Universiti Teknologi Petronas - Malaysia
The International Geology Course Programme
Petroleum and Regional Regional Geology Northeast Java Basin, Indonesia”
“
7 – 11 11 December 2015
EXCURSION G GUIDE B BOOK prepared by :
Salahuddin Husein
DEPARTMENT OF GEOLOGICAL ENGINEERING, UNIVERSITAS GADJAH MADA Yogyakarta, 2015
Agenda DAY -1 Monday, 07/11/15 07.30 - 08.00
Registration
08.00 - 08.30
Opening; Introduction; Photo session
08.30 - 09.45
Lecture 1: Petroleum Exploration in NorthEast Java Basin
09.45 - 10.00
Coffee break
10.00 - 12.00
Lecture 2: Petroleum exploration and production
12.00 - 13.00
Lunch and praying break
13.00 - 14.30
Lecture 3: Regional Geology of East Java
14.30 - 14.45
Coffee break
14.45 - 16.00
Lecture 4: Overview on the geological excursion of NE Java Basin
DAY -2 Tuesday 08/12/15 07.00 - 08.30
Program
Activity
Instructor
Head of Department
Dr. Sugeng Sapto Surjono
Dr. Jarot Setyowiyoto
Salahuddin Husein, Ph.D.
Salahuddin Husein, Ph.D.
Location / Instructor
Depart from Yogyakarta to Bayat
08.30 - 11.30
Field observation 1: Basement structure and initial basin in-filling sedimentation of Wungkal-Gamping Formation
11.30 - 11.45
From Bayat to lunch location
11.45 - 13.00
Lunch and praying break
13.00 - 15.00
From Klaten to Alaskobong
15.00 - 16.00
Field observation 2: Kerek Formation at Kendeng fold-thrust belt
16.00 - 18.00
From Alaskobong to Purwodadi
18.00 - 19.30
Check-in, dinner
Hotel Griya Laksana, Purwodadi
19.30 - 21.00
Discussion on the basement geology and Kendeng fold-thrust belt
Salahuddin Husein, Ph.D.
DAY -3 Wednesday 09/12/15 06.00 - 07.00
Breakfast and check-out
07.00 - 08.00
Depart from hotel to Bledug Kuwu
08.00 - 09.00
Field observation 3: Mud volcanism of Rembang Zone
09.00 - 11.30
From Bledug Kuwu to Blora
Activity
Gunung Gajah
Javanese traditional fried chicken Bu Mayar Restaurant
Alaskobong
Location / Instructor Hotel Griya Laksana, Purwodadi
Bledug Kuwu
11.30 - 13.00
Lunch and praying break
13.00 - 13.30
From Blora to Polaman
13.30 - 14.30
Field observation 5: Depositional system of Ngrayong reservoir sandstones
14.30 - 15.00
From Polaman to Braholo
15.00 - 16.30
Field observation 4: Stratigraphy of Tawun and Ngrayong Formation and deformation of Braholo Fold
16.30 - 17.00
From Braholo to Blora
17.30 - 19.30
Check-in, dinner
Hotel Kencana, Blora
19.30 - 21.00
Discussion on the mud volcanism and Rembang petroleum system
Salahuddin Husein, Ph.D.
DAY -4 Thursday 10/12/15 06.00 - 07.00
Breakfast and check-out
07.00 - 08.00
Depart from hotel to Ledok Field
08.00 - 11.00
Oil field visit
11.00 - 11.30
From Ledok to Gadu
11.30 - 12.30
Field observation 4: Fluvio-laharic deposit of Kabuh Formation
12.30 - 13.00
From Gadu to Cepu
13.00 - 14.00
Lunch and praying break
14.00 - 18.00
From Cepu to Solo
18.00 - 19.00
Dinner
19.00 - 21.00
From Solo to Yogyakarta
21.00
Activity
Polaman
Braholo
Location / Instructor Hotel Kencana, Blora
Ledok Field
Karangtengah
Tunas Asri Restaurant, Cepu Taman Sari Restaurant, Solo
Check-in
DAY -5 Friday 11/12/15
Gajah Restaurant, Blora
MM Hotel UGM, Yogyakarta
Program
Instructor Salahuddin Husein, Ph.D.; Sarju Winardi, M.T.; Rahmadi Hidayat, M.Eng.
08.00 - 10.00
Overview and Wrap-up
10.00 - 10.15
Coffee break
10.15 - 10.30
Closing ceremony
Head of Department
10.30 - 17.00
City tour
Guided by assistants
Excursion Map
Polaman
Bledug Kuwu
Braholo
Ledok
Alaskobong Karangtengah
UGM
Bayat
: Fieldtrip day-1 : Fieldtrip day-2 : Fieldtrip day-3
The Team 1. 2. 3. 4. 5.
Salahuddin Husein, Ph.D. (Lead Instructor) Rahmadi Hidayat, M.Eng. (Instructor) Muhamad Nasir Lukman (Assistant) Hafiz Fatah Nur Aditya (Assistant) Mohamad Sakur (Assistant)
UTP – UGM Field Trip
Petroleum Geology and Regional Geology Northeast Java Basin December 8 – 10, 2015
Trip Guide Prepared by Salahuddin Husein
A. Introduction 1
This geologic excursion is crossing several physiographic zone of Java, perpendicular to major east-west structural trends of the island (Figure 1). Therefore, studying physiographic expression is a transient approach to understand geological formation of a particular region and its tectonic history. This excursion starts at the northern fringe of Southern Mountains Zone, a peneplained Eocene-Miocene volcanic belt. Successively the excursion runs to the north, crosses the Solo Zone, a low land formed by modern volcanic belt of Java Island, on to Kendeng Zone, a rolling and hilly area composed of inverted deeper marine sediments. To the north, the journey continues to Randublatung Zone, a narrow depression formed at the edge of Sundaland continental platform, which is dotted by astonishing mud volcanism. The excursion will be concluded in the Rembang Zone, an inverted continental platform which hosts the Northeast Java Basin, a prolific basin in the Java Island.
Figure 1.
Physiographic zonation of central and eastern part of Java Island (Pannekoek, 1949; van Bemmelen, 1949).
The excursion is covering the history of Java Island throughout Kenozoic, which was built upon a complex interaction between subduction-related volcanism and stable continental platform, and tying them to its hydrocarbon potential (Figures 2, 3).
1
Physiography refers to specific features and attributes of land surface, which resembles unique geologic processes and their controlling rock formation. Physiographic zonation of Java Island was established by Pannekoek (1949) dan Van Bemmelen (1949). Page 1 of 17
Figure 2. Tectonic elements of East Java.
Figure 3. Stratigraphy of East Java. Page 2 of 17
B. Southern Mountains
The Southern Mountain of East Java is a mountainous region occupying the southern part of eastern Java Island, aligned in ESE-WNW trend from Parangtritis of Yogyakarta to Ujung Purwo of Banyuwangi. It is an Eocene – Middle Miocene volcano-sedimentary basin that conformably overlying by Middle Miocene – Pliocene limestones, and later was uplifted and block-faulted so their formation gently dipped to the south (Husein & Srijono, 2007). Although it has interesting geologic history and the occurrence of certain petroleum system components, so far exploration over thhe Southern Mountains has resulted in little indication of possibilities. Potential source rocks of Eocene marine shales were observed in Jiwo Hills, a small basement window exposed to its northern foot. Apparently, understanding long-lived volcanism and magmatism throughout Cenozoic becomes the substantial success key in exploring this region.
Physiography The western part of Southern Mountains was studied in detail by van Bemmelen (1949), which is divided into three geomorphic units (Figure 4). The southern part composed of karst topography, dominated by connical hills which is known as a region of thousand hills ( Gunung Sewu – Javanese language), and bounded directly by Indian Ocean. To the north of Gunung Sewu, lying a circular flat terrain, known as the Wonosari Plateau. The northern part composed by remnants of volcanic edifices, known as Baturagung Ridge, which is bounded to the north by the Solo Zone.
Figure 4.
Physiographic zonation of western part of Southern Mountains (Husein & Srijono, 2007; after van Bemmelen, 1949).
Surface rivers eroding the Southern Mountains commonly flowing to the south and joining the Indian Ocean, most of them crossing the Gunung Sewu karstic morphology as sub-terranean drainage system. Northern flank of the northern ridges are dissected by rivers flowing to the Page 3 of 17
north, to the Solo Zone. To the west of Pacitan Bay, a dried, incised river valley assumed as the ancient Bengawan Solo runs southward to the Indian Ocean, before was pirated and turned northward to the Solo Zone by Late Pleistocene block-faulting of Baturetno Graben (Husein & Srijono, 2007).
Stratigraphy The following stratigraphy of Southern Mountains were compiled from numerous researchers (van Bemmelen, 1949; Sumosusastro, 1956; Surono et al ., 1992; Jurusan Teknik Geologi FT UGM, 1994; Novian et al ., in prep.; Figure 3): 1) Metamorphic rocks: as the oldest rock unit that exposed in the Jiwo Hills, composed of phyllites, mica schists, calc-silicate schists, and marble. Bothé (1929) found Orbitulina in a boulder of schist, suggesting this unit formed within Late Cretaceous to Paleocene. Epidoteglaucophane schists were also found adjacent to serpentinite. Some Eocene carbonates were metamorphosed into garnet-wollastonite skarn by diabasic intrusion. 2) Wungkal-Gamping Formation: unconformably overlying the metamorphic rocks, composed of quartz conglomerates, polymict breccias, quartz sandstones, calcareous sandstones, calcareous siltstones, intercalated by nummulitic limestones. Their age was Early to Middle Eocene (P8-P13), deposited in a transgressed shallow marine platform. Gradually, volcanic materials increased toward the next volcaniclastic formation. 3) Kebo-Butak Formation: deposited during Middle Eocene to Early Miocene (P12-N4), composed of pebbly sandstones, siltstones, claystones, tuff, and shales. Basaltic lavas with pillow structures were found in the lower part, whilst the upper part was characterized by intercalation between polymict breccias and sandstones. Numerous Late Oligocene intrusion of diorite, diabase, andesite porphyry, and basalts, were f requently observed in Jiwo Hills. 4) Semilir Formation: conformably interfingered deposited over Kebo-Butak Formation during Late Oligocene to Early Miocene (N3-N8), composed of lapilli tuff, tuffaceous sandstones, autoclastic breccias, and polymict breccias, with calcareous tuffaceous sandstones in the upper part. Andesitic lava occurred in the lower part, whilst lignite and carbonaceous siltstones were found in the middle part. U-Pb age dating indicates 20 Mya (Early Miocene) for its deposition (Smyth, 2005). Their environment ranges from terrestrial to deeper marine in a volcanic island arc setting. 5) Nglanggran Formation: interfingered with Semilir Formation in Early Miocene (N5-N6), composed of polymict conglomerates, pebbly sandstones, tuffaceous sandstones, andesitic breccias, tuff, and basaltic lavas. 6) Sambipitu Formation: conformably overlying Nglanggran Formation in the late Early Miocene (N6-N9), composed of interbedded of calcareous sandstones with tuffaceous siltstones, with increasing carbonate components upward. 7) Oyo Formation: deposited in upper to lower bathyal environment during the late Early Miocene to early Middle Miocene (N9-N12), thus interfingered with Sambipitu Formation, this formation composed of intercalation between muddy allochem limestones and tuffaceous sandstone , gradually developed into foraminiferal limestones in the upper part. 8) Wonosari Formation: interfingered with Oyo Formation from late Early Miocene to Late Miocene (N10-N18), composed intercalation between limestones and sandy limestones, and cross-bedded limestones, deposited in carbonate platform. In Jiwo Hills, this formation was
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intruded by diabase. 9) Kepek Formation: interfingered with the upper part of Wonosari Formation, deposited during Middle to Late Miocene (N17-N18) in middle bathyal to platform environment, composed of intercalation of tuff with claystones, gradually covered by limestones. 10) Quaternary deposit: composed of fluvio-volcanic sediments from Merapi Volcano since Pleistocene to Recent.
Stop site Observation on the Southern Mountains will be located in Gunung Gajah, Bayat (Figure 5). A nonconformity contact of Paleogene deposit upon metamorphic basement was exposed. Phyllite was block-faulted and overlied by basal breccias of Early Eocene (P8). Overlying sequences was deposited after short hiatus by calcareous sandstone-siltstones of Middle Eocene (P10-P11), containing large foraminifera of Nummulites that were randomly scattered, accompanied plutonic quartz and metamorphic fragments. Diabasic sills have intruded those Eocene sequences, where it might be account on the occurrence of graphite in the outcrop. In the upper part, as indicated by age of P13, volcanic quartz have come about, and gradually became more dominant as carbonate component diminished, suggesting that they were conformably developed into Kebo-Butak Formation.
Figure 6. Nonconformity contact between nummulitic limestones and phyllite.
C. Solo Zone 2
Solo Zone (sensu lato ) is an open depression in the median of Java Island, aligned in ESEWNW, trending from Solo to Banyuwangi, which is further divided into three sub-parallel compartments (van Bemmelen, 1949), from north to south: 1) Ngawi sub-zone, a narrow depression between Kendeng Zone and the Modern volcanic arc. 3 2) Solo sub-sone (sensu stricto ), intermontane plains between present-day volcanoes (Figure 7). 2
In a broader sense (Latin) Page 5 of 17
3) Blitar sub-zone, a depression between the Modern volcanic arc and the Southern Mountains Range.
Figure 7. Lawu Volcano, located to the east of Solo City, one of main volcanoes occupying the Solo Zone
(photo taken from http://agosaga.blogspot.co.id).
At the present-day, Solo Zone is an active sedimentary basin with fluvial environment receiving sediment supply from the adjacent active volcanoes, Southern Mountains Zone, and Kendeng Zone. A number of large rivers is flowing through this zone and depositing their deposit along the way, i.e. the Bengawan Solo, Bengawan Madiun (which later joined Bengawan Solo in Ngawi City), and Brantas River. At the end of their journey, those large rivers actively formed broad deltas in the coastal strip of Surabaya and Gresik. Stratigraphically, this zone was assumed to be similar with Kendeng Zone.
D. Kendeng Zone
Kendeng Zone is composed of elongated hills aligned in E-W direction, lying directly to the north of Ngawi sub-zone. This zone was formed by marine sedimentary sequences that has been 4 intensively deformed onto anticlinorium , with the length of fold-thrust belt reached 250 km and maximum width of 40 km (de Genevraye & Samuel, 1972), stretching from Ungaran Volcano in its western end to Mojokerto lowland in its eastern end. However, this zone is still extended in subsurface in the Madura Strait. Tectonic deformation were more intense in the western part, and getting weaker to the east. In the central part, this anticlinorium was intruded by Pandan Volcano of Early Pleistocene (Lunt et al ., 1998). As fold pattern around the volcano have been symmetrically deflected, it is assumed that Kendeng deformation took place at the same time with Pandan volcanism, around Late Pliocene to Early Pleistocene. As rock formation dominated by soft, fine grained marine sediments, erosional processes is so intense and were being able to inverse the existing morphology, such as anticlinal valleys and synclinal hills. These soft rock formation also made Bengawan Solo possible to maintain its direction while perpendicularly crossing the anticlinorium belt along 15 km length, and made this river as an antecedent stream during the Plio-Pleistocene tectonic uplift. Although the Kendeng Zone is known as the deepest sedimentary basin in the eastern Java, 3 4
In a strict sense (Latin) A series of anticlines and synclines so grouped that taken together they have the general outline of an arch, and those minor folds were superimposed each other. Page 6 of 17
however the occurrence of petroleum system is still poorly understood. Complex thrust faults have been responsible to poor seismic interpretation.
Stratigraphy Main references in composing the stratigraphy of Kendeng are publication of de Genevraye & Samuel (1972) and Pringgoprawiro (1983) (Figure 3). Kendeng was formed deep marine sediment at its lower part, and getting shallower upward, and finally become terrestrial deposit. Their sediments mainly of clastic turbidites, carbonates, and volcaniclastics. 1) Pelang Formation: as the oldest formation exposed at the surface with an age of Late Oligocene – Middle Miocene (N3-N9), composed of marl with bioclastic limestones, without any occurrence of volcanic material, and have been deposited during regression system (Irwanto et al ., 2015). 2) Kerek Formation: conformably interfingered with the upper part of Pelang Formation during Middle to Late Miocene (N8-N17), composed of interbedded between claystones, marls, calcareous tuffaceous sandstones, and tuffaceous sandstones. Sedimentary structures in any places suggest that this formation was mainly deposited in turbidity current. 3) Kalibeng Formation: conformably overlying the Kerek Formation during Late Miocene to Pliocene (N16-N21), in the lower part composed of massive marl that rich in planktonic foraminifera and frequently disrupted by volcanic sandstones, which developed as debris deposit in the western part (Banyak Member) and as turbidite deposit in the eastern part (Atasangin Member). The upper part of this formation were composed of limestones (Klitik Member) graded to marls and bluish cl aystones. 4) Pucangan Formation: deposited as laharic facies and black clay, conformably overlying the Kalibeng Formation, suggesting regression stage up to terrestrial enviroment. 5) Kabuh Formation: deposited in fluvio-lacustrine environment and composed of cross-bedded sandstones with conglomerates and tuffs. 6) Notopuro Formation: mainly as laharic deposit during Late Pleistocene. 7) Bengawan Solo terrace deposits: composed of polymict conglomerate. Stop site Observation on the Kendeng Zone will be located in Alaskobong, Sumberlawang, and Karangtengah, Ngawi. 1. Alaskobong A railcut cliff exposes a folded Kerek Formation in an inverted synclinal hill (Figure 8). It is composed of tuff that gradually overlying by tuffaceous marls and calcareous sandstones, deposited in outer shelf during lower Middle Miocene (N8-N9). At the northern end an anticline occurred in association with a reverse fault. The axial plane of anticline is N102°E/89°. At the median of synclinal hill, the axial plane N64°E/78°. Those attitudes of fold axis suggests the occurrence of conical (non-cylindrically) fold (Figure 9).
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Figure 8. Photo mosaic of Alaskobong outcrop and its zoom in on syncline and anticline, facing to east
(Husein et al ., 2008).
(a)
(b)
Figure 9. Graphic solution on axial plane of anticline (a) and syncline (b), by following method of Marshak
and Mitra (Husein et al ., 2008).
2. Karangtengah A sand quarry exposes a section of fluvio-laharic deposit of Kabuh Formation (Figure 10). It is mainly composed of volcanic sandstones with trough and planar cross-bedding structures, which contain pebbly, rounded fragments of pumice and andesites. Conglomerates were deposited in channel structures. Those fluvio-laharic deposit were separated into lower and upper conglomeratic sandstones by one meter thick, massive tuff. The upper sandstones were reddish in color, scouring into the tuff layer.
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Figure 10. Planar cross-bed volcanic sandstone of Kabuh Formation, camera facing to east.
E.
Randublatung Zone
Randublatung Zone is a narrow depression between Kendeng and Rembang zones, covering area of Purwodadi, Cepu, and Bojonegoro. It is an undulating lowland with some gentle folds trending east-west such as Dander and Ngimbang hills (van Bemmelen, 1949). Sedimentation within this zone have been active since Late Tertiary until present-day, receiving sediments from adjacent Kendeng and Rembang zones. Drainage system within this zone are divided into Lusi and Bengawan Solo system. In the western part, sedimentation were done by Lusi River that later joins Serang River to form Serang Delta, producing the prograding coastal plain of north Java. In the eastern part, Bengawan Solo runs to the east to form Ujung Pangkah Delta in the northern part of Gresik. Stratigraphically, this zone was assumed to be similar with Rembang Zone. A remarkable feature in the Randublatung Zone is the occurrence of mud volcanism (Figure 11). Although their mechanism is still poorly understood, those mud volcanism were good indication for the existence of petroleum system.
Figure 11. Eruption of Kuwu mud volcanism (photo taken from https://picture.triptrus.com).
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Stop site Observation on the Randublatung Zone will be located in Kuwu mud volcano. It is an active mud volcano, with periodic eruption of mud, water, gas, and rock fragments. A variation of gas were observed from biogenic gas, petroleum gas, as well as dry condensate gas (Burhannudinnur et al ., 2012). Mud volcano forms typical morphology of pie, salsa, and pool (Figure 12). The Kuwu mud volcano is a large pie with a diameter of 60 m, which is composed by several smaller pie aligned in particular direction. Erupted water exhibits pH of 6.5-7.0 with temperature ranges 30-32 °C and high Na, Cl, and Mg content. Erupted mud contains smectite, kaolinite, quartz, and feldspar (Burhannudinnur et al ., 2012). Paleontologic analysis indicates the oldest mud material from N7-N9 interval (Early to Middle Miocene), suggesting their source from Tawun Formation.
Figure 12. Dried gryphone and p ond in Kuwu mud volcano.
F.
Rembang Zone
The Rembang Zone composed several anticlinoria aligned in E-W direction, stretching from Purwodadi to Madura Island. It is further divided into North Rembang Anticlinorium and South Rembang Anticlinorium, which is also known as Cepu Antiklinorium (Van Bemmelen, 1949). Those two anticlinoria are separated by Lusi River in the west and K ening River to the east. Anticlines are the main structural traps in hydrocarbon exploration over the region (Figure 13). Folds in Rembang Zone were arranged in left-stepping en-echelon pattern, controlled by several ENE-WSW sinistral basement faults with small lateral slip (Husein et al ., 2015). This pattern can be noticed in the North Rembang Anticlinorium from Dokoro Fold to Lodan Fold, whilst in the South Rembang Anticlinorium stretches from Jamprong Fold to Gabusan Fold. Fold formation took place in the Late Pliocene, during deposition of Lidah Formation, by a N-S regional compressional stress, which was re-oriented to NW-SE direction when approaching the basement faults (Husein et al ., 2015). Surface erosion in the zone is not as rigorous as Kendeng Zone, due to higher resistance of limestones to erosion, so that there were no occurrence of topographic inversion. Denudational
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processes were only carried out by smaller rivers which brought their sediments directly to the Java Sea, and they are not able to form considerable deltas. The Rembang physiographic zone, is a geomorphic expression for the North East Java Basin, a prolific basin in the island of Java. Although it contributes only a small amount to the 1.3 millionbarrel oil per day of national production, has a significant means to the oil industry. It has the first well drilled in Indonesia (Ledok #1). The well is located about 20 km west of Cepu and it was drilled in 1893. This well marked the beginning of oil i ndustry in Indonesia. Since then, numbers of oil field have been developed in the region (Kawengan, Ledok, Semanggi - Nglobo and other smaller fields). Some of those fields are currently still in production. Currently, local peoples mine the oil using a traditional method.
Figure 13. Physio-tectono-stratigraphic zonation of East Java with oilfields (Nachrowi & Koesoemo, 2003).
Stratigraphy Stratigraphy of Rembang Zone mainly follows publication of Pringgoprawiro (1983). Based on sub-surface data from hydrocarbon exploration, the oldest Tertiary stratigraphic unit is Ngimbang Formation, but it was never exposed at the surface (Figure 3). 1. Pre-Tertiary Basement: Wells data indicate that basement rock comprises accretion wedges of gabro, ophiolites, metamorphic rock, and metavolcanic/volcanic, igneous, cretaceous sediments, which is lithified sequences of sandstonelshale with some chert. 2. Ngimbang Formation: The middle Eocene sediment comprises of a sequence of sandstone, siltstones and shales unconformably overlies the cretaceous basement. This sequence is overlaid unconformably by the middle to late Eocene series of the fluvial, deltaic, lacustrine and marginal marine deposits of the middle to late Eocene Ngimbang Formation. This sequence can be divided into 2 (two) main sedimentary units, which are the Ngimbang clastic and Ngimbang carbonate.
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3. Kujung Formation: The formation is dominantly composed by marl, interbedded with thin bedded green fossiliferous sandstone and limestone, and contains larger foraminifera, algae and coral debris. It was deposited in the deep open marine during the Late Oligocene (P18N3). 4. Prupuh Formation: The formation is constituted by interbedded reefal bio-calcarenite, biocalcilutite and bluish grey marl. They were deposited in the outer neritic during the Late Oligocene – Early Miocene (N3-N5). 5. Tuban Formation: The formation is formed by monotonous black-grey claystone, and several thin bedded bioclastic limestone where algae, coral debris decrease their size and their quantity. The planktonic foraminifera analysis shows the age of Early Miocene (N5-N9). 6. Tawun Formation: The lower part of the formation is dominated by black-grey claystone to marl, and changes gradually to grey siltstone in the upper part. The uppermost part of the Tawun Formation is the so-called Ngrayong Member (but more renowned as Ngrayong Formation), presented by bedded quartz sandstone and micaceous, lignitic shale containing mollusc and intercalated by thin bedded orbitoid limestone, and laterally changes in facies westward to black pyritic claystone. Frequent facies changes are occurred in this member, since it was deposited in the transitional environment during Early to Middle Miocene (N8N13). 7. Bulu Formation: The formation is constituted by bedded grainstone and wackstone which its bioclastic content is consisted of large forams, algae, echinoderms, coral fragments, and mollusc in the lower part. This formation was deposited in the shallow open marine, where reef buildups were developed in certain areas, during Middle Miocene (N13-N14). 8. Wonocolo Formation: It composed by massive grey fossiliferous sandy marl and intercalated thin bedded grey fossiliferous calcarenites. It was deposited during Late Miocene (N15-N16), in the outer neritic environment. 9. Ledok Formation: The formation is composed by interbedded thickening upward glauconitic fossiliferous greenish grey calcareous sandstone, and thinning upward fossiliferous greenish grey sandy marl. They were developed in the upper part of the formation, indicating of the outer to inner neritic environment. The age of the formation is Late Miocene (N16-N18). 10. Mundu Formation: The formation is dominated by fossiliferous greenish grey marl, while in the upper part of this formation is presented by interbedded fossiliferous greenish grey sandy calcarenite and greenish grey sandy marl, the so-called Selorejo Member (or also long known as Selorejo Formation). The formation was deposited during the Late Miocene to Pliocene (N19-N21) in the outer neritic environment. 11. Lidah Formation: This formation consists of bluish claystone and bedded marl with intercalations of sandstone and lenses of coquina, deposited during Pleistocene (N22-N23). The lower part of this formation is supposed to have been deposited in middle to outer sublittoral environment. Shallowing upward sequence occurred in the upper part of this unit, where a shallow water environment prevailed, indicated by the presence of molluscan bearing layers. 12. Paciran Formation: Composed of reefal limestones, partially converted to karst topography and chalky limestones, deposited from Pliocene (N19) up to Recent.
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Stop sites Observation on the Rembang Zone will be conducted in 2 locations: i.e. Polaman and Braholo. 1. Polaman Ngrayong Formation is well exposed in this abandoned quarry (Figure 14). The lower part composed of shales that gradually changed to sandstones. Gastropods and pelecypods become more abundant upward. Amber and lignite are also frequently observed. The upper part was composed of cycloclypeus limestones, which suggests that Ngrayong deposition had shifted from tidal flat to shallow marine platform.
Figure 14. Outcrop of Polaman quarry, showing gradational changes of calcareous sandstones to
cycloclypeus limestones, camera facing east.
2. Braholo On this 31 meter height rocky cliff, transition from Tawun to Ngrayong formartion can be observed (Figure 15). The lower part were composed of rudstones that gradually changed to quartz sandstones. Aerial exposures have been assumed to be occurred before deposition of Ngrayong, as diagenetic processes were prevailed on the limestones, as suggested by good porosity and intense cementation. This location also exposes anticline axis of Braholo Fold, with its associated faults.
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Figure 15. Braholo outcrop, showing gradational change from limestones to siliciclastics and to mixed -
siliciclastics and limestones at the uppermost part. Camera fasing southwest.
Exploration history Hydrocarbon exploration in the Northeast Java Basin has been started since 1871, based on surface geology and oil or gas seepages in the area, exploration drilling were conducted and finally resulted the discovery of the Kuti Gunug Anyar Oil Field (1888), Lidah Oil Field (1889) in the Surabaya area, and the Ledok Oil Field (1893) and Kawengan Oil Field (1894) in Cepu area. Following these discoveries, during the same decade, more than 25 oil fileds have been discovered and developed in the Cepu - Surabaya area, but most of them were now abandoned. In the 1970s, Pertamina conducted modern and intensive exploration in Cepu and surrounding areas, several exploration wells such as Kujung-I, Dermawu-I, Purwodadi-I, Dander-I and Jatirogo-1 have been drilled but failed to discover oil or gas in commercial quantities. In the 19801s, Pertamina in Cepu area, JOB-Stanvac in Gundih area and JOB-Trend in Tuban area, run geological and geophysical surveys and drilled several exploration wells but again they failed to discover oil or gas. In the 19901s, modern sequence stratigraphy concept was world widely applied in the petroleum industries. Based on this modern concept, Pertamina, Humpuss, JOB-Santa Fe and JOBHuffco Brantas, started again to explore the onshore area of the Northeast Java Basin. Finally, JOB-Santa Fe discovered oil in Tuban area, Pertamina found gas in Rembang area and JOB Huffco
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Brantas found gas in Porong area, while Humpuss is still continuing exploration in Cepu and surrounding areas. In the 20001s, Humpuss and Mobil-Oil, finally discovered the oil reserve in commercial quantities within "the reservoir of the Prupuh reefal limestone formation" in the Kalitidu area, south of Bojonegoro - East Java.
Oil and Gas Fields For the time being, only 5 (five) oil fields and l (one) gas field in the onshore area of the Northeast Java Basin are still being kept in production, i.e.: Kawengan oil field (Pertamina), 2. Ledok oil field (Pertamina), Nglobo - Semanggi oil fields (Pertamina), Traditional Wonocolo oil field (Pertamina), Soko-Tuban oil field (JOB-Santa Fe), and Balun gas field (Pertamina). The other oil fields, which have been developed and produced since 1888 until 1900's, have already abandoned. Kawengan is the biggest oil field in the Northeast Java Basin, the cumulative production reached 150 million cubicmeter in the 1990's. The production layers are the upper part of the Tawun Formation (the Ngrayong sand) and the lower part of the Wonocolo formation. The quartz sand of the Ngrayong formation is the main reservoir, it is porosity range from 16-21% permeability 31 - 165 milidarcy, with water saturation (Sw) more than 90% with the depth varies from 400 - 600 meter in Kawengan-Wonocolo area. Structurally, Kawengan is an asymmetric anticline with NW - SE direction. The south flank is bounded by the reserve fault with the same direction. Several normal faults with NE -SW directions devided Kawengan anticline into several blocks or culminations and become the boundaries of production system. They are Kidangan (east), Ngudal, Wonosari, Kawengan and Wonocolo (west). Balun is the only gas field in the Northeast Java Basin (onshore) structurally is a simple symmetric anticline, east-west direction the reservoir is the globigerenid sand of the Selorejo Formation in a depth around 400 - 500 meter. The Selorejo formation composed almost entirely of sand size planktonic foraminifera and has major type of interparticle primary porosity, with porosity range from 28-42% permeability from 53-963 milidarcy. There is no agreement on what could be the source rocks for the oil. Weeda (1958) suggested the thick marine Early Miocene Tawun clays are the most likely source rocks. However, unpublished geochemical analyses suggest the oils were derived from a non -marine source, which would have to be looked for deeper in the section.
Ledok Traditional Oil Mining Starting from 1989, in the Ledok Oilfield, located at Sambong District, rights for old oil wells exploration was given to a group of local people (known as "miner") through cooperation contract with Pertamina Employee Cooperation Body "Patra Karya" (Figure 16). These group of miner will work under Patra Karya and get compensation fee from each litre oil they got. It is claimed that almost 90% abandoned wells of Pertamina was succesfully recovered by traditional methods. In the beginning, their production rate only about 600 litre per day, but recent numbers grow to 30,000 litre per day. This incredible number is even larger than what Pertamina could have done. Salary rate was set by Pertamina in 2004 as IDR 215 per litre oil. If the miner group could reach production level of 1 million litre per month, they will get approximately IDR 200 million. With member of 220 people, assuming those salary equally divided, each member will get IDR 900,000 per month, or IDR 30,000 per day work. These number are 14 times larger than gross-
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income rate of Sambong District year 2003 of IDR 778,000 per month, or even more if comparing to minimum salary level of Blora Regency year 2003 of IDR 366 ,000. Thus, from economic point of vi ew, these traditional oil mining have multi pliers effect, which is providing local job, income source, decreasing poverty level, and preventing urbanisation. Although it requires capital of about IDR 10 million to operate a traditional oil mining, people with group could share the expenses during first year operational. Apparently, people of Ledok Village could take economic advantage, as shown in 2004 by numbers of autocar vehicles (35 unit) and could send their children to uni versities (there was 81 bachelor-degree holder).
Figure 16. (upper left) scenery of traditional oil mining in Wonocolo Village (photo : Alan Yudhis); (lower
left) final residue of traditional distillation. One well could produce 2-3 barrel oil of diesel and gasoline; (right) wooden oil rig.
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De Genevraye, P. and L. Samuel (1972) Geology of The Kendeng Zone (Central & East Java). st Proceedings of the Indonesian Petroleum Association 1 Annual Convention and Exhibition, p. 17 – 30. Husein, S., K. Kakda, dan H.F.N.Aditya (2015) Mekanisme Perlipatan En-Echelon di Antiklinorium Rembang Utara, Prosiding Seminar Nasional Kebumian ke-8 Jurusan Teknik Geologi Fakultas Teknik Universitas Gadjah Mada, Yogyakarta, GEO41, pp 224-234. Husein, S., A. Mustofa, A. Matikayuda, dan I. Sudarno (2008). Kompleks Lipatan Alaskobong: laboratorium alam geologi struktur. Prosiding Seminar Nasional “Tantangan dan Strategi Pendidikan Geologi dalam Pembangunan Nasional” , Jurusan Teknik Geologi FT UGM, Yogyakarta, 12 hal. ISBN 978-979-17549-0-3. Husein, S. and Srijono (2007). Tinjauan Geomorfologi Pegunungan Selatan DIY/Jawa Tengah: telaah peran faktor endogenik dan eksogenik dalam proses pembentukan pegunungan. Prosiding Seminar Potensi Geologi Pegunungan Selatan dalam Pengembangan Wilayah, Pusat Survei Geologi, Yogyakarta, 10 pp. Irwanto, H., S.E. Hapsoro, G.D. Zoenir, M. Maha, dan J. Setiawan (2015) Tinjauan Ulang Terhadap Posisi Stratigrafi Formasi Pelang. Prosiding Seminar Nasional Kebumian Ke-8 Departemen Teknik Geologi FT UGM, pp. 371-381. Jurusan Teknik Geologi FT UGM (1994) Geologi Daerah Pegunungan Selatan: Suatu Kontribusi. Kumpulan Makalah Seminar Geologi dan Geotektonik Pulau Jawa, Sejak Akhir Mesozoik hingga Kuater. Jurusan Teknik Geologi Fakultas Teknik Universitas Gadjah Mada, Yogyakarta, 23 hal (extended abstract ). Pannekoek, A.J. (1949) Outline of the Geomorphology of Java. Reprint from Tijdschriftvan Het Koninklijk Nederlandsch Aardrijkskundig Genootschap, vol. LXVI part 3, E.J. Brill, Leiden, pp. 270-325. Pringgoprawiro, H. (1983) Biostratigrafi dan Paleogeografi Cekungan Jawa Timur Utara, Suatu Pendekatan Baru. Desertasi Doktor, Institut Teknologi Bandung. Novian, M.I., P.P. Utama, dan S. Husein (2013) Penentuan Batuan Sumber Gununglumpur di Sekitar Purwodadi Berdasarkan Kandungan Fosil Foraminifera. Prosiding Seminar Nasional Kebumian ke-6, Jurusan Teknik Geologi FT UGM, Yogyakarta, pp. 519-534. Novian, M.I., D.H. Barianto, S. Husein, dan Akmaluddin ( in preparation) Peta Geologi Lembar Wonosari dan Semanu, Provinsi Daerah Istimewa Yogyakarta. Pusat Survei Geologi, Bandung. Smyth, H. (2005) Eocene to Miocene Basin History and Volcanic Activity in East Java, Indonesia. PhD Thesis, University of London, 470 p. Sumosusastro, S. (1956) A Contribution to The Geology of Eastern Djiwo Hills and The Southern Range in Central Java. Department of Geology, Faculty of Science, Universi ty of Indonesia. Surono, B. Toha, dan Ign. Sudarno (1992) Peta Geologi Lembar Surakarta-Giritontro, Jawa. Pusat Penelitian dan Pengembangan Geologi, Bandung. Van Bemmelen, R.W. (1949) The Geology of Indonesia, vol. I.A. General Geology . Martinus Nyhoff, The Hague.
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