepung -S elorej o (Kaling es) Form ation of Late Pliocene age. Among those formations, the Kalitik-Ngepung-Selorejo (Kalinges) Formation is very interesting not only for the variety of facies present but also for the range of the depth and depositional environments (shallowing upward) recorded. This formation will be the focus of this paper. Several Late Pliocene carbonate outcrops have been visited, observed and measured, and more than 50 samples collected and analyzed in the laboratory. The final database included information such as lateral distribution, thickness variations, texture, sedimentary structures, dominant particles and their composition and organiclfossil content of the samples or

44 formations. Further petrophysical studies and laboratory analyses of porosity and permeability have also been completed to aid in reservoir characterization. Based on the field observations and laboratory data, both qualitative and quantitative, we have attempted to analyze and interpret the depositional environments and facies, and to outline their implications for hydrocarbon reservoir potential.

GEOLOGICAL SE'ITING The northeast Java Basin is a Tertiary foreland basin. Approximately three quarters of the basin is onshore and the rest lies offshore below the Java Sea. The basin is bounded by a volcanic belt or geanticline in the south, the Karimunjawa and Bawean arc in the north, the Pati and Cirebon troughs in the west and the Madura ridge and Madura basin in the east. Based on tectonic characteristics, stratigraphy, paleogeography and hydrocarbon potential, the northeast Java Basin can be divided into three east-west trending tectono-physio-stratigraphic zones. These are the Kendeng zone in the south, the Rembang Zone in the north and the Randublatung Zone in the centre.

The Kendeng Zone This corresponds to the Kendeng hills (Van Bemmelen, 1949) or the labile deep basin. It is a hilly region, created by the very intensive tectonism that elevated the Kendeng anticlinorium . The general anticlinal trend is east-west, with overthrust and thrust faults of the same strike orientation. There are few indications of normal faulting. The thrust and overthrust faults usually do not penetrate into the basement, resulting in a detached structural style. The Kendeng zone is dominated by volcanoclastic rocks and mark with few intercalations of shale, sands and carbonates. The rocks are interpreted as having been originally deposited in a deep sea basin environment.

The Randublatung Zone This corresponds to the Blora, Cepu, Ngimbang and

Dander hills (Van Bemmelen, 1949) or the transition zone. The zone is gently deformed and is structurally lower than the Kendeng and Rembang zones. Anticlines in this zone show a broader range of orientation, and trends vary from E-W to NNW-SSE. Reverse faults strike in similar directions but normal faults usually trend in a NE-SW direction. The zone is dominated by normal faults, which usually extend into basement thus creating a basement involved structural style. Dominant lithologies are marls and clays, with intercalations of sands, calcarenites and carbonates which also act as an important reservoir facies. These sediments are thought to have been deposited on an undulating continental slope.

The Rembang Zone This extends to the northern margin of Java and is separated from the Randublatung zone by the Lusi trough in the west, the Kening trough in the middle and the Solo trough in the east. It is a hilly zone with a level of tectonic deformation intermediate between those of the Randublatung and Kendeng zones. The Rembang anticlinorium consists of many superimposed anticlines. The general trend of anticlines in this zone varies from E-W to NNW-SSE. Reverse faults have the same direction, while normal faults usually trend NE-SW. This zone is dominated by normal faults with little throw. These faults usually extend into the basement suggesting that this zone also has a basement involved structural style. Dominant lithologies are sands and carbonates with intercalations of mark and clays. This stratigraphic sequence is interpreted as having been deposited on a continental shelf.

LATE PLIOCENE CARBONATE DEPOSITS Definition or Scope of Discussion In the Kendeng zone, Late Pliocene carbonate deposits are represented by the Kalitik and Ngepung Limestones and were formally named as a member of the Sonde Formation by Harsono P. (1983). Age equivalent carbonate deposits in the Rembang and Randublatung zones have been named the Selorejo Member (Harsono, 1983) or the Selorejo Formation (Sabardi Musliki, 1990) (Figure 1).

45 In the Tuban area, in the eastern portion of the Rembang zone (Figure 2), the Paciran Formation is a thick (150 m) sequence of carbonates that have persisted since early Pliocene (N.19) through the Holocene. This formation will not feature in the discussion below which will be limited to the Late Pliocene carbonate deposits of the Kalitik-NgepungSelorejo or Kaliiiges Formations.

Lateral Distribution :md Thickness Vaiation The Kalitik-Ngepung-Selorejo or Kalinges Formations accumulated in response to the Pliocene (2.9 MA) global sea level fall. They lie unconformably on top of the Kalibeng - Mnndu or Kalimu Formations, and are covered by the Pleistocene clays of the Lidah Formation. The Kalitik and Ngepuiig Formations are distributed in the southern part of the Kendeng zone and outcrop from the Sangiran area in the west to the Jombang area in the east. Possible subcrops are suspected beneath the alluvial plains of the Ngawi - Brantas area. The thickness of the Kalitik and Ngepung Limestone varies from 25 m thick at the Mantingan area in the west to 250 m thick in the Nganjuk area to the east (Figure 3). The Selorejo Formation varies from hard bedded limestone in the Pati area to interbedded hard and soft or consolidated Aimtones in the Cepu area Planktonic foraminif sra form the main component of these limestones and L!IC thickness of the formation ranges from 20 in i n the Cepu area to more than 130 m in the Pati area 11

determine six major facies and to assess the depth and energy levels that prevailed during deposition.

Mad Facies (1) The Marl facies is represented by the deposition of thick massive mudstones or marls of the KalibengMundu or Kalimu Formations, which occurred under the open marine (bathyal) conditions that existed throughout the entire basin during the Pliocene. This formation is considered as a "basement of deposition" for the overlying carbonate facies which show gradations of energy, depth of deposition and carbonate mud content.

Globigenna .Mads (2) and Globigenna Limestone Facies (3) Because of tectonic activity and the consequent sea level fall in the Late Pliocene (2.9 MA), several bathymetric highs developed within the area. Bottom currents greatly influenced the deposition of the Globigerina Limestone facies (3) which is largely restricted to these highs (Figure 3 ) . This limestone is composed almost entirely of sand size planktonic foraminifera and is highly porous. Whereas the foraminifera1 limestone facies was deposited on the tops of the highs, the Globigerina Marl facies dominates the flank areas. It differs from the Globigerina Limestone facies in having a higher (>50%) mud and matrix content, reflecting the lower energy and greater depth of deposition.

Reef Limestone Facies ( 5 ) About 70% of the Selorejo Formation crops out in the Pati area (northwest part of the Rembang zone), and in the Lusi and Blora areas (southwest part of the Rembang zone). The rest remains in the subsurface in the Cepu and Surabaya areas (Randublatung zone) and is possibly present under the alluvial plain of the PatiRembang area.

DEPOSlTIONAL ENVIRONMENT AND FACES Interpretation of the depositional environments of the Late Pliocene carbonate deposits was derived from texture, sedimentary structures, dominant particles and their composition. From this, it has been possible to

Owing to their special requirements, reef and reefal buildups are not developed everywhere. The Reef Limestone Facies is well developed around the Pondok-Dero, Kalitik, and Ngawi areas. Elsewhere, there are few indications of reef buildups having become established. The reef limestone facies represent buildups in shallow water depths (*20m), categorized as littoral-neritic bathymetric zones, rich in corals, algal and other shallow water fauna and flora. The limestones are massive, with few or poorly established bedding planes, and are generally lacking in sedimentary structures. They are typically hard and tight with no indication of primary porosity (Figure 5).

46 Limestone Dehiis Facies (4) Wave activity along the front of the reef bodies caused serious abrasion of skeletal material which was deposited as a limestone debris facies, below wave base, in the lower foreslope environment The limestone debris facies is the result of high cnergy processes acting in a generally low energy. open marine, carbonatc producing environment The lith~logy is thdt of a clastic limestone, containing h g m c n t s of cord and pelecypods. and sometimes showing bedding planes Mollusc Limestone Facies (6) Thc Mollusc Limestone facies is found in the shallow. lower energy, b ~ c h reef lagoonal environment, removed frotn thc: Lone of high wave energy The lithology I S dominated bv mud rich carbonates in v iidi molluscs. notab15 gastropods and peleq pods. arc common and \\here biolurbation (Figure 6) is soiiietimes present

DIS CU sSION The Late Pliocene carbonate deposits are very homogeneous and dominated by the Globigerina Limestone facics i n the Reinbang and Randublatung zones The thickness is barely 20 in in the centre (Balun section. Randublatimg Lone) but thickens to., more than 100 m i n thc north (Larangan section, Rembai:g zone) and south (Sonde section. Keiideng zone)

It can be interpreted that during the late Pliocene, the Kembang and Randublatung L O ~ C Soccupied an open marine. bathyal to outer neritic environment. where scdimeiitation occurrcd under below wave base conditions The Balun area was a bathqmetric high and probably expcrienced submarine erosion from bottom current activity In the southeni part of the Northeast Java Basin (Kendeng zone), the late Pliocene carbonate deposits are represented by SIX different facies types The thickness varies from Lero or no deposition in the west (Gendingan section) to more than 125 m in the east (Dodol section) It is interpreted that. during the late Pliocene, the

paleogeography of the Kendeng zone varied from terrestrial (surface erosion in Gendingan section), to lagoonal (Mollusc Limestone and Reef Limestone facies in the Ngawi and Dero sections), to open marine environment (Globigerina Marl and Marl facies in the Miana and Dodol sections). Based on the foregoing data, it may be concluded that the Late Pliocene carbonate deposits record a gradual change in depositional environment from deep to shallow (shallowing upward sequence) This 1s represented by the succession Mark (l), Globigerina Marls (2), Globigerina Limestone (3), Limestone Debris (4), Reef Limestone (5) and Mollusc Liinestone (6), as shown in Figure 7 However, it i s hard to find a complete shallowing upward sequence at any one location. It may not be developed or else it has been covered by alluvium and soil, or weathcred. Nevertheless, the various measured sections reported in this paper can be combined to demonstrate the existence of such a shallowing upward sequence

RESERVOIR POTENTIAL Among those facies discussed above, the best reservoir potential is to be found in the Globigerina Limestone facies. In the Surabaya area this facies belongs to the upper part of the Mundu (GL) Formation or the lower part of the Lidah (MT) Formation. Good quality oil reservoir accumulations in this facies were proven in the now abandoned Lidah, Kruka: Kuti Guiiung-Anyar, Bogomiring and Metatu oil fields. In the Cepu area, the Globigerina Limestone facies of the Selorejo Formation is also known as a good qaality gas reservoir. The Balun gas field, for example, is still on production with a yield of f 400 MMSCFiday. Laboratory analysis indicates high values of porosity and permeability in samples collected from such intervals. Porosities in the Lidah oil field are 20 to 40%, with permeabilities reported to be 67 md. In the Balun gas field porosities are in the range 28 to 42%, with permeabilities between 53 and 963 md (Figure 8). The other facies, based on megaseopic observations and sample description, tend to have very poor reservoir potential: as was confirmed by very low porosity measurements and microccopic examination.

47

CONCLUSION AND RECOMMENDATIONS A Sea level drop towards the end of the Pliocene (2.9 MA), in the northeast Java Basin, is indicated by sharp lithological changes from deep marine marl facies of' the Kalibeng-Mundu or Kalimu Formations to shallow marine carbonate facies of the Kalitik-Ngepung-Selorejo or Kalinges Formation. Sections, compiled from several locations, of the Late Pliocene carbonate facies of the Kalitik-NgepungSelorejo or Kalinges Formation can be interpreted as a shallowing upward sequence recorded in the passage from Marl, Globigerina Marl, Globigerina Limestone to Debris Limestone, Reef Limestone and Mollusc Limestone facies. Among these, the Globigerina Limestone facies has the best reservoir potential with hydrocarbon accumulations and production in the Kuti, Lidah, Kruka, Metatu and Bogomiring (oil fields) and Balun (gas field). Paleontological studies and detailed bathymetric evaluation and measurements are still needed to interpret the age and the depth of the depositional environment within some facies. Paleogeographic studies and detailed subsurface mapping of the carbonate facies will be required to locate the best remaining hydrocarbon accumulations and reservoir potential.

A CKN OWLEDGML+JTS The authors would like to thank Mr. Soemaryo (The Head of Oil and Gas Training Centre or PPT MIGAS CEPU), who has given the opportunity and facilities to do the research and preparation of this manuscript for publication and presentation. Special acknowledgment is made to Robert K. Park and John W. Armon (MAXUS Southeast Sumatra

Inc.), who have given valuable discussion, guidance and suggestions during the preparation of this manuscript . Acknowledgment is also made to friends and colleagues, and others who have helped contribute data, offered information and discussion in order to finalize the paper.

REFERENCES Bambang Soetantri, Luki Samuel, G.A.S. Nayoan, 1973, The Geology of The Oil Fields in Northeast Java Basin, Proceedings of the Indonesian Petroleum Association, 2, 149-175. Harsono Pringgoprawiro, 1983, Biostratigrafi dan Paleogeografi Cekungan Jawa Timur Utara, Suatu Pendekatan Baru, Desertasi Doktor, ITB Bandung, 1983. Sabardi Musliki, 1990, The Pliocene Selorejo Formation and Its Hydrocarbon Prospects in Cepu and Surrounding Areas, Proceedings 19th Indonesian Association of Geologist (IAGI) Convention, Bandung, 1990. Sabardi Musliki, 1996, Paleogeographic Interpretation Based on Lithostratigraphic Units and Relative Sea Level Changes During Plio-Pleistocene Period in the Northeast Java Basin, Proceedings International Symposium on Geology and Environment, Chiang Mai, Thailand, January 1996. Schiller: D.M., Seubert, B.W., Sabardi Musliki and Mardan Abdullah, 1994, The Reservoir Potential of Globigerenid Sands in Indonesia, Proceedings of the Indonesian Petroleum Association, 2311, 1 89-212. Van Bemmelen, R.W., 1949, The Geology of Indonesia, The Hague, Government Printing Office, Netherland, 997 P.

48

S

a W O l W R l N A L S T ?ACES OF THE SELOREJO FORMATION.

FIGURE 1

131

E

A

Line- of Cross Section.

MARL 011 CLAT FACIES OF SONDE OR LIOAU ORM MAT ION

LUCRGEO AREA

.

- Late Pliocene Paleogeography of the Northeast Java Basin, (after Sabardi Musliki, 1996).

1

1

rLCn DcEYl

P

L I

.o C

E N E

---e

-

-

M i 0 C

E N E

FIGURE 2

- Plio-Pleistocene Lithostratigraphic correlation of the Kendeng, Randublatung and Rembang zones.

49 YORTH

-.

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

LARA*.AN

(,I x11

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

DALUY

TCYIYUN.

NUWIN

MNOCR

-1

SOUTI.

aomoc

YOAWI

- 0

I

-.5.

-

PLLISTOCLME aN FACILS.

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DEBRIS LIMESTONE F'IES.

EIfj3

RLlSTOCENE HK. FACIES.

GLOB. LIMESTONE FACIES.

PLIOCENE VOC FACIES.

OLOB. MARL FACIES.

MOLUSC L M S T O N E FACIES.

M A R L TACKS.

REEF

00

LIMESTOI(E R Q E S .

FIGURE 3a - Lithofacies correlation of the Late Pliocene carbonate deposits of the Rembang zone (Larangan and Ngawen sections), Randublatung zone (Temengeng, Balun and Dander sections) and Kendeng zone (Ngawi and Sonde sections). WEST

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

(c-

3 7>,)0Kn DLRO

SOYOC

YOLUSC LIYCSTONL IACILS.

............................................. KARUtdJATl

YIANA

I-

EAST OOOOL

M A R L IAClCS.

REEC LIMCSTOUE IACIES

~~

~

~

FIGURE 3b - Lithofacies correlation of the Late Pliocene carbonate deposits of the Kendeng zone, in the southern part of the Northeast Java Basin.

a

This thin section shows planktonic foraminifera as the dominant component of the Globigerina Limestone facies in this sample taken from Gadu vjllage. Note the associated interparticle primary porosity.

Outcrop of Late Pliocene age, Globigerina Limestone facies in Gadu village, Randublatung zone, about 5 km NW of Cepu Oil City.

Globigerina Limestone facies in Blungun village, Rembang zone, location is 15 km N W of Cepu Oil City.

- Sharp contact between the marl of the Mundu Formation and

0

cn

~

FIGURE 5 - Photographs of reef components in the Reef Limestone facies, Kalitik Limestone in Kalitik village, Ngawi area, within the Kendeng zone. It is located 40 km south of Cepu Oil City. (a) Closeup of the reef buildup, (b) Closeup of a head coral, (c) Closeup of a branching coral.

-~

~

ZS

53

1:: -4

ux 0 Llm 0 r,

1

I

L rnI

1 M

c

*rl

a

L,

* m

A

e

m

n

E

O

I

I n

2

.L

n

d

%

W

b

m w3

w

H

3 crr

w

E

L

Selorei o (1 layer)

Balun (Gas Field)

-

Glob. Lst.

Upper part of GL/Mundu (2 layers)

Kruka (Oil field)

FIGURE 8

Glob. Lst.

Upper part of GL/Mundu (1 layer)

Bogomiring (Oil field)

n

400 - 500 I

(8 = 28-42 % K = 53-963 md I

Discovery : 1932 Still in Production Prod. = +- 400 MM SCF/Day

Discovery : 1929 Abandoned : ? Cumm.Prod.:1660.10 3 M 3 ,

110 f

Discovery : 1901 Abandoned : 1908 Cumm.Prod.: 8,9.103 M 3

-

645

k

.

.

Discovery : 1900 Abandoned : 1922 Cumm.Prod.: 9,5.103 M 3

Discovery : 1893 Abandoned : 1946 Cumm.Prod.:2568.10 3 M 3

-

20-40 % 67 md

650

= =

f

8 K

Discoverv : 1894 Abandoned : 1919 Cumm.Prod.: 55.103 M 3

1241

Discovery : 1888 Abandoned : 1939 Cumm.Prod.: 108.103 M3

-

-

-

I

I

385 - 645

732

-+ 300

I

I

The reservoir potential and reservoir characteristics of the Late Pliocene carbonates in the Northeast Java Basin (from Bambang Soetantri et al, 1973 and Schiller et al, 1994).

Glob. Lst.

Glob. Lst.

Upper part of GL/Mundu ( 5 layers)

Sekar Korong (Oil field)

Glob. Marl & Glob, Lst.

Glob. Lst

Tambakromo & GL/Mundu ( 3 layers)

Upper part of GL/Mundu (1 layer)

L i d a h (Oil Field)

Glob. Marl & Glob. 1st

Metatu (Oil field)

Lower part of Tambakromo ( 3 layers)

Kuti-Gununganvar (Oil field)

I

1

I

t