petroleum geology (fieldtrip) of cepu area

FIELDTRIP REPORT [ ]

CHAPTER I

GENERAL DESCRIPTION

I.1. General Description of fieldtrip area

North East Java basin is classified on back arc basin based on

morphotectonic. This area is relatively stable area form influence of tectonic, so

thick sediments are filled, quiet environment are created, deep marine environments

develop and the most important is occurence of organic material for source rock

qualification. Tectonics activity created some structural trap both which are very

important in petroleum system due to accumulation of hidrocarbon. Generally, East

Java basin is prospect basin based on its petroleum system.

Fig I.1.1 Physiography of North East Java basin (Satyana et al, 2004)

Gambar 2. Profil Cekungan Jawa Timur Utara.

Fig I.1.1.2 Nort East Java basin cross section

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CHAPTER II

REGIONAL GEOLOGY OF CEPU AREA

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 occurrence of fault and fold because

of compressive force yield fractures, faults and weak zones trending southeast-

northwest, southwest-northeast and north-south.

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II.1.2. Randublatung Zone

Randublatung Depression is physiographic depression due to tectonic

activity lies between Kendeng Zone and Rembang Zone, formed on Pleictocene.

This depression has west-east trend. The narrowest part of this area is located on

Cepu Area, then become wide to east direction as far as sedimentation distance of

Madura strait and until now sedimentation in this one is still occur. Randublatung

Depression 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 Quarternary.

II.1.3. Rembang Zone

Rembang zone is high elevated area separated by Randublatung Area and

parallel 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 :

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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,

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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

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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.

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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.

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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.

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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.

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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)

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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.

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Rembang-Madura-Kangean fold and thrust belt

Sunda -

Arjuna normal faults

North Seribu

Fault

fold and faults

200 KMS

reverse

thrust or

no

rmal

fa

ult

s

f o l d s

faults

antithetic strike

- slip faults

en echelon folds and faults

extension fractures

Structural analysis of Java using strain ellipsoid kinematics (Wilcox et al., 1973)

NE

C

CE


CHAPTER III

FIELDS DESCRIPTION

III. 1. Mrapen Stopsite

Mrapen is located in Grbongan district, and about 28 km on the west from

Purwodadi. Mrapen is one region in Randublatung Zone.

Location 1 Eternal Fire

The first stop site is located at Mrapen, Purwodadi and it is a part of

Randublatung depression. This place have alrady well known as a quite famous tourism

object, by its eternal flame, commonly called Mrapen. The flare is quite difficult to be

seen because mostly it’s consists of methane or CH4. This eternal flame is produced by

gas seepage in the subsurface. This gas predicated as biogenic gas, that produced by

biopolymerytation processes. Biogenic gas dominated by methane (CH4). This gas can

reach to the surface because the gas has path and the path is geological structures like

fault. The source rock of this gas is pedicated from theh Tawun Formation that migrated

to the reservoir of the Selorejo Formation.

Actually, methane can be formed when sub surface rocks are decomposited. The

methane also associated to the nature of HC materials, the evidance of existance o this

phenomenon can be prooved by putting bumale materials such as paper on the rocks

where the gasses exhausted. We would considered that the gas in this location is the

same whit the gas in the oil field if we dont know the origin of the gas in this location.

Genetically, gasses are derived from 2 processes.

a. Biogenic gas or biogas

b. Thermogenic gas or thermogas

This kind of gasses can be formed here in consedired as biogenic gas, to know

the genetic processes there should be an isotop analysis so we will know it’s C atoms.

Organics materials is decomposited by microorganism anaerobic materials there 3

elements influenced the processes; decomposition of organis materials at law

temperature, anaerobic microorganism and in rocks that are lack of water.

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Biogenic gas is considered useful and this kind of gas could become an

alternative fuel to subtitute the oil and LNG function someday. VICO is one of iol

company that make a specific study about this gas (geochemistry aspect),because :

1. Geologically, easy to predict.

2. In a large area, can be found in a shallow depth

3. Always found in large ammount.

The gasses formed by transported burried organic material in a basin/ depression

and structurally subside as result of continous seccesion on the deposit after all.

Biopolimeration processed took place yielding biogenic gas that is not associated to

hydrocarbon. If geopolimeration happended (in a deeper part), thermogenic gas that is

associated to HC is formed, the 2 gasses can be found as methane (CH4) come out frpm

rock fractures, the observation of these gasses can be dome by making surface mapping

and stratigraphy.

Fig. III.1.1 Eternal flame of Mrapen area, gas leakage manifestation

Location 2, Sendang Dudo

It is located about 25 metres from location 1 to the south, but still in the area of

thermal flame tourism object. Sendang Dudo is pond that made by spring and have

unique phenomenon. On the pond we can see bubble, and this phenomenon caused by

gas that has same process with the eternal flame, that the energy from subsurface get

Group 5 13


into surface because any some crack as tool for the energy pass way. The spring is

indicate that this location is on the zone.

According to geochemistry research, the gasses are : Clorida 62 ppm, shulphate

400 ppm, CO2 2000 ppm and H2S 1,6 ppm. These gasses are biogenic gas whit complex

CO2 in high temperature, and connected to the seepage of methane. Sendang Dudo

yielding gasses.

There are some opinions of the gas causes :

1. Structural control or there are fractures where the gasses can come up into the

water.

2. Seapage of gasses in the shallow area.

3. In has some depth wiyh deformation of spring.

This last opinion actually is the combination between the former opinions,

means that because of the structural deformation, gas can make a gas seepage through

the fracture that cause by that deformation, and than the seepage was overlied by spring

which is also the result of the deformation processed.

Fig. III.1.2 Bubble gas outgoing form pool in Mrapen area, gas leakage manifestation

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III. 2. Stopsite 2 Bledug Kuwu

Bledug Kuwu observatory site

Location

Bledug Kuwu is in Purwodadi region, approximately 30 km eastside of first

observatory stations (Mrapen and Swimming Sendang Dudo). This Areas is a

phenomenal site because of the gases bursts activities which emerged poking into

surface which bring mud material.

Geomorphology

The geomorphology of the observatory site is dominated by low relief land.

Fig. III.2.1 Mud volcano in Bledug Kuwu

Lithology

The lithology of the observatory site is consist of ejected mud from mud

volcano. Around the observatory site there was alluvial deposit which lay on lidah

formation.

Structural geology

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There was probable fault or fracture in subsurface where this structural geology

being a way for mud, water formation and gasses ejected to the surface.

Potential

This area has some potential as geological tourism site and salt factory. The

outpouring air formation that contain salt material is used by people around there. The

negative potential of this phenomena, if the volume of mud was ejected to the surface is

very huge, this area will be like mud volcani in sidoarjo area.

Bledug Kuwu Phenomena could be a manifestation there was probability of

hidrocarbon accumulation in subsurface. The outburst of bledug kuwu contain gasses

mixed with mud and formation water. Firstly, there was a diapiric shale with high

pressure; and because of plioplistocene tectonic event, it forms fractures. This fracture

make the pressure release and then the mud was ejected to the surface. There are some

condition can form mud volcano,

o Thick layer of uncompacted shale.

Sedimentation from shale was very quickly so material shale is not

compacted perfectly and consequently formed undercompacted shale (nature

plastis).

o High pressure of formation water or fluids from subsurface

Shale that formed in the mud volcano is usually a undercompacted

shale. Order this shale can exit if threre are enough tremendous

pressure. There are wo types pressures regarding shale they are:

Hydrostatic pressure that depends at depths layout shale, increasingly

within layout shale hence bigger depressurised.

Pressure overburden derived from other materials stacked on top of

shale.

o Gas / fluids accumulations.

Existence fluid from water formation excessive causing shale changed

become liquid. Fluid can originated from zone faults represents exit shale

filled by water formation and gas. This causes shale mingled formation ater

and gas.

o Way the accumulation to exit.

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Fractures that formed by regional tectonic through the accumlation diapiric

shale can be a way fot it to the surface.

Fig. III.2.1 Eruption of mud and gas from vents periodically in Bledug Kuwu area

Characteristics:

– High saline.

– Low temperature.

– Classified into biogenic gas.

– Main lithology is shale form Lidah Formation, shallow depth.

– Connate water is bursted from eruption vents.

– Explosions are periodically.

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III. 3. Stasiun Pengamatan Ngampel

This stop site location in district Ngampel sub-province Blora, or precisely at S

06 54 ' 12'' and E 111 26' 28,3''. location of Reconnaissance stays at South edge S.

Broholo flowing from occidental direction eastwards. Morphology as a whole in the

form of hill bank with dip of caster 450 - 700 degrees which more alike like anticline

limb.

Lithology at this district consisted of assorted, for example limestone, sandstone,

shale. observation about lithology only can be done is patch up is caused by time which

unable to be adequate since have already nearing night. limestone at the location having

colour rather white brass, medium fairish until coarse, and age relatively young because

laying in outmost division of the anticline limb. Limestone has dip of layer equal to N

80 E / 48. This limestone conceived of Orbituidal Calcarenite. Anticipated deposition

from this limestone is shallow Sea district because its scale is being medium until

coarse.

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Figure II.3.3.1 Anticline limb contained by limestone dominantly

Under from limestone there is sandstone lithology. this sandstone has is medium

size and coarse dominance, interbedded structures, lapped over by clastic material, rose

colored, what is estimable effect of oxidation. This litollogi then at intervals with set of

black Clay lithology, fairish of Clay, contains mineral gypsum having colour white, has

laminated structures. lithology like that way indicates both the lithologies as reservoir

and seal rock exploration field of oil in district Cepu because seen from the grain size is

medium until coarse ( for sandstone reservoir) and then Clay layer is top of it. Both of

lithology has dip N78E/48.

Both types of this lithology is member from upper Tawun Formation or more

knowledgeable with Ngrayong Formation. Interpretation of its deposition area is namely

sea area with depth of skin-deep that is then experiences derivation of sea water face so

forms lagoon. In lagoon district this is formed Clay containing mineral gypsum while at

condition of shallow Sea is formed sandstone. The case happened repeatedly so is

formed restating between both the lithologies. The Black Clay lithology actually has a

real abundance organic material content, but because level of the maturity which has not

fullfilled (immature) causes the unit has not can yield hydrocarbon.

Figure III.3.3.2 Sandstone interval with blackclay member of Ngrayong Formation

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More and more towards lithology fold axis more and more old in age. oldest

lithology met is shale is having colour black, has lamination sewer structures, very

smooth fairish, has layer dip N75E/50. Set of this lithology is member from Ngimbang

formation which age Lower Oligocene. According to organic material content data at

this finite lithology of 0,5% and level of its(the maturity which have been enough

causing makes this lithology as source rock from exploration field of district gas and oil

Cepu.

As a whole, history of the forming of this district having beginning of from the

forming of Ngimbang (shale) at lacustrine area or delta environment because rich of

organic material. Then is formed lithology claystone which is member of Tawun

formation. Initial condition of Tawun formation is a deep sea environment in at the age

of Middle Miocene. Because happened derivation process of sea water face that is

continuously causes condition of its deposition turns into shallow Sea and formed

sandstone and lack clay at lagoon area. Event of second deposition restating of this

lithology clearly codified at outcrop in stop site location. The deposition also

terminating set of Tawun formation. Topmost division of Tawun formation called

Ngrayong formation. Then is precipitated after of limestone orbituidal as result of

increase of sea water face returns. Limestone is member from formation Bulu. Then,

happened tectonic event of north direction of causing south formed Northern East Java

anticlinorium having direction West-East. This anticlinorium which becomes trap for

hydrocarbon in exploration district of oil in Cepu.

III. 4. PPPT MIGAS Cepu 4th Stopsite

Pusdiklat Cepu oil is the government agency under the Department of Energy

and Mineral Resources. These training centers have the task of carrying out oil and gas

education and training sector on the basis of Oil and Natural Gas Minister of Energy

and Mineral Resources. Pusdiklat migas has the program, which are;

Improved operations of oil field in order to enrichment oil reserve

Change and repaired the equipment in oil and gas and also oil manufacture in

order to complete to safety of work.

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To improve the function of oil field training, pusdiklat migas has some facilities,

they are laboratory of drilling, laboratory of exploitation and laboratory of production.

But in this time, we only visited to the laboratory drilling.

Laboratory of drilling are consist of drilling’s rig, rig “well service”, “tool

house”, laboratory of drilling’s simulation and drilling’s equipment. In this laboratory,

we did three activities, such as observation of the drilling process operation in the

laboratory of drilling’s simulation, observation of the installation of pipe in the drilling

process in the drilling’s rig and drilling’s equipment.

First location is laboratory of drilling’s simulation. In this room, we observated

the operator that was simulating the operation of technical equipment that control the

mechanical equipment in the drilling process. The technical equipment that was

simulated in this room is a important component in the process of exploration,

especially the drilling process. Drilling processes were simulated in the second location.

This location is drilling’s rig, the location is in outside. In this location, there is a small

rig used for the simulation of drilling by the trainees. Some trainees seem to do the

installation of pipes which in turn is inserted into the boreholes. There are 5 systems in

the drilling processess; power system, rotaring system, circulating system, hoisting

system, and BOP system. Each system has their own function.

a. Power system

This system has function as a source of power to move all the components

for drilling processess.

b. Rotaring system

This system has a function to rota ring the whole drilling system.

c. Circulating system

This system has a function to inject the drilling mud in order to avoid blow-

outs. In this processess we have to keep the mud pressure is litlle bit higher

than formation pressure.

d. Hoisting system

This system has a function to raise and lower the lifting mechanism.

e. BOP system

This system has a function to avoid the blow out.

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Figure III.3.4.1 Situation in the drillings rig at Pusdiklat migas Cepu.

The third location that we were visited is laboratory of drilling equipments. In this location we learn about the function all the equipments of each drlling system component.

a. Hoisting system

1. Elevator : to carried forward the pipe

2. Slip : to prevent the casing, drill collar, and drill pipe so as not to fall.

Figure III.3.4.1 Hoisting systemsb. Bit

1. Cone bit/ Roller bit : to penetrate a weak-hard formation

2. Diamond bit : to penetrate a medium- very hard formation

3. Wing bit : to penetrate a weak formation

4. Core bit : to take the core sample

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Figure III.3.4.3 various bit; [a] cone/roller bit, [b] diamond bit, [c] wing and core bit.c. BOP system

1. Annular : to close the existing borehole string

2. Blind rem : to close borehole without string

3. Cementing equipment : include centraliser, drill hole cleaner, cementing

head.

4. Pump

5. Indicator sensor

Figure III.3.4.4 BOP systemd. Fishing Tools

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a. b.

c.


To take the pipe that fall or break in the drill hole.

Figure III.3.4.5 Fishing toolse. Miling Tool

To smooth the rough borehole.

III.5 Kawengan oilfield 5th Stopsite

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.

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Figure III.3.5.1 Oil Derrick, primary recovery by oil pumpLedok field

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.

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CHAPTER 4

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

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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

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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.

Fig. IV.5.1 Petroleum Play of Cepu Area

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CHAPTER V

CONCLUSION

1. Gas in Mrapen area is classified into biogenic gas and is a product of Lidah

formation.

2. Biogenic are characterized by shallow place of occurence, shallow overburden,

high concentration of CO2.

3. Eternal flame of Mrapen area is manifestation of working petroleum system of

Rembang area in gas form.

4. Bledug Kuwu phenomenon is mud volcano manifestation caused by

everpressured subsurface shale resulting enough pressure to explode mud and

gases.

5. Characteristics of Bledug Kuwu are:

Mud volcano manivestation

Lithology consists of mainly overpressured shale

High salinity, due to connate water from Lidah formation

Gas product is classsified into biogenic gas.

Water and mud explosion occur periodically, depend on pressure

accumulated below surface.

6. Ngampel area is lithology contact or boundary between Ngrayong Unit from

Tawun formation and Bulu Formation in which contact is conformity.

7. Ngrayong member is main reservoir in Northeast Java basin, especially on

quartz sandstone reservoir.

8. Quartz sandstone form Ngrayong formation are loose, have high porosity and

permeability.

9. Bulu limestone didn’t develop well due to its tight character so hidrocarbon will

tendence to be accumulated on Ngrayong sandstone.

10. Carbonaceous shale of Ngrayong formation has good TOC, but low thermal

maturity, so its potential as source rock is not enough good.

11. Most oil form Ngrayong formation is refined and extracted in Pusdiklat Migas

Refinery.

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12. Drilling simulation and tool equipment are available in Pusdiklat Migas Training

Centre Office.

13. Ledok Oil field produce mainly oil from Ngrayong reservoir.

14. At this time, watercut value on Ledok field is about 96%.

15. Problems in ectracting oil form Ngrayong rerservoi is sandstone character of

Ngrayong which is loose, so water flooding or other enhanced oil recoovery will

make grains of loose sandstone mix with oil so pipes will be plugged.

16. Main source rocks in Cepu oilfield are Kujung, Tuban, Ngimbang and Ngrayong

unit (Tawun formation).

17. Main reservoirs in Cepu oilfield are Ngrayong sandstone, Bulu limestone and

Kujung limestone.

18. Main seal in Cepu oilfield are Tuban and Kujung formation.

19. Main trap in Cepu oilfield is struut\ral trap in anticline form.

20. The other trap in Cepu area is fault and stratiraphic trap (reef etc).

21. Mifration timing in cepu area was begun on Middle Miocene and late Pliocene

until late Pleistocene.

22. Petroleum system in Cepu is working properly.

23. Northeast Java basin has good hidrocarbon prospesct both in oil and gas.

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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

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PERHAPI, The 30th HAGI, The 30th IAGI and The 30th 14th PERHAPI Annual

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Bumi, Dept. Of Geological Engineering, UGM, Yogyakarta.

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petroleum geology (fieldtrip) of cepu area

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