Northern part of Java area (East Java in this case) is a stable back arc basin. This condition resulted thick sedimentary deposit which had good hydrocarbon potential. Tectonic development occured in this area, marked by uplifting, faulting and folding as in present condition.
Fig.1 Profile of Nothern part of East Java Basin
II.1. Regional Geomorphology of Cepu Area
II.1.1. Kendeng Zone
Kendeng Zone is moutain range located in the north side of Ngawi sub-zone, oriented to west-east direction. This mountain range consist of deep marine sediment which had been faulted and folded intensively forming anticlinorium. North margin is bordered by Randublatung Depresion, then south margin is bordered by volcanic mountain (Solo Zone). This mountain lies from Ungaran Mountain on the west side to Ngawi and Mojokerto Area. Rest of this mountain is still can be tracked until subsurface on Madura strait.
Fig. II.1 Phisiography of Central and East Java (Bemmelen)
Kendeng Mountain on west part’s wide is about 40 km, then become narrow to the east with length approximately 250 (de Genevreye & Samuel, 1972). Characteristic of Kendeng Zone is line of hills with low elevation, wavy hill morphology elevated on 50-200 m. This west-east lineament reflects some folding and faulting trending west-east. Folding and thrusting intensity in this mountain have big intensity in west part and become weak in east part. Thrust fault make the unit boundary to be structural boundary. The occurence of fault and fold because of compressive force yield fractures, faults and weak zones trending southeast-northwest, southwest-northeast and north-south.
II.1.2. Randublatung Zone
Randublatung Depression is physiographic depresion due to tectonic activity lies between Kendeng Zone and Rembang Zone, formed on Pleictocene. This depresion has west-east trend. The narrowest part of this area is llocated on Cepu Area, then become wide to east direction as far as sedimentation distance of Madure strait and until now sedimentation in this one is still occur. Randublatung Depresion generally can be classified into synclinal valley morphology unit consist of Cepu Area and Bojonegara. Bengawan Solo river flow in this area, form meandering system. Most of Randublatung Area is filled by fine grain clastic sediment such as clay, silt from Lidah Formation aged Quartenary.
II.1.3. Rembang Zone
Rembang zone is high elevated area separated by Randublatung Area and paralel to Kendeng Zone. This area consist of anticlinorium trending west-east due to tectonuc activity happen on late Tertiary. This foleded mountain has enough high elevation, average is less than 500 m. Some of those anticlines are young anticlines mountain and have not been eroded intensively. Those exogenic activitiy make the mountain look like back of hill. Structurally, Rembang Zone is fault zone located between carbonate Shelf on north of Java (Java Sea) and deep basin located on south of Rembang (Kendeng Basin). Litology consists of mix silisiclastic which is mixing of shallow carbonate and clastic from continent, clay and deep sea marl. Oil field had been found in this area and operated since early 20th century.
II.2. Regional Stratigraphy of Cepu Area
II.2.1. Mandala Kendeng Stratigraphy
Sedimentary rock which fill Kendeng Basin consist of turbidite clastic, carbonate, and deep marine volcaniclastic, especially on lower part of deposits. On vertical sucession, more shallow deposit will develop to the top anf finally non-marine deposits will be formed on the top.
Stratigraphy of Kendeng Zone can be divided into 9 Formation from oldest to youngest. The formations are :
Fig. II.2.1.1 Regional stratigraphy of Kendeng Zone (Harsono, 1983)
a. Pelang Formation
Pelang Formation is the oldest formation located in Mandala Kendeng Area. Lithology of this formation consists of clayey marl with bioclastic calcarenite lenses which contain many large foraminifera. Lithology on top and bottom of this formation is unknown due tectonic activity which deform the formation so the top and bottom of this formation is difficult to be defined. The outcrop located on up-thrust area and directly verge with Kerek Formation. Kerek Formation is youger than Plang one, and aged N4-N9 (Middle Miocene).
b. Kerek Formation
This Formation is deposited on the unconformity on Pelang Formation. Lithology of this formation sonsists of interbedded clayey marl, sandy marl, calcareous tuff, and tuffaceous sandstone which show flysch characteristic.This formation can be divided into 3 members from the oldest to the youngest :
· Banyuurip Member
This member consists of interbedded clayey marl, marl, clay, tuffaceous calcareous sandstone, and tuffaceous sandstone. Thick of deposit reach 270 m and this deposit is deposited on N10-N15 (Middle Miocene).
· Sentul Member
This member consists of lithology which has same characteristic with Banyuurip Member. The difference is pointed to the occurence of bed which contain thicker tuffaceous material . Total thickness of this deposit is about 500 meter and aged N16 (Early Upper Miocene).
· Kerek Limestone Member
This emmber consists oof interbeded of tufaceous limestone and clay and tuff bedding . Thickness of this formation reach 150 meterand aged N17 (Middle Late Miocene).
c. Kalibeng Formation
This Formation is unconformable on Kerek Formation and divided into 2 parts : Lower Kalibeng Formation and Upper Kalibeng Formation. Lower Kalibeng Formation consist of massice marl which develop into depp marine volcanic deposit, characterized by turbidity structure. These facies are named Atasangin Member with thickness is about ± 600 meter. Based on bentonic foraminifera association, this formation is formed on marine environment on depth 1000 meter. The age of this formatin is N17-N21 (Upper Miocene-Pliocene).
d. Banyak Formation
This Formation is intefingering with Kalibeng Formation. Lithology of this formation consist of tufffaceous sandstone, thick gravelly sandstone, calcareous sandstone with clay and marl.
e. Klitik Formation
This formation is conformable with Kalibeng Formation. Lithology forming this formation are bioclastic limestone with marl interbedded.
f. Sonde Formation
Secong part of Kalibeng Formation is also called by Sonde Formation or Upper Kalibeng. This formation can be divided into a member called Klitik Member which consist of well bedded marly calcarenite limestone. On the top of deposit, facies are breccia with fragmen carbonate gravelly limestone and carbonate cements, sandy marl deposit which develop into clayey marl. Thickness of this formation is 27-589 meter and aged N9-N21 (Pliocene). This formation is deposited in shallow marine environment, on the shelf margin near shore.
g. Danar Formation
This formation is unconformable with Sonde Formation. There is facies changing to the east from Danar Formation to Pucangan and Lidah one eastward. This formation consists of lahar deposit and vlack clay with interbedding of diatome. The thickness is about 61-480 meter, aged N21 (Late Pliocene). Lithology of this formation generally is formed on shallow marine environment and develop into non marine (fresh water environment).
h. Kabuh Formation
This formation conformable on Danar one. Lithology of this formation are non-volvanic sandstone and conglomerate interbeding. Thickness of this formation reach 100 meter, deposited on fluvial system environment. This fluvial deposit are charecterized by occurence of crossbedding structure, lake deposit, mollusca fresh water fossils, and Pithecanthropus skull fossils.
i. Notopuro Formation
This Formation is deposited conformable on Kabuh Formation. Lithology of this formation consists of interbeddiing tuff and tuffaceous sandstone, laharic breccia and volcanic conglomerate. Interbedding of volcanic breccia with andesite and pumice fragments as lenses are also founded. This pumice characterize deposits of Notopuro Formation. Depositional Environment of this formation is non-marine, aged Late Pleistocene with total thickness more than 240 meter.
II.2.2. Mandala Rembang Stratigraphy
Rembang stratigraphy are consist of some formation below from the oldest to the youngest
Fig. II.2.2.1 Stratigraphic coloumn of East Java basin (Mudjiono and Pireno, 2001)
II.2.2.1. Ngimbang Formation
This formation consists of shale with silt interbedding, fine sandstone, limestone and coal. Depositional environment of this formation is on delta system, lacustrine and shallow marine on Eocene until Lower Oligocene.
II.2.2.2. Kujung Formation
This formation consists of shale with clay interbedding. On some area, there are clastic and reef limestone found spotted. Depositional Environment of this formation is deep marine until shallow marine on Late Oligocene until Early Miocene.
II.2.2.3. Tuban Formation
This formation consists of claystone beds with limestone interbedding. There is facies change southward in which lithology will grade into shale and claystone facies (Soejono, 1981 on Handbook of Petroleum Geology 2006). Depositional Environment is deep-middle neritic.
II.2.2.4. Tawun Formation
This formation consists of silty shale with limestone interbedding. On top deposit, sandstone facies with little clay inside dominate. Locally, there is spotted limestone found. Top of Tawun Formation is also called as Ngrayong Member. Depositional Environment of this formation is open marine until shallow marine in the top formation. This formation is formed on Middle Miocene (N9-N13) (Rahardjo & Wiyono, 1993, on Handbook of Petroleum Geology, 2006).
II.2.2.5. Ngrayong Member
This member is also called as “Upper Orbitoiden-Kalak” by Trooster (1937). Van Bemmelen (1949) named this emmber as “Upper Rembang beds”. “Ngrayong Sandstone” name has been introduced by Brouwer (1957). He proposed local type of this sandstone based on quartz sandstone found in Ngrayong Village, Jatirogo, in which that member are composed of sandstone mainly with intercalation of coal and sandy clay.
Harsono (1983), describe Ngrayong as member of Tawun Formation, consists of orbitoid limestone and shale in lower part. In the upper part, there are sandstone with limestone intercalation and lignite. This unit is aged on Middle Miocene, N9-N12. Depositional environment of this unti is fluvial or submarine based on outcrop found in northside of Village (Jatirogo, Tawun). On south side of village, this depositional environment of formation change into marine environment. Marine deposit of this formation on Ngampel Area show shallowing upward pattern from shoreface to beach. This unit also show hiatus on mouth of Java Sea. Ngrayong Unit is main reservoir of Cepu Oilfield, but there is shale occurence in the south and east of this field. Total thisckness of this unit is varied, average is more than 300 m.
II.2.2.6. Bulu Formation
This formation consists of bedded clastic limestone and sandy limestone. Locally, chalky limestone and marl will be found spotted. Crossbedding limestone is often to found. Depositional environment of this formation is open marine on Middle Miocene (N13-N14).
II.2.2.7. Wonocolo Formation
This formation consist of yellow-brown marl, contain glauconite. There are interbedding of calcarenite and claystone. Depositional environment of this formation according to Purwati (1987, on Handbook of Petroleum Geology, 2006) is deep neritic until middle bathyal and formed on Middle Miocene – Upper Miocene (N14-N16).
II.2.2.8. Ledok Formation
This formation consist of green and red sandstone, glauconitic type with intercalation of calcarenite and claystone. Crossbedding structure will be found intensively in sandstone. This formation is deposited due to regresion phase from Wonocolo Formation on Upper Miocene (N17).
II.2.2.9. Selorejo Formation
This formation formed by Selorejo Beds acording Trooster, 1937. This formation had been classified into member of Lidah Formation by Udin Adinegoro (1972) and Koesoemadinata (1978). This research had been done since Harsono (1983) didn’t continue his observation about the occurence of unconformity between Lidah Formation and Mundu Formation. He classified this member into Mundu Formation. Local type of this member located on Selorejo Village, near Cepu. Lithology character is shown by weaker and harder bed boundary, remain some glauconites. This unit depositional environment is deep marine, based on foraminirefa analysis.
II.2.2.10. Lidah Formation
This formation consists of blue limestone, bedded marl and coquina limestone lenses. There are sandstone intercalation especially quartz sandstone contain glauconite and seaa mollusca. In are where this formation develop into reef limestone, the unit is called Dander Member.
II.3. Regional Geology Structure of Cepu Area
East Java Basin generally are formed by main primary structure from south to north. Kendeng Zone and Madura strait lie in west-east direction, especially are characterized by fold, fault and thrust one. Southern part of Rembang Zone and Randublatung are characterized by disharmonic fold type. Beside that, this zone is characterized by dome structure associated with fault structure as like as anticline in Ngimbang. Northen part of Rembang Zone and Madura are characterized by uplifted and eroded anticlinorium structure on Plio-Pleistocene. This structure is associated with strike slip fault system trending northeast-southwest. The fault system continue until South Kalimantan Area.
There are 2 primary geological structure in East Java Basin:
a. Northeast-southwest structural pattern, also called by Meratus type.
b. West-east structural pattern, also called by Java type.
Fig. II.3.1 Regional structural pattern in Indonesia (Satyana, 2005)
Kendeng Zone on Early Miocene is active tectonized area. In tectonic regional framework, Kendeng Zone is included on the part of East Java basin. This basin has occured extensional tectonic regime on Paleogene and result many normal faults in high-low morphology form. On Neogene, East Java basin was compressed by compressional regime and caused reactivation on normal faults. and yield many thrust faults. Those faults cut Neogene sediment and resulted thrust faults. These thrust faults can be tracked in west part of Kendeng area. On east part of Kendeng area, there are many anticlines with plunged axis to the east. Fold and thrust intensity will be decreased to the east, then dissapeared on the south. The phenomenon explain that there are only anticlines in north part which can be tracked until Surabaya. These fold pathway verge directly with volcanic arc and only separated by Ngawi Alluvium.
PETROLEUM SYSTEM
IV.1. Source rock
Source rock is rock which have potential to create and expel hidrocarbon in enough quantity.
There are 4 formation which act as source rock, they are : Ngimbang Formation, Kujung Formation, Ngrayong formation and Tuban Formation. Generally, they have shale which can create kerogene. Beased on maturity, all of them had been mature thermally. Three formation above had been being mature on Paleogene, except Tuban Formation which formed on Miocene age.
Ngimbang Formation has good source rock, prooved by occurence of good TOC (1%-4%) on shale and carbonaceous limestone also 40% for coal). Source rock on this formation is classified into effective source rock because this source has supply economic hydrocarbon (95% oil and gas) in NE Java basin.
Kujung Formation has classified into possible source rock due to no identification by geochemical method to some factor (TOC, Ro, and kerogen type).
Tuban Formation has been classified into effective source rock due to high content of TOC (0,53-2,3 %). Lithology consists of clastic limestone as intercalation. Kerogen type is classified into type III (gas prone), then on the lower part of this formation, kerogen is belong to type II and II (oil and gas prone).
Ngrayong formation (lower) has become good source rock with local TOC greater than 1 %. Organic matter is prdominantly form terestrial. Maturity of this source rock is belong to submature, due to lack of thermal maturity because of burial time and intensity in Central deep.
IV.2. Reservoir rock
Reservoir rock is rock that can accumulate hidrocarbon. On Nort East Java absin, there are 3 formation act as reservoir rock, they are Kujung formation, Ngrayong formation and Mundu Formation.
Kujung formation is divided into 3 part, Kujung I, II and II. Kujung I consist of shelfal equivalent to deep water Prupuh limestone of the East Java / Madura basin. It has the best reservoir characteristic of North East Java basin. Kujung II consist of limestone form one of main target in central deep and provide reservoir. Limestone of high energy and reefal facies oversteep basement on the flanks but are fine grained in the basin centre. Primary reservoir quality is fair, but the performance can be enhanced by some technique like fracturing to are near reactiveted faults zone. Kujung III consist of mixed shlefal clastics and carbonates. Sand are medium to soarse and locally tuffaceous and are interbedded with muds, micritic limestone and lignite. This formation can be enhanceh by fracturing technique.
Ngrayong formation consists of sandstone and carbonate and reservoir target especially is quartzsandtone. This sandstone is positioned on 600-700 m below sea level. Average porosity is 18% and almost 70% of total oil production in North East Java basin is produced in this field. Sand in this area is uncompacted and has excellent reservoir quality. However, reservoir is low, production drawdown will be limited and reserves will depend largely on abandonement pressure. This sands are present in the central deep, but the unit is carbonate dominated.
Mundu formation that has Selorejo limestone act as reservoir of gas. The depth is ranged about 300 m. Porosity is ranged between 28-42% while permeability is ranged 55-903 mD with thickness is about 0-50 m.
IV.3. Seal
Seal rock is rock which act as seal, usually impermeable and has fine grain like marl, shale or clay so hidrocarbon accumulated under this seal cannot migrate or leak to the other place. Primary seal in North East Java basin is thick shale facies of Tuban Formation. Tuban shale provides primary top seal to the underlying Kujung and Ngrayong reservoir. Shale of the formation generally drapes over the Kujung formation (reef) and effectively capped any trapped hydrocarbon below it.
IV.4. Trap
Trap is geological feature which can accumulate hidrocarbon after they have migrated. Generally, traps are divided into 2 varian, sratigraphic and structural trap. Most trap in North East Java basin is structural trap, mainly is anticline with simple variation. The other type are faults and stratigraphic traps. Faults trap will give some risk, due to proper timing of forming and occurence of seal bed. Anticline trap formed on Plio-Pleistocene tectonic phase compression thrust-folding. Anticlines which have northwest axis direction have become the most effective hidrocarbon trap. Stratigraphy trap on this area is oriented to onlap forming from shale related to reefal limestone. On some other field, there are some stratigraphic trap like patch reef complexes with some structural control which probably initiated reefal growth. Generally, traps in this basin are cut by thrust fault rending northeast-southwest and this faults act as oil-water boundary.
IV.5. Proper Timing of Migration
Proper time of migration is time in which hidrocarbon migrate from source rock into reservoir rock so the hidrocarbon will be accumulated into right trap and no leaked occur. Based on tectonic history, there are 2 phase of tectonic period. Firts is Middle Miocene tectonic phase (after deposition of Ngrayong formation), second is Plio-Pleistocene. Source rock from Ngimbang formation had been mature on late Miocene. Based on this fact, first migration occured on this period. Them, second migration occured after first phase Plio-Pleistocene tectonic. On that time, oil from Kujung formation has been mature. Migrasi hidrokarbon atau perpindahan hidrokarbon pada Cekungan Jawa Timur Utara dapat terjadi secara lateral maupun secara vertikal. Generally, lateral migration in this basin will oriented to north direction because of some thickening of sediment northward, same with sedimentation direction.
OILFIELD
More than 25 oil field have been discovered and development in Cepu area until Surabaya. But most of them have been leaved. Now only 5 oil field (Kawengan, Ledok, Nglobo, Semanggi , Wonocolo) and gas field (Balun) that have been produced by Pertamina (Oil Company of Indonesia Government). Kawengan oil field is the biggest oil field in North East Java Basin. Cumulative production in this field can achieve 150 million cubic meters at 1990. Upper Tawon formation, Ngrayong sandstone, and bottom Wonocolo formation are layers that produced hydrocarbon. Now, oil production use pump system because the formation pressure decrease. Pertamina doesn’t use EOR method (Enhanced Oil Recovery). Oil in this area is collected and dispersed by water.
Figure III.3.5.1 Oil Derrick, primary recovery by oil pump
This observation site still under the head of Pertamina operational at Kawengan oil field. In this location, oil intake is used by modern method (land shore). There are 2 type of tool that is used to oil intake within the earth.
The first tool is mechanic type. This type is old tool, so the depth obtained isn’t high and the result of oil intake isn’t optimal. The second tool is hydraulic machine type. The technology of this tool newer than the first tool, so the depth obtained is high. This tool capable of reaching depths of 700 meters. This tool also able to display data recording (result of oil intake) digitally.
REFERENCES
Anonym, 2006, Indonesia Basin Summaries (IBS), The Gateway to Petroleum Investment in Indonesia, Inamata Series, Inonesia Metadata Base.
Assistants of Petroleum Geology, 2010, Panduan Field Trip Geologi Minyak dan Gas Bumi daerah Purwodadi, Blora, and Cepu Jawa Tengah, Dept. Of Geological Engineering, UGM, Yogyakarta.
IOG Chronicle magazine ( Indonesian Oil and Gas Information). 2001. Cepu : Will We Ever Know the Truth? (page 34 – 36). Jakarta.
IPA Newsletter. 2002. 28th Annual IPA Convention & Exhibition. Jakarta.
Koesoemadinata, R. P., 1980, Geologi Minyak dan Gas Bumi, ITB, Bandung
Koesoemadinata, RP, 1980, “Geologi Minyak dan Gas Bumi” , Jilid 1 , Penerbit ITB, Bandung
Pulunggono, A. dan Martodjojo, S., 1994, Perubahan Tektonik Paleogene – Neogene
Merupakan Peristiwa Tektonik Terpenting di Jawa, Proceeding Geologi dan
Geotektonik Pulau Jawa, Percetakan NAFIRI, Yogyakarta
Satyana, A.H., Erwanto, E., dan Prasetyadi, C., 2004, Rembang-Madura-Kangean-Sakala (RMKS) Fault Zona, East Java Basin: The Origin and Nature of Geologic Border, Proceeding Indonesian Association of Geologist, 33rd Annual Conference.
Styana, A.H., 2005, Structural Indentation of Central Java : Regional Wrench Segmentation, Proceeding Joint Convention Surabaya 2005-HAGI-IAGI-PERHAPI, The 30th HAGI, The 30th IAGI and The 30th 14th PERHAPI Annual Conference and Exhibition, Surabaya.
Setyowiyoto, J. & Assistants, 2002, Petunjuk Praktikum Geologi Minyak dan Gas Bumi, Dept. Of Geological Engineering, UGM, Yogyakarta.
