-
IPA 89-11.10
PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Eighteenth Annual
Convention, October 1989
STRUCTURAL DEVELOPMENT OF HYDROCARBON TRAPS IN THE CEPU OIL
FIELD NORTHEAST JAVA, INDONESIA
N. Soeparyono * P.G. Lennox * *
ABSTRACT
Reinterpretation of 18 local and 7 regional seismic lines in
northeast Java, numerous exploration wells and their integration
with newly me'asured stratigraphic sec- tions has enabled a new
structural model to be developed for the Cepu oil fields. The
generally shallow water hey-clastic sequence developed in a rifting
back-arc basin with many northeast-southwest oriented basement
faults.
Deformation in the early Middle Miocene caused reactivation of
the basement faults in the Nglobo- emanggi area with wrenching and
the initial develop- ment of flower structures. This deformation
caused areally restricted erosion of the main reservoir rocks in
this area. Later Pliocene deformation accelerated the development
of the flower structures in the Nglobo- Semanggi area which were
reflected at the surface as a series of en echelon,
hydrocarbon-bearing anticlines. The Tambakromo-Kawengan area
underwent minor north over south thrusting along east-west oriented
lis- tric reverse faults with detachment at shallow depths and the
development of hydrocarbon-bearing folds exist in the subsurface
north of the Tambakromo-Kawengan structure. Such folds would be
related to imbricate blind thrusts parallel to the Tambakromo-
Kawengan thrust.
The wrench structures and detached compressional structures
forming hydrocarbon-bearing folds in the Cepu Oil Fields are
probably a result of the transference of stresses due to oblique
subduction at the Java Trench during the Neogene to
Pleistocene.
INTRODUCTION
Reinterpretation of twenty-five seismic lines, Bou- guer anomaly
gravity maps and surface mapping have enabled new tectonic models
to be proposed LO explain the formation of the Cepu Oil Fields.
*
** Sesksi Diklat Eksplorasi. PPT-Migas, JI. Sarogo I , Cepu,
Central Java, Indonesia Department Of Applied Geology, University
Of New South Wales, PO Box I . Kensington. Australia, 2033
The Neogene-Pliocene sequence was deposited in a back-arc basin
oriented northeast-southwest. This basin was deformed twice with
the development of a regional unconformity and two areally
restricted unconformities (Sabardi, 1988), open folding and fault
reactivation. This reactivation caused the cover sequence to
develop en echelon folds. flower structures and detachments within
incompetent units within the sequence (Soeparyono 1988).
Previous models for the tectonic development of nor- theast Java
have been based on the Moody and Hill( 1956) wrench faulting model
(Situmorang el af., 1976), the intrusion of diapiric shale
(Soetarso and Suyitno, 1976) and the effects of detached
compressional structures (Lowell. 1979). This paper details ihe
evidence for some oil fields being formed in en echelon folds over
flower structures reflecting wrench movement on palaeofaults and
other oil fields being developed in asymmetric faul- ted-folds n i
t h listric faults soling in detachment surfaces at depth
reflecting compression. This compression may represent transmission
of the stresses generated in the subduction zone (Froidevaux et
al., 1988) once the pre- sent plate disposition had become
established in the Ter- tiary (Katili and Reinemund, 1984).
GEOLOGY
The Cepu Oil Fields are located in northeast Java and are being
exploited by the Indonesian government through PT hligas and more
recently Pertamina (Fig. 1). The twenty-setfen oil fields were
discovered from 1894 on\\.ards by Bataafsche Petroleum Maatschappij
(BMP) oil company and Nederlandsctie Koloniale Petroleum
Maatschappij (NKPM). Only five fields are still in pro- duction
(Soetarltri ef a/., 1973). Over 150 MMbbls of oil hnve been
extracted from the quartz sandstone and cal- carenite reservoir
rocks over the two stratigraphic inter- vals within the Upper
Miocene and Pliocene epochs.
Remapping of some structures plus five measured sec- tions have
enabled refinement of the stratigraphic column constructed by
Samuel & Gultom (1984) as
IPA, 2006 - 18th Annual Convention Proceedings, 1989
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140
shown in Fig. 2. This study has shown that the Early to Middle
Miocene regional unconformity at the base of the Ngrayong Member is
infact areally restricted and loca- ted at the top of this member.
The Pliocene unconfor- mity between the Selorejo Member and Lidah
Forma- tion has been relocated above the Mundu Mombar (Sa- bardi,
1988).
The productive Mio-Pliocene formations are located above
pre-Tertiary pelagic sediments, melange, serpen- tinite, acid
volcanics, schists and phyllites which do not outcrop in this area.
The Ngimbang Formation which does not outcrop consists of basal
coarse sandstone, shale and minor coal seams which changes
upsection into predominantly micrograined limestone with minor mads
and sandstone (Fig. 2). This formation represents deposition in a
fluviatile to shallow marine environment during transgression upon
a pre-Tertiary basement. The Kujung Formation consists of clay,
shale or marl intercalated with sandy calcarenites representing
deposi- tion in a deep water environment. The Ngimbang and Kujung
Formations thicken to the east reflecting the presence of a deeper
basin in this area at this time (see Fig. 4). Middle Miocene
Orogeny caused development of a basement high near Tuban with
resultant weathering of the Tuban Formation and some parts of t h e
K u j u n s Formation.
Stratigraphy
A single well, surface exposures and seismic traverses have
enabled development of models of the lateral and vertical
configuration of the three economic formations in the Cepu Oil
Fields. Proposals to rehabilitate the Cepu Oil Fields in the mid
1970s resulted in the shooting of 18 new seismic lines (Cu 2-20) of
181 km total length (Fig. 3). These lines supplement a number of
regional seismic sections which can be tied to suitable wells both
east and west of the Cepu Oil Fields. These seismic lines were
divided vertically using five seismic reflectors which have been
defined in stratigraphic terms using ties to wells. These seismic
reflectors correspond to the middle part of the Wonocolo Member and
the top of the Ledok Member, Ngrayong Member, Prupuh Member and
basement rocks.
The Tuban Formarion
The Tuban Formation consists of fine clastic sediments which are
thicker in the northern and eastern parts of the basin and of
uniform thickness in the western part of the basin (Fig. 2). The
Tuban Formation is subdivided into the Tawun Member (lower) and
Ngrayong Member (up- per). The Tawun Member consists of grey
carbona- ceous shale and calcarenite. The calcarenite is rich in
or- bitoidal foraminifera suggestive of a shallow water envi-
ronment. The Ngrayong Member is a poorly cemented quarrz sandstone
(and represents the main reservoir
(75%) of the Cepu Oil Field). Seismic records indicate the
presence of local deltaic facies within the Ngrayong Member
suggesting southeastern progradation (Fig. 4). A deeper starved
basin on which slope deposits accumu- lated was located in the
southwestern part of the study area. There is also evidence of a
Middle Miocene tecto- nic uplift over a restricted area because of
erosion of the Ngrayong Member. Downlapping seismic facies corres-
pond to erosional truncation on the upper part of the Ngrayong
Member in this area.
Kawenangan Formation
The Kawengan Formation consists of four confor- mable members
and it outcrops extensively over the Cepu Oil Fields (Figs 2,4). It
is composed dominantly of mark interbedded with thin sandy
calcarenite at its base, becoming well bedded calcarenite towards
its top.
The Kawengan Formation lies in part unconformably upon the Tuban
Formation. Its basal member consists of well bedded, trough cross
bedded calcarenite indicative of a high energy, shallow platform
type environment (Figs 2,4). The basal member is overlain by
thickly bed- ded marl which is in turn overlain by glauconitic
calcare- nite. The lithology, cross-bedding and biogenic structu-
res are indicative of shallow marine conditions. The uppermost
member of the Kawengan Formation consists of monotonous
unstratified marl with abundant small foraminifera1 fossils
indicative of a bathyal environ - ment.
Lidah Formation
The extensively outcropping Lidah Formation con- sists
predominantly of clay or marl (Fig. 2). The basal member, the
Selorejo Member, has a very restricted distribution in the Cepu Oil
Fields. This may reflect eit- her subsequent erosion or wedging out
of facies in this direction or restricted sediment supply to this
area. It was deposited under shallow marine conditions. The fol-
lowing Tambakromo Member consists largely of blue clay with some
sub-millimetre thick streaks of very fine grained quartz sandstone
and was deposited under shal- low marine conditions. The Tambakromo
Member is distinguished from the overlying calcareous Turi Mem- ber
because the Turi Member consists of intensively weathered marl. The
Turi Member is everywhere uncon- formably overlain by recent
alluvial deposits.
Palaeogeography
The palaeogeographic map shown in Figure 4 was derived from a
much larger fence diagram covering a 20 x 50 km area centered on
Cepu which was constrwted using data from seven oil wells and a
twenty-five line seis- mic grid (Fig. 3). This map aetails the
lateral and vertical lithological variations in the Tuban and
Kawengan For-
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14 1
mations. A landmass lay to the northwest of Cepu (Sun- daland of
Katili, 1974j: The palaeoshoreline probably was oriented
northeast-southwest and lay to the north of Cepu. Shallow marine
deposition in the northwest gra- des to open marine basinal
deposits in the southeast. The local deltaic facies within the
Ngrayong Member and the presence of a starved basin in which slope
deposits accu- mulated in the southwest suggest localized
shallowing parallel to the palaeoshoreline. In the northeast,
carbo- nate facies were being deposited suggesting reworking of a
carbonate shelf at this time. The presence of thick, shallow,
offshore carbonate deposits northeast of Cepu may be due to shells
being derived from reefs north and northeast of Cepu. In this
respect, reefal environments have been present throughout northeast
Java not only within the Oligocene Kujung Formation but also in the
Neogene-Pleistocene Paciran Formation in the Tuban area.
There were coarser sediments in the north which grade south into
finer sediments. Alternation between sandstone, calcarenite and
sandy marl are predominan- tly developed in the northern part of
the area and repre- sent cyclic and reciprocal sedimentary
processes bet- ween the clastic detritus from a hinterland and the
rewor- ked carbonate detritus from reef atolls or patch reefs. The
bulk of the clastic detritus is quartz, consistent with a granite
hinterland northof Java island at this time (Katili, 1974).
The calcarenite to the north is surrounded by marl in the south.
Thus these facies changes are consistent with a reef framework
grading down slope into beds of calcare- nite and grading to marl
in the deeper parts of the basin. The thick shale and marl in the
southern part of the area indicates continuing stability of the
shelf during Neogene deposition.
TECTONIC SETTING
The present day tectonic features in east Java reflect its
development as a convergent zone between the Eura- sian plate and
Indian Ocean-Australian Plate (Katili, 1974, Hamilton 1975, Lowell,
1979. Katili and Reine- mund, 1984). There has been a rotation of
the magmatic arc from being oriented northeast-southwest in the
Late Mesozoic to east-west in the Tertiary in northeastern Java
(Katili and Reinemund.1984). Given this magmatic arc rotation the
Cepu area would have changed from a Mesozoic fore-arc setting to a
Tertiary back-arc setting. The pre-Tertiary basement lithologies
are indicative of an accretionary complex whilst the
northeast-southwest oriented basement faulting and horsts and
grabens arc consistent with rifting in a fore-arc setting suckas
those developed elsewhere adjacent to subduction zones (Beck, 1983,
Jarrard, 1986). The plate vectors indicate that convergence was not
always at right angles, even at
the present time, which would aid sinistral strike-slip movement
on any northeast trending faults in this nor- theast Java basin
(Lowell 1979).
The northeast Java basin is considered to consist of four blocks
differentiated on the basis of fold orienta- tion, faulting and
outcrop distribution (Fig. 5a). Block l consists of an area covered
by the oldest surface sedi- mentary rocks reflecting the presence
of an elevated basement in this area. Block 2 in the western
section of the oil fields exhibits en echelon, east-west trending
folds with Neogene rocks in their cores (Fig. 5c). It is separa-
ted from Block 3 by a major sinistral fault (Fig. Sb & c). The
evidence for this major sinistral strike-slip fault includes the
differences in the nature of hydrocarbons either side of this line,
the differences in the trend of folds and the presence of abundant
similarly trending faults in the basement. Block 3 in the eastern
half of the oil fields has mainly northwest-southeast trending
folds (Fig. 5c). Block 4 to the south of the other blocks exhibits
a lower basement. contains extensive alluvial deposits and has
east-west trending, en echelon folds (Fig. Sc). The development of
the oil fields structures involved a pre-Late Oligocene basement
configuration in which Block 1 basement \vas higher than basement
in any other block (Fig. 5a). Shear couple movement, possibly due
to oblique subduction, resulted in reactivation of the deep-seated
northeast-southwest trending palaeofault o n the eastern edge of
Block 2. This was accompanied by conjugate, sinistral, strike slip
movement and formation of flower structures and en echelon folds in
Block 2 as described below. In Block 3 northwest-southeast tren-
ding reverse faults and folds were initiated as described in the
section below on the Tambakromo Mawengan structures.
Previous Studies
Martins (1951 ) axial-surface. thrust-reverse faulting model for
the Kawengan Anticline was the first to explain this structures
form. The present study results are consistent with the reverse
fault elements of Martins (1951) cross section, although this study
shows that at depth the faults sole out on a subhorizontal
detachment within the Tuban Formation. Lemigas and Beicip (1969)
considered that dog-legs in the generally east- west trending oil
producing anticlines nere due to shea- ring caused by re-activation
of northeast-southwest tren- ding palaeofaults during the
Pleistocene deformation. This may p:titly explain the fold
pattern.
The two major fold trends identified in the Cepu area were
ascribed to the Neogene and Pelistocene orogenies (Soetantri et
al., 1973). Soetantri et a/ . . (1973) proposed that DI resulted in
the north\vest-southeast trending folds and rare east-west trending
folds; while D2 resul- ted in east-west trending folds only (Figs 1
& 5 ) . The two
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142
structures which are the focus of this study, the
Kawengan-Tambakromo structure and Nglobo- Senianggi structure, were
formed durifig D1 and DI(?)- D2 respectively according to this
model. This folding model for the Kawengan structure contrasts with
the diapiric shale model proposed by Soetarso & Suyitno (1976)
for this structure. The diapiric shale was derived from the
Wonocolo Member and Upper Tuban Forma- tion.
Other models for northeast Java have emphasized the impor- tance
of faults in controlling the development of folds (Situmorang et
al., 1976). East-southeast to west- northwest oriented 2nd order
dextral faults develop east- west oriented drag folds and these are
deformed by 3rd order sinistral, northeast- southwest oriented
faults.
Nglobo-Sernanggi
Well developed flower structures can be identified from two of
the five north-south seismic lines across the crestal region of the
Nglobo-Semanggi structure (Fig. 6, Harding, 1985). These are
positive flower structures because the upward spreading faults have
reverse sepa- ration on most of their elements. These structures
reflect a convergent wrench zone where there is strike-slip motion
on the fault system (Harding and Lowell, 1979). The isochron map of
the Prupuh Member indicates dominantly northeast-southwest trending
reverse faults (Fig. 7d), while the overlying sequence shows only
nor- mal faulting at approximately right angles to the fold axes
(Fig. 7a-c). This basal reverse fault pattern is consis- tent with
the flower structures while the normal fault pat- tern in the cover
sequence reflects accommodation in the stretched hinge zone of the
anticlines.
There is a significant diffcrcnce between thc thickncss of the
Tuban Formation over the crest of these flowcr structures compared
with the limbs (Figs 6 8: 8). This differential thickness cannot be
accounted for using displacements on faults within the flower
structure; and is visible on both sections normal and parallel to
the east- west trending fold axes (figs 6 & 8). The Tuban
Forma- tion thickens over the crest of folds as a probable result
of its original deposition in a graben at this locality between the
northeast-southwest trending normal faults in t h e pre-Tuban
basement. These normal Paults have been reactivated during
transpression in the Neogene to their present listric, reverse
form. The development of anti- forms and synforms in the overlying
Tuban Formation reflects the amount and type of strike- slip
displacement on these faults and the sense and degree of overlap of
these faults. If the faults are curving or have dog- legs there
will develop either pop-ups or pull apart basins (Aydin and Nur,
1985). The basement reverse faults cha- nge strike from
northeast-southwest to eastnortheast- westsouthwest between the
Nglobo and Semanggi Antic-
lines and sinistral movement on these faults would result in
flower structures with pop-ups (Figs 6,9).
There is some suggestion in one seismic sectior. of detachment
within the Tuban Formation beneath the Ngloho anticline (Fig. 8b).
Further processing of the other seismic sections may clarify
whether such detac- hments are more commonly developed in the Tuban
For- mation reflects the ability of the pre-Tuban faulting to
accommodate the imposed Neogene transpression. If these basement
faults do not substantially reactivate as strike-slip faults during
Neogene transpression, then the Tuban Formation will detach to
accommodate deforma- tion. In the case of the Nglobo anticline it
owes its exis- tence to development of a flower structure in the
base- ment and lower Tuban Formation and possibly some deep-level
detachment within the Tuban Formation (Fig. 8b). This detachment
within the Tuban Formation was probably triggered by the thicker
section of Tuban Formation in this former graben and the presence
of the bounding reactivated faults. This detachment within the
.Tuban Formation soles at about 4 km unlike that dedu- ced from
the Tamhakronio-Kawengan structure which soles at 1.5 km.
In areas of transpressional tectonics i t is likely that ste-
pover between subparallel overlapping faults will result in the
development of flower structures as an accommo- dation mechanism
during strike-slip movement (Aydin and Nur. 1985). A model for the
development of the ste- povers in the Nglobo-Semanggi area is shown
in Fig. 9
The eight seismic lines and the basal isochron map indicate the
north-northeast trending basement faults causing the
Nglobo-Semanggi anticlines must have slip- ped in a sinktral sense
during the Pleistocene deforma- tion. This would be consistent,
using the strain ellipse method of Wilcox ef a/., (1973), with a
major sinistral fault trending northeast. This fault forms the
boundary between the eastern and western oil fields (Fig. 5 ) . The
Nglobo-Semanggi structure is but one of the east-west oriented.
open. upright folds in the block west of the regional fault. Folds
to the east of the regional fault are generally northwest-southeast
oriented, open, upright faulted structures
Tarnbakrorno-Kawengan
Using the ten seismic sections it is possible to deter- mine
that there is a fundamental difference between the shallow and deep
structures in the Tambakromo- Kawengan area.
There is a half-flower deep structure affecting the Pru- puh
Member and the lower part of the Tawun Member but not the overlying
cover sequence (Fig. 10). The major reverse fault within the half
flower structure has a
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143
vertical displacement of 160 m which I S much less than the 550
m throw calculated for the major reverse faults in the
Nglobo-Semanggi area. The Prupuh Member shows abundant generally
nor1 heast-southwest trending faul- ting, probably as a reflection
of basement faults of a simi- lar trend which were reactivated
during the Neogene orogeny (Fig. l lc) . In contrast to the
faulting which dominates the deep structures, the shallow
structures occur as both faults and folds.
Whereas the open Kadewan and Tambi anticlines affect the top of
the Ledok Member and Middle part of the Wonocolo Member, the gently
plunging Tamba- kromo anticline involves the top of the Ngrayong
Mem- ber. Thus the Tambakromo anticline formed during both Neogene
and Pleistocene orogenies whereas the Kade- wan and Tambi
anticlines were affected only by the Pleis- tocene deformation. The
Kawengan anticline differs from the other folds in this eastern
block in that it is an open asymmetrical fold with a southern more
steeply- dipping limb dissected by a reverse fault which soles into
a detachment within the Tuban Formation.
The isochron map of the top of the Ledok Member shows the
northwest-southeast trending culmination of the Tambakromo
anticline and a single normal fault at right angles to the fold
axis (Fig. 1 la). The isochron maps for the middle of the Wonocolo
Member and top of the Ngrayong Member show a major reverse fault in
the nor- theast corner of the map (Fig. 1 Ib & c). Since the
nor- theast-southwest trending normal fault identified at the top
of the Ledok Member isochron map occurs on these two other isochron
maps and in all cases terminates against the northwest-southeast
trending reverse fault, this implies that the reverse fault formed
after the normal fault. The northwest-southeast trending reverse
fault identified from the middle of the Wonocolo Member and top of
Ngrayong Member isochron maps does not out- crop the surface. The
isochron map for the top of the Prupuh Member shows many
northeast-southwest tren- ding normal and reverse faults (Fig. 1
Id). This fault pat- tern is similar to the comparable isochron map
in the Nglobo-Semanggi area (Fig. 8d). This reflects the perva-
sive northeast-southwest trending basement faulting gen- erated in
the Cretaceous fore-arc and re-activated during defor- mations in
the Neogene and Pleistocene when the northeast Java basin was in a
back-arc setting. The absence of the northwest- southeast trending
reverse fault in the Prupuh Member, which was recogni- zed in the
Wonocolo and Ngrayong Members indicates this reverse fault is
detached within the Tawun Member.
The deep structures were formed during the Middle Miocene
deformation whereas the shallow structures, which are important for
oil field formation, formed during the Middle Miocene and Early
Pleistocene defor- mations. Thus some deep seated faults were
formed and
other reactivated during the Middle Miocene deforma- tion whilst
rare half flower structures and folds in the cover sequence were
formed during Early Pleistocene deformation (Fig. 10) . The
restricted angular unconfor- mity upon the Ngrayong Member supports
such a two phase deformation model.
CONCLUSIONS
Anticlines are the dominant structural trap in the Cepu Oil
Fields. The western and eastern oil fields differ markedly in their
structural style (Fig. 12). The Nglobo- Semanggi oil field would be
a mainly basement-involved structure whereas the
Tambakromo-Kawengan oil field would be a detached structure (Lowell
1979). Wrench faulting appears to be a minor factor in the
development of the Tambakromo-Kawengan structure whereas it was
crucial in the formation of the Nglobo-Semanggi structu- re.
Nglobo-Semanggi oil fields are characterized by the presence of a
flower structure at approximately 5-6 km depth and, at the surface,
by en echelon fold patterns (Fig. 12a). There is some suggestion of
detachment wit- hin the Tuban Formation at about 3 km beneath the
Nglobo Anticline. Further processing would be required to verify
how often such detachments occur in this area. The Kawengan oil
field is mainly characterized by asym- metrical folding formed
above a listric, detached, reverse fault at 1.5-2 km depth. There
is a possibility that blind thrusts north of the present
Tambakromo- Kawengan structure caused further anticlinal closures
containing hydrocarbons.
The Cepu Oil Fields contain both wrench and compres- sional
structures which formed hydrocarbon traps. These structures
probably reflect transpressional stres- ses associated with oblique
subduction in the Java trench during the Neogene to
Pleistocene.
ACKNOWLEDGMENTS
The authors are grateful to Associate Professor Evans for his
comments on numerous seismic sections during the course of this
study. Dr M. Johnstone, Esso, is than- ked for reviewing the
manuscript. N. Soeparyono was in receipt of an Australian
Development Training Award, Australian Internal Development
Assistance Bureau during this study. This paper is published with
the kind permission of PPT-MIGAS. Mrs M. Kadar drafted the figures
and Mrs M. Clark and Miss K. Jones typed the manuscript.
REFERENCES
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strike-slip tectonics. In: Strike slip deformation, basin formation
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SEPM 3 1 , 3 5 4 3 .
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Beck, M.E. 1983. On the mechanism of tectonic trans- port in
zones of oblique subduction. Tectonophy- sics 93, 1-1 1.
Froidevaux. C. , Uyeda. S. and Uyeshima, hf. 198s. Island arc
tectonics. Tectonophysics 148, 1-9.
Hamilton, W. 1975. Subduction in the Indonesian region.
Southeasi Asia Petroleum Exploration Society Proceedings 2,
37-40.
Harding, T.P. 1985. Seismic characteristics and iden- tification
of negative flower structures, positive flower structures and
positive structural inver- sion. Bulletin of the American
Association of Petroleum Geologists 582-600.
Harding, T.P. and Lowell, L.D. 1979. Structural styles. their
plate tectonic habitab and hydrocarbon traps in petroleum
provinces. Bulletin of the Ameri- can Association of Petroleum
Geologists 101 6- 1058.
Jarrard, R.D. 1986. Terrane motion by strike-slip faulting of
forearc slivers. Geology 14, 780-783.
Katili, J .A. 1974. Geological environment of the Indo- nesian
mineral deposits, a plate tectonic approach. Publikasi Teknik .
Seri Geoiogi Eko- nomi 7,16 pp, Geological Survey of Indonesia.
Katili, J.A. and Reinemund. J.A. 1984. Southeast Asia: Tectonic
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International Union of Geological Sciences 13, 68p.
Koesoemadinata, R.D. 1969. Outline of geolo;
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A G E
M.Y.B.P. WSHORE
QUAT. I JL IOAH . PLIOCENE
( SAMUEL AND GULTOM, 1984)
Y woLlocoLo I - - -
3 1-3- - - 3 =
3o 1y , -_ - .. . 35j- - 13 - . . . - . . . z - . . . _
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LITHOLOGY
[XI Sandy shale I3 sands Reefal 3 platform -carbonates
FORMATION AND
MEMBER OEPOS iTlON
CYCLE
m
r
E A S E M E N T
(MOOIFIED BY THE AUTHOR)
[=?[ Shales marls 8 mudstone
t-1 cool seam
FIGURE 2 - The standard and modified stratGraphic coiamns for
the Cepu Oil Fields. The new column is based on additional measured
section and remapping of the geology (Soeparyono,l988,
SabardiJ988). The modified columb is divided into depositional
cycles.
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147
-
148
FIGURE 4 - A paleographic reconstmction for the Northeast Java
Basin during the Middle - Upper Miocene.
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149
Q . L AT M- OL IGOCN
0 PLANTUNGAN
NGLO B 0 - SEMANGGI
KAWENGAN qj @ NORTHERN KENOENG
I b. MIOCENE-PLIOCENE
c. LOWER ?LISTOCN
FIGURE 5 - Proposed tectonic subdivision of Cepu region based on
the age of outcropping rocks,fold trends and the nature of the
hydrocarbon traps.
-
150
TOP LEDOK MEMBER MIDDLE WONOCOLO M E M B E R
.TOP NGRAYONG M E M B E R
FIGURE 6 - Seismic lines Cu-l9(aj and Cu-17(b) over the Nglobo-
Semanggi structures. See Fig. 3 for the localities. Regio- nally
continuous reflectors with stratigraphic assignment are labelled.
Both show positive flower structure indica- tive of wrench
tectonics.
-
n
I 110
30
I
LE
GE
ND
--2
80
0--
- Tw
o w
ay t
ime
cont
our
in m
illis
econ
ds
(co
nto
ur
inte
rval
50
met
res
1
C,H
Lo
w o
nd h
igh
area
in s
urfa
ce
h
me
t
111
30
111
30
- C U
7
Sels
mlc
lin
e or
ient
atio
n
- -u
U
ndef
lned
fau
lt,
norm
al f
ault
-&
-_
L4
U
ndef
ined
B o
bser
ved
reve
rse
faul
t
FIG
UR
E 7
., Is
ochr
on
map
s of
th
e N
gloh
o-Sc
iiian
ggi
strl
lctt
lrc
:It
diff
eren
t stra
tigra
phic
lev
els;
a)
top
of L
edok
Mem
ber,
17
) M
iddl
c W
oiio
colo
Mcm
hcr.
c) t
op of
Ngr
nyon
g M
ciii[
>cr :I
n({
d) to
p of
Pru
puh
Mcr
nber
. T
hc le
gend
on
d) is
ap
plic
able
for
all
thcs
c is
oclir
on n
i;ips
.
-
ZZi
-
153
Ci.
b.
I , . ,
F IGURE 9 - Model for the development of the Nglobo - Semanggi
structures in the stepover between numerous overlapping
transcurrent faults.
-
154
CU-8 cu-9 T - * I-
KADEWAN TOP LEDOti MEMBER MIDDLE WONOCOLO MEMBER TOP NGRAYONG
MEMBER
TOP PRUPUH MEMBER
FIGURE 10 - Flower structure near thc Kadc\vati :intidinc in the
Tam- bakromo-Kawcngan arc;\ in seismic line Cu- 10. See Figure 3
for the locality o f this northcast- southwest oriented seismic
line.
-
155
L E G E N D - CU 7 Seismic the or lenta t ion - moo---Two way
tlme contour In m i l l i s e c o n d s - - Undeflned fault ,
normal fault
(contour Interval 50 m e t r e s )
L, H Low a n d hlgh a r e a in s u r f a c e Undeflned Et o b s
e r v e d r e v e r s e fault
FIGURE 11 - Isochron maps of the Tambakromo-Kawengan structure
at different stratigraphic levels; a) top of Ledok Member, b)
Middle Wonocolo Member, c) top of Ngrayong Member and d) top of
Prupuh Member. The legend in d) is applicable for all these
isochron maps.
-
156
T U B A N FM.
\ 0 I
SURFACE / TUBA N FM.
2 3 4
km
g--- I I KUJUNG B NGIMBANG FM.
FIGURE 12 - Model showing the different structural styles in the
NgIobo-Semanggi and Tambakromo-Kawengan oil fields. The former
structure was formed during wrenching and reactivation of basement
faults whereas the latter was formed during compressional tectonics
with .detachment in the cover sequence.
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