BARITO BASIN

BARITO BASIN

Tectonically the Barito Basin is located in the southeastern boundary of Schwanner Shield, South Kalimantan. This basin is bounded by Tinggian Meratus in the East and in the North part is separated from the Kutai Basin by the form of Adang Fault, to the South it still opens to the Java Sea, and to the West is limited by the Sunda Exposure.

The Barito Basin is an asymmetric basin, has a front basin (foredeep) at the easternmost part and is a platform on the western side. The Barito Basin began to form in the Late Cretaceous, after collisions between microcontinent Paternoster and Southwest Kalimantan (Metcalfe, 1996; Satyana, 1996).

At the Early Tertiary extensional deformation occurred as a result of convergent tectonics, and produced the Northeast - Southeast rifting pattern. Rifting was then the site of sedimentation of lacustrine and alluvial fan from the lower Tanjung Formation originating from the Horst region and filling the graben section, then followed by deposition of the upper Tanjung Formation in the transgression relationship.

At the beginning of the Oligocene, a lifting process occurs followed by deposition of the Lower Berai Formation which covers the upper Tanjung Formation in harmony in the regression relationship. In the Early Miocene it was followed by the deposition of massive limestone units of the Berai Formation.

During the middle Miocene a process of removal of the Meratus complex occurred which resulted in a simultaneous regression cycle with the deposition of the lower Warukin Formation, and in several places showed symptoms of local (hiatus) inconsistency between the upper Warukin Formation and the lower Warukin Formation.

This appointment continues until the Middle Miocene End which ultimately results in regional inconsistencies between the upper Warukin Formation and the Upper Miocene Dahor Formation - pliocene.

The last tectonic occurred at the time of Plio-Pliestosen, the entire area was raised, folded and broken. The structure axis is parallel to Tinggian Meratus. Upward faults are formed with a slope to the east, breaking tertiary rocks, especially the Tinggian Meratus areas.

Barito Basin Geology

STATIGRAFI

Barito Basin Stratigraphy, Kutai Basin, and Tarakan Basin. (Courtney, et al., 1991, op. Cit., Bachtiar, 2006).

The stratigraphic sequence of the Barito Basin from old to young are:

• Tanjung Formation (Eocene - Early Oligocene)

This formation is composed of sandstones, conglomerates, claystone, coal, and basalt. This formation is deposited in the neritic litoral environment.

• Berai Formation (Late Oligocene - Early Miocene)

The Berai Formation is composed of limestone interspersed with claystone / shale at the bottom, in the middle consists of massive limestone and at the top it repeats itself again into the intersection of limestone, shale, and sandstone. This formation is deposited in the middle lagoon-nile environment and covers in harmony the Tanjung Formation which is located at the bottom. Both the Berai Formation, and Tanjung have a thickness of 1100 m near the Cape.

• Warukin Formation (Lower Miocene - Middle Miocene)

The Warukin Formation is deposited on the Berai Formation and is covered in inconsistency by the Dahor Formation. Most have been exposed, especially along the western part of Tinggian Meratus, but in the Tanjung and Kambitin regions have eroded. Only to the south of Tanjung which is still below the surface.

This formation is divided into two members, namely the lower part of Warukin (clastic member), and the upper part of Warukin (coal member). The two members are distinguished based on their lithological arrangement.

Warukin the lower part (clastic member) in the form of nesting between marl or slime clay with thin sandstone inserts, and thin limestone at the bottom, while at the top is alternating sandstones, clays, and coal. The coal has a thickness of no more than 5 m, while the sandstone can reach a thickness of more than 30 m.

Warukin the upper part (coal member) with a maximum thickness of ± 500 meters, in the form of sandstone interbreeding, and claystone with coal inserts. The thickness of the coal seams reaches more than 40 m, while the sandstones are not very thick, usually containing fresh water. The Warukin formation is deposited in an innerneritic - deltaik environment and shows a regression phase.

• Dahor Formation (Upper Miocene - Pliocene)

This formation consists of alternating between sandstones, coal, conglomerates, and shales deposited in a litoral environment - supra litoral.

Reference:

• Bachtiar, A., 2006, Slide Kuliah Geologi Indonesia, Prodi Teknik Geologi, FITB-ITB
• Satyana, A.H., 2000, Kalimantan, An Outline of The Geology of Indonesia, Indonesian Association of Geologists, p.69-89.

NORTH WEST JAVA BASIN

NORTH WEST JAVA BASIN

A. BRIEF REGIONAL GEOLOGICAL REVIEWS

The North West Java Basin has been known as the main hydrocarbon province in the Pertamina DOH JBB area, Cirebon. This basin is located between the Sunda Shelf in the North, the Perlipatan - Bogor Line in the South, the Appointment of Karimun Jawa in the East and the Seribu Island Exposure in the West. The North West Java Basin is influenced by the North-South block faulting system. North-South faults divide the basin into graben or several sub-basins, namely Jatibarang, Pasir Putih, Ciputat, Rangkas Bitung and some basement heights, such as Arjawinangun, Cilamaya, Pamanukan, Kandanghaur-Waled, Rengasdengklok and Tangerang. Based on the stratigraphy and its structural patterns, and its location in the pattern of subduction arc from time to time, it turns out that the West Java basin has experienced several times of sedimentation and tectonic phases since the Eocene to the present (Martodjojo, 2002).

 

Location of the West Java Basin (Northwest Java Basin)

B. TEKTONOSTRATIGRAPHY AND GEOLOGICAL STRUCTURE

The North West Java Basin consists of two areas, namely offshore in the North and onshore in the South (Darman and Sidi, 2000). The entire area is dominated by extensional faults with very little compressional structure. The basin is dominated by rifts which are associated with faults which form several deposited structures, including the main depositors, namely the Arjuna Sub-Basin and the Jatibarang Sub-Basin, as well as other deposers such as: Ciputat Sub-Basin, Pasirputih Sub-Basin. The depositors are dominated by Tertiary sequences with a thickness exceeding 5500 m.

The important structures in the basin consist of various elevation areas that are associated with broken anticlines and high blocks (horst block), folds on the part that descends on the main fault, keystone folding and hits the bedrock height. The compressional structure only occurs at the beginning of the formation of the first rift which is relatively northwest-southeast in the Paleogene period. This fault will be active again in the Oligocene. Tectonics of West Java are divided into three tectonic phases starting from Pre-Tertiary to Plio-Pliostosen.

 

The tectonic phase is as follows:

1. First Tectonic

In the Late Tertiary Cretaceous, North West Java can be classified as 'Fore Arc Basin' with structural orientations ranging from Cileutuh, Bogor Sub-Basin, Jatibarang, Muriah Basin and West Florence Basin which indicate control of the 'Meratus Trend'. The Paleogene (Eocene-Oligocene) Period is known as the Extensional Rifting Paleogene. In this period a horizontal shear fault occurred in the main Sundanese krataon due to the Indian Plate collision with the Eurasian Plate. These faults initiated the formation of Tertiary basins in Western Indonesia and formed the North West Java Basin as a pull apart basin.

This extension of tectonics forms half gnraben systems and is the first phase of rifting (Rifting I: fill phase). The sediment deposited on rifting I is called synrift sediment I. The initial rifting basin was formed during fragmentation, rotation and movement of the Sundanese palace. Two trends of normal fault caused by the development of rifting-I (early fill) trending N 60o W - N 40o W and almost N - S are known as the Sunda fault pattern. During this time lacustrin and volcanic deposits formed from the Jatibarang Formation which covered the low lows that existed. This sedimentation process continues with the discovery of the Talangakar Formation transition deposits. This system is then terminated with the carbonate deposited in the Baturaja Formation.

2. Second tectonic

The second tectonic phase occurs at the beginning of Neogen (Oligo-Miocene) and is known as Neogen Compressional Wrenching. It is characterized by the formation of shear faults due to the compressive force of the collision of the Indian Plate. Most of the fault shifts are reactive to normal faults formed in the Paleogene period.

A new subduction route was formed in southern Java. The Early Miocene volcanic pathway is currently located off the southern coast of Java. This row of volcanoes produces submarine volcanic deposits which are now known as "old andesite" which are spread along the southern part of Java. This tectonic pattern is called the Javanese Tectonic Pattern which changes the old tectonic pattern that happened previously to be trending east-west and produces an upward fault system, starting from the south (Ciletuh) moving north. This fault pattern is in accordance with the upward fault system behind the arc or known as the "thrust fold belt system".

3. Last Tectonics

The final tectonic phase that occurs is in the Pliocene - Pleistocene, where there is a process of compression again and forming structural traps in the form of rising faults on the southern path of the North West Java Basin. The upward faults formed are the faults rising in Pasirjadi and the faults rising in Subang, whereas in the northern path of the North West Java Basin a downward fault is formed in the form of a downward fault Pamanukan. As a result of the structural trap, the process of hydrocarbon migration occurs again.

 

(Physiographic transverse incision of the basin and volcanic arc of West Java)

C. REGIONAL STRATIGRAPHY

(StratigraphyTable of North West Java Basin )

The general stratigraphy of North West Java from old to young is as follows:

1. Basement rock

Bsement rocks are andesitic and basaltic igneous rocks that are Middle Cretaceous to Upper Cretaceous and Pre-Tertiary metamorphic rocks (Sinclair, et.al, 1995). Environment Deposition is a surface with the rest of weathered tropical vegetation (Koesoemadinata, 1980).

2. Jatibarang Formation

This unit is an early synrift deposit, mainly found in the central and eastern part of the North West Java Basin. In the western part of this basin, there is not much (very thin) appearance of the Jatibarang Formation. This formation consists of tuffs, breccia, agglomerates, and base conglomerates. This formation is deposited in fluvial facies. The age of this formation is from the Late Eocene to the Early Oligocene. In some places in this formation oil and gas are found in tuff fractures (Budiyani, et al., 1991).

3. Talang Akar Formation

The next syn rift phase is precipitated by Talang Akar Formation above the Jatibarang Formation. In the beginning fluvio-deltaic functioned to marine phaises. The lithology of this formation is preceded by the intersection of sandstone sediments with nonmarine flakes and is terminated by interrelationships between limestone, shale, and sandstones in marine facies. At the end of sedimentation, the Talang Akar Formation is characterized by the end of synrift sedimentation. This formation is expected to develop quite well in the Sukamandi area and its surroundings. The sedimentation of this formation occurs from the Oligocene to the Early Miocene.

4. Baturaja Formation

This formation is deposited in harmony above the Talang Akar Formation. Sedimentation of the Baturaja Formation which consists of limestones, both in the form of exposures and those that develop as a reef post-shifting phase buildup which regionally covers all clastic sediments of the Talang Akar Formation in the North West Java Basin. The development of reef limestones is generally found in high elevations. However, it is now known as an inner area. This formation was formed in the Early Miocene - Middle Miocene (mainly from the association of foraminifera). The formation environment of this formation is in shallow sea conditions, the water is quite clear, sunlight is present (especially from the abundance of foraminifera Spriroclypens Sp).

5. Upper Cibulakan Formation

This formation consists of alternating between shale with sandstones and limestone. Limestone in this unit is generally clastic and limestone reefs that develop locally. This limestone is known as the Mid Main Carbonate (MMC). This formation is deposited in Early Miocene-Late Miocene. This formation is divided into 3 Members, namely:

• Massive

This member is deposited incongruously above the Baturaja Formation. The lithology of this member is the intersection of claystone with sandstones which have a fine-medium grain size. In this massive hydrocarbon content is found, especially at the top. In addition there are fossils of planktonic foraminifera such as Globigerina trilobus, bentonic foraminifera such as Amphistegina (Arpandi and Patmosukismo, 1975).

• Play

The Main Member is deposited in harmony above the Massive Member. The constituent lithology is claystone interspersed with sandstones which have fine-medium grain size (glauconitic). At the beginning of its formation develops limestones and also the sand cage, where in this section the Main Member is divided again called the Mid Main Carbonat (Budiyani et al, 1991).

• Parigi

Pre Parigi members are deposited in harmony above the Main Member. The lithology is the intersection of limestone, dolomite, sandstone and siltstone. This member was formed in the Middle-Miocene Middle Miocene Late and was deposited in the Middle-Neritik Inner Environment (Arpandi & Patmosukismo, 1975), with shallow sea fauna and glauconitic sandstones present.

6. Parigi Formation

This formation is deposited in harmony above the Upper Cibulakan Formation. The constituent lithology is mostly clastic and reef limestones. The sedimentation of these limestones extends throughout the North West Java Basin. The depositional environment of this formation is the shallow-middle sea (Arpandi & Patmosukismo, 1975). The lower limit of the Parigi Formation is characterized by gradual changes from the mixed carbonic facies rocks of the Upper Cibulakan Formation to the carbonate rocks of the Parigi Formation. This formation was deposited in the Late Miocene-Pliocene Period.

7. Cisubuh Formation

This formation is deposited in harmony above the Parigi Formation. The constituent lithology is claystone interspersed with sandstones and side shales. The age of this formation is from Late Miocene to Pliocene - Pleistocene. Formations are deposited in shallow marine environments which are increasingly upward becoming litoral-parallic environments (Arpandi & Patmosukismo, 1975).

D. BASIN SEDIMENTATION

The initial period of sedimentation in the North West Java Basin begins at the Middle Eocene - Early Oligocene (transgression phase) which produces sedimentation of volcanic land - shallow sea from the Jatibarang Formation. At that time volcanism increased. This is related to the interaction between plates in the southern part of Java Island, as a result the unstable regions often experience tectonic activity. Volcanic materials from the east begin to be deposited.

The next period is the transgression phase that takes place during the Late Oligocene - Early Miocene which produces deltaic trangresive transition sediments to shallow seas which are equivalent to the Talang Akar Formation at the beginning of the period. The basin area consists of two different environments, the western part is paralic, while the eastern part is the shallow sea. Furthermore, volcanic activity diminishes so that the areas become somewhat stable, but the Ciputat basin children are still active. Then the seawater inundated the land which took place during the Early Miocene starting from the northwest and continuing to the southeast inundating several heights except the Tangerang high. From these heights the resulting clastic sediments are equivalent to the Talang Akar formation.

At the end of the early Miocene the basin area was relatively stable, and the western part of Pamanukan was a shallow platform, where the carbonate developed well so that it formed the equivalent of the Baturaja formation, while the eastern part was a deeper base. In the Middle Miocene, which is a regression phase, the North West Java Basin is deposited with shallow marine sediments from the Upper Cibulakan formation. The main sediment source from the Upper Cibulakan formation is thought to originate from the north-northwest direction. At the end of the Middle Miocene back away from a stable area, limestone developed well. This good development is due to very weak tectonic activity and the environment in the form of shallow seas. Late Miocene - Pliocene (regression phase) is the phase of formation of the Parigi and Cisubuh Formations. The condition of the basin area undergoes a slight change where the sea conditions diminish into the parallel environment.

In Pleistocene Period - Alluvium is marked for the removal of the main axis of Java. This appointment was also followed by increased volcanic activity and also followed by the formation of the main structure of Java. The removal of the main axis of Java ends suddenly, affecting sea conditions. The coarse grains are deposited incongruously above the Cisubuh Formation.

 

Reference:

• Jopie, dkk, 2001. PSDM ENHANCES REEF INTERPRETATION IN JATILUHUR BLOCK, WEST JAVA, Proceedings of the Indonesian Petroleum Association, 32nd Annual Convention, Vol.1 p.31-43
• Amril, A., Sukowitono., Supriyanto., .1991. Jatibarang Sub Basin – a half Graben Model in the Onshoe of North West Java. IPA Proceedings, 20^th Annual Convention, Jakarta. hal 279-307.
• Arpandi, D., Patmosukismo, S., .1975 The Cibulakan Formation as One of the Most Prospective Stratigraphic Units in the Northwest Java Basinal Area. IPA Proceeding. Vol 4th Annual Convention. Jakarta
• Budiyani,S., Priambodo, D.,Haksana, B.w.,Sugianto,P., .1991. Konsep Eksplorasi Untuk Formasi Parigi di Cekungan Jawa Barat Utara. Makalah IAGI. Vol 20^th, Indonesia. hal 45-67.
• Darman, H. dan Sidi, F.H.,. 2000. An Outline of The Geology of Indonesia. IAGI. Vol 20th. Indonesia
• Gordon, T. L., .1985. Talang Akar coals Ardjuna subbasin oil source. Proceedings of the Fourteenth Annual Convention Indonesian Petroleum Association, v.2. hal. 91-120.
• Hamilton, W., 1979, Tectonics of the Indonesian Region. USGS Professional Paper, 1078.
• Hunt, J.M., .1979. Petroleum Geochemistry and Geology. xxi+617 pp., 221 figs. Oxford: Freeman.
• Noble, Ron A.,. 1997. Petroleum System of Northwest Java Indonesia. Proceeding IPA. 26th Annual Convention. hal: 585 – 600.
• Reminton. C.H., Nasir. H.,. 1986. Potensi Hidrokarbon Pada Batuan Karbonat Miosen Jawa Barat Utara. PIT IAGI XV. Yogyakarta
• Sinclair, S., Gresko, M., Sunia, C.,. 1995. Basin Evolution of the Ardjuna Rift System and its Implications for Hydrocarbon Exploration, Offshore Northwest Java, Indonesia. IPA Proceedings, 24^th .Annual Convention, Jakarta. hal 147-162. 
• Purnomo Edy, dkk, 2001.  PALEOGENE SEDIMENTATION OF THE JATIBARANG SUB-BASIN AND ITS IMPLICATION FOR THE DEEP PLAY PETROLEUM SYSTEM OF THE ONSHORE NORTHWEST JAVA, Proceedings of the Indonesian Petroleum Association, 37th Annual Convention, PG-02
• https://www.slideshare.net/RichardNetherwood/pet-geol-indonesia-55727765?from_action=save

EAST JAVA BASIN

EAST JAVA BASIN

Location of the East Java Basin

GEOMORPHOLOGY

This zone includes the northern coast of Java which stretches from Tuban to the east via Lamongan, Gresik, and almost the entire Madura Island. It is a plain area that is undulated with hilly terraces that run west-east and intersect with alluvial plains. The average width of this zone is 50 km with the highest peaks of 515 m (Ivory) and 491 (Mounds). Carbonic lithology dominates this zone. Accessibility is quite easy and the land character is hard.

The Rembang line consists of folds shaped like the Anticlinorium which extends west - east, from Purwodadi City through Blora, Jatirogo, Tuban to Madura Island. The morphology in the area can be divided into 3 units, namely Low Morphology Unit, corrugated hills and Morphological Unit steep hills, with hilly ridges generally extending West-East direction, so the river flow pattern is generally almost parallel (sub-parallel) and partly patterned dendritic. The main river that passes through the investigation area is S. Lusi, which flows towards the Southwest, through the City of Blora and empties into Bengawan Solo.

STRATIGRAPHY

According to Sutarso and Suyitno (1976), physiographically the research area is included in the Rembang Zone which is part of the North East Java Geosyncline sedimentation basin. This basin was formed in the Late Oligocene, which traversed East - West, almost parallel to Java (Van Bemmelen, 1949). According to Koesoemadinata (1978), the northern part of the East Java basin is more geosyncline with Tertiary sediment thickness may exceed 6000 meters. One thing that is typical of the northern part of the East Java basin is East-West and is seen as a Young Tertiary tectonic phenomenon.

Three stages of orogenesa have been known to influence the deposition of the Cenozoic rock series in Indonesia (Van Bemmelen, 1949). The first occurred between the Late Cretaceous - Middle Eocene interval, the second in the Middle Eocene (Intramiocene Orogeny) and the third occurred in Plio-Pleistocene. Orogenesa that occurs in the Middle Miocene is characterized by important events in the distribution of sediments and the spread of flora and fauna, especially in the western part of Indonesia and also causes a regression phase (sea shrinkage) that occurs in a short time in Java and the Java Sea area. The Middle Miocene orogenesis phase is also characterized by a hiatus in the Cepu region and is characterized by facies changes, from transgression facies to regression facies throughout the Rembang Zone. In addition to the above, the orogenesa phase is characterized by the emergence of several Pre-Tertiary bedrock in the island of North Java (Van Bemmelen, 1949).

A striking difference regarding the lithological properties of sediments in the Kendeng Mandala, Rembang Mandala, and Java Sea Exposure is sediment. Kendeng Mandala is generally filled with turbidite current deposits which always contain pyroclastic rocks with napal interludes and carbonate rocks and are deep sea deposits. In general, these sediments are folded firmly and have a hood to the north, while the Mandala Rembang shows rocks with high levels of sand in addition to increasing carbonate levels and the disappearance of pyroclastic deposits. Mandala Rembang sediments give the impression of shallow sea deposits not far from the coast with a non-uniform depth of the seabed. This is caused by the existence of block faults which result in facies changes and form a high or low area. The area off the coast of the Java sea is generally occupied by exposure deposits which consist almost entirely of carbonate deposits.

The Mandala Rembang according to the Tectonic system can be classified into the retro arc back arc (Dickinson, 1974) which is filled with thick and continuous Kenozoic sediments ranging from Eocene to the Pleistocene. Eocene age deposits can be seen from the wellbore data (Pringgoprawiro, 1983).

Tertiary Litostratigraphy in the northern part of the East Java Basin has been studied by geologists including Trooster (1937), Van Bemmelen (1949), Marks (1957), Koesoemadinata (1969), Kenyon (1977), and Musliki (1989) and many experiencing development in its stratigraphic arrangement. The confusion of the lithostratigraphic unit has been discussed in detail by Pringgoprawiro (1983) where the sediment deposits in the North East Java Basin are included in the Mandala Rembang stratigraphy from the order of the young, namely the Ngimbang Formation, Kujung Formation, Prupuh Formation, Tuban Formation, Tawun Formation, Tawun Formation The Bulu Formation, the Ledok Formation, the Mundu Formation, the Tongue Formation and the youngest sediment are referred to as the Undak Solo deposit. The Ngrayong Formation Tawun member from Pringgoprawiro (1983) was upgraded to the Ngrayong Formation by Pringgoprawiro, 1983. Members of the Selorejo Mundu Formation (Pringgoprawiro, 1983) were upgraded to the Selorejo Formation by Pringgoprawiro (1985) and Djuhaeni and Martodjojo (1990). While the Lidah Formation has three members, namely the Tambakromo Member, Malo Member (commensurate with Dander Members from Pringgoprawiro, 1983) and Turi Members (Djuhaeni, 1995).

Picture of the Mandala Rembang Stratigraphy Column (Harsono Pringgoprawiro, 1983)

Details of the stratigraphy of the North East Java Basin from the Rembang Zone compiled by Harsono Pringgoprawiro (1983) are divided into 15 (fifteen) units namely Pre-Tertiary Rocks, Ngimbang Formation, Kujung Formation, Prupuh Formation, Tuban Formation, Tawun Formation, Ngrayong Formation, Bulu Formation, Wonocolo Formation, Ledok Formation, Mundu Formation, Selorejo Formation, Paciran Formation,

Tongue Formations and Solo Steps. The discussion of each unit from old to young is as follows:

1. Tawun Formation

The Tawun Formation has a aligned position above the Tuban Formation, with the Tawun Formation boundary characterized by soft rock (claystone and marl). The lower part of the Tawun Formation, consists of claystone, sandstone, sandstone and lignite limestone, while at the top (Ngrayong member) consists of sandstones which are rich in mollusks, lignite and upward found quartz sand containing mica and iron oxide. The naming of the Tawun Formation was taken from the village of Tawun, which was first used by Brouwer (1957). The Tawun Formation has a wide spread in West Mandala Rembang, from the type location to the East to Tuban and Rengel, while to the West the rock unit can still be found in South Pati. Deposition environment of the Tawun Formation is a protected shallow exposure, not too far from the beach with a depth of 0 - 50 meters in the tropics. The Tawun Formation is the main oil reservoir in the Rembang Zone. Based on the existing fossil content, the Tawun Formation is estimated to be from the Upper Early Miocene to the Middle Miocene.

2. Ngrayong Formation

The Ngrayong Formation has a position aligned above the Tawun Formation. The Ngrayong Formation is composed of quartz sandstones with claystone silt, silt, lignite and bioclastic limestone. In its sandstones sometimes it contains shells of marine mollusks. Deposition environment The Ngrayong Formation in a shallow area near the coast which is increasingly above its environment becomes a littoral, lagoon to sublittoral edge. The thickness of the Tawun Formation reaches 90 meters. Because it consists of quartz sand, the Tawun Formation is a potential oil reservoir rock in the North East Java basin. Based on the existing fossil content, the Ngrayong Formation is estimated to be Middle Miocene.

3. Bulu Formation

The Bulu Formation is harmoniously above the Ngrayong Formation. The Feather Formation was originally known as the 'Platen Complex' with a stratigraphic position located in harmony above the Tawun Formation and the Ngrayong Formation. The lithological features of the Fur Formation consist of interlacing between limestone and calcarenite, sometimes with claystone inserts. In thin-layered limestone sometimes shows large-scale cross-structure and shows the presence of napal. In the sandstone limestone shows a quartz mineral content of up to 30%, large foraminifera, algae, Bryozoa and echinoids. This formation is deposited in shallow marine environments between 50 - 100 meters. The thickness of this formation reaches 248 meters. The Fur Formation is estimated to be the upper Middle Miocene.

4. Wonocolo Formation

The location of the Wonocolo Formation was not stated by Trooster, 1937, possibly from the village of Wonocolo, 20 km northeast of Cepu. The Wonocolo Formation is located in harmony above the Fur Formation, consisting of sandy marlene with calcarenite inserts and sometimes claystone. In the marl, it often shows a parallel laminate structure. The Wonocolo Formation is deposited in open sea conditions with a depth of between 100 - 500 meters. The thickness of this formation is between 89 meters and 339 meters. The Wonocolo Formation is estimated to be of the late Late Miocene to the late Middle Miocene.

GEOLOGICAL STRUCTURE

In the present (Neogen - Resen), a tectonic pattern that develops in Java and its surroundings, especially the North East Java Basin is a convergent zone, between the Eurasian plate and the Indian plate - Australia (Hamilton, 1979, Katili and Reinemund, 1984, Pulonggono, 1994).

Tectonic evolution in East Java can be followed starting from the Late Cretaceous (85-65 million years ago) to the present (Pulonggono, 1990). In summary, the East Java basin experiences two periods of time which cause the relative direction of the magmatic pathway or its tectonic pattern to change, namely in the Paleogene (Eocene-Oligocene) era, which is oriented towards the Northeast - Southwest (in the direction of the Meratus pattern). This pattern causes the North East Java Basin, which is a back arc basin, experiencing strain tectonic regimes as indicated by Pre-Tertiary bedrock lithology showing North-West-Southwest trending accretion pattern, which is indicated by fault fault orientation in bedrock, horst or faults factoring and graben or fault ladder. And in the Neogen era (Miocene - Pliocene) changed to relatively East - West (in line with the length of Java Island), which is a compression tectonic regime, resulting in fold geological structures, factoring faults and causing elevated North Java basins (Orogonesa Plio - Pleistocene) (Pulonggono, 1994). Especially in the North East Java Basin, data that supports both tectonic patterns can be seen from seismic data and from revealed structural data.

According to Van Bemmelen (1949), the North East Java Basin, namely the Kendeng Zone, the Rembang Zone - Madura, the Java Sea Exposure Zone and the Randublatung Depression Zone. The condition of folding structures in the northern part of the East Java Basin is generally West - East, while the fracture structure is generally in the Northeast - Southwest direction and there are several upward faults in the East - West direction.

 Regional structure of the East Java Basin

The Rembang - Madura mountain zone (Northern Java Hinge Belt) can be divided into 2 parts, namely the North (Northern Rembang Anticlinorium) and the South (Middle Rembang Anticlinorium).

The northern part has experienced lift which is stronger than in the south so that erosion occurs until the Tawun Formation, sometimes even down to the Lower Kujung. In the southern part of this area lies the Banyubang, Mojokerep and Ngrayong structures.

The southern part (Middle Rembang Anticlinorium) is marked by two positive paths that are clearly adjacent to Cepu. On the positive north, there are important oil fields in East Java, namely the fields: Kawengan, Ledok, Nglobo Semanggi, and also include the anticline - the Ngronggah, Banyuasin, Metes, Divinity and Tambakromo anticlines. In the southern positive lane there are anticlinal-anticlinal / Gabus, Trembes, Kluweh, Kedinding-Mundu, Balun, Tobo, Ngasem-Dander, and Ngimbang High structures.

Along the Rembang Zone path forms a folding structure that can be divided into 2 parts, namely:

1. The East, where the general direction of the anticline axis extends from the North West to the Southeast.
2. The western part, where each axis has a West-East direction and in general the anticlines subdue either westward or eastward.

Reference:

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