Cepu 3D Variations in Carbonate Build Up Morphology

8/18/2019 Cepu 3D Variations in Carbonate Build Up Morphology

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IPA07-G-122

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION

Thirty-First Annual Convention and Exhibition, May 2007

CEPU 3D SEISMIC - VARIATIONS IN OLIGO-MIOCENE CARBONATE BUILDUP

MORPHOLOGY

Agung B. Cahyono*

Carl F. Burgess*

ABSTRACT

In 2001, Mobil Cepu Ltd. acquired a very large,

high quality onshore 3D seismic survey in the Cepu

Block, East Java Basin, which provides a unique

opportunity to understand the depositional and burial history of an extensive carbonate platform.

Figure 1 shows the location of the Cepu Block and

3D seismic survey.

Oligo-Miocene age isolated carbonate build-ups are

the major proven reservoirs that have recently been

discovered in the Cepu area. Several significant oil

and gas discoveries have been found in these

reservoirs including the Banyu Urip oil field which

is one of the largest oil fields found in Indonesia

since the 1970's (Figure 2).

Carbonate build-ups in the Cepu Block have diverse

morphologies ranging from steep-sided, narrow

pinnacles to broad platform deposits. These

buildups developed on a single larger isolated

platform that began to be deposited during the

Lower Oligocene. Through the Upper Oligocene

and Lower Miocene, carbonate deposition ceased

over significant areas of this platform while other

areas continued to grow as carbonate deposition

kept up with relative sea level rise. This process

eventually resulted in several separate and isolated

carbonate buildups that drowned at different timesand have distinct morphologies, mostly related to

the underlying extensional faults and relative

subsidence rates across the platform.

The thickest of these carbonate buildups are up to 2

km thick. The thicker buildups drown in Lower

Miocene time and are covered by Middle Miocene

clastics that are low quality seals. However in

certain cases, these result in large oil accumulations

because trapped gas is held in overlying clastic

reservoirs or is leaked due to a top seal with low

capillary-entry pressure. Other areas of the Cepu

* Mobil Cepu Ltd

platform drowned earlier during the Oligocene.

These carbonate deposits have different

morphology, lower reservoir quality and more clay-

rich seals with high capillary-entry pressure, and

commonly contain large gas columns.

Documenting the variations in Oligo-Miocene

carbonate sedimentation has been instrumental in

understanding these hydrocarbon accumulations and

should have relevance for similar reservoirs in other

Indonesian basins.

INTRODUCTION

The Cepu platform is one of several isolated

carbonate platforms located in the southwestern

portion of the East Java Basin. During the late

Oligocene and early Miocene this basin wascharacterized by the evelopment of shallow water,

reefal carbonate deposits (Fulthorpe and Schlanger,

1989). The morphology and growth history of the

Cepu carbonate platform was strongly influenced by

underlying extensional fault patterns related to

Eocene extensional faulting when clastic deposition

dominated much of the East Java Basin.

A 3D seismic survey was acquired over the

southeastern Cepu Block area. The 3D seismic

clearly images reservoir geometries of an isolated

carbonate platform from origination, growth anddrowning (Figure 2). The 3D seismic has been

invaluable for the study of carbonate development

through time and illustrates the relative effects of

local and regional structural subsidence, eustatic

change in sea level, and environmental factors such

as prevailing winds and oceanographic currents.

Good seismic quality at depth also permits the

imaging of deeper structure in some areas and

shows the influence of Eocene faulting on carbonate

development.

The objectives of this paper are to (1) describe and

illustrate the variations in Oligo-Miocene carbonate

morphology of several buildups in the Cepu area,



(2) discuss controls on the stratal architecture of this

carbonate and understand the reservoir quality, and

(3) understand top seal performance related to the

type of hydrocarbons present.

METHOD

Mobil Cepu Ltd. acquired a very large, high-quality

onshore 3D seismic survey over most of the Cepu

Block in 2001. This survey represents the largest

onshore 3D survey ever acquired in Indonesia and

the largest ExxonMobil has acquired globally. Pre-

Stack Time Migration processing was performed

providing relative amplitude, AGC, and angle-stack

cubes for seismic interpretation. Data quality is

excellent across the majority of the survey area, but

some imaging challenges were found in areas with

shallow gas, steeply dipping near surface stratarelated to thrust faults, and large urban centers. The

most significant data quality problems are related to

shallow trapped gas which causes amplitude and

frequency attenuation.

Data from several exploration and appraisal wells

have been integrated with the seismic data to study

carbonate buildup development through time for

several buildups located within the Cepu area. The

study was supported by absolute age data from87

Sr/28

Sr isotope analyses in the carbonate intervals

and biostratigraphic analysis in the overlying clasticsection.

OBSERVATIONS

Carbonate reservoirs in the Cepu Block first

developed during the Oligocene (possibly as early

as the Eocene) and continued throughout the Early

Miocene. Most shallow-water carbonate deposition

seems to have taken place on a large basement high

or horst blocks that appear to have been created by

Eocene extensional faulting. Minor extensional

faulting continued along the edges of the basementhigh throughout the end of the Oligocene creating

small differences in tectonic subsidence that seem

to have a major influence on the shape and

stratigraphy of the various isolated Cepu carbonate

buildups over time.

Carbonate Geometry of Selected Buildups

Cendana

The Cendana buildup sits on a structural high

controlled by a NE-SW trending fault (Figure 3A).

It consists of three narrow-pinnacle buildups in a

cone-shaped feature which is broad at the base and

much narrower at the top. Several seismically

defined sequences prograde southward, but at a

larger scale each sequence stacks in a

retrogradational pattern. The overall stratigraphic

pattern is retrogradational or backstepping (Figure

3D). The buildup consists of three basic seismic

facies: platform, in situ buildup and detrital apron.

The in situ carbonate buildup and platform facies

developed along the foot-wall high. Detrital

carbonate facies are interpreted to be deposited in

deeper water as carbonate debris was shed from the

platform and built up due to wave and wind action.

This material was distributed mostly north and

south of the main buildup. An intra-platform

seaway channel is observed on 3D seismic east of

Cendana. This seaway forms the lateral

hydrocarbon seal between the Cendana and Banyu

Urip buildups.

Jambaran

Jambaran is a long, narrow buildup trending

approximately 10 km in the east-west direction

(Figure 3B). This orientation is related to a system

of down-to-the south normal faults located 1-2 km

to the south. Three depositional sequences are

observed in the Jambaran well: flooding, build-up

and final drowning (Figure 3E). These sequences

can also be defined on the seismic data. The build-

up facies has the best reservoir properties while theflooding and drowning sequences consist of lower

porosity intervals. The Jambaran buildup is

separated from the adjoining Alas Tua platform on

the east by a deep shale-filled seaway. This

relationship is an example of the complex paleo-

bathymetry of the Cepu platform.

Alas Tua

The Alas Tua trend consists of two broad, low relief

carbonate buildups separated by a deep shale-filled

intraplaform seaway (Figure 3C). The Alas-Tuacarbonate interval has not yet been penetrated by

wells, but 3D seismic interpretation of this area

defines a wide area of interpreted Lower Oligocene

platform carbonate with complex morphology quite

distinct and different from the younger buildups of

the Cepu platform. The Alas Tua trend exhibits a

very thin carbonate section at the crest of a large

faulted basement high (Figure 3F). This seems to

indicate very limited sedimentary accommodation

along the southern edge of the Cepu Platform and

this thin trend probably represents an area of

sedimentary bypass at the shelf-slope break. This

shelf-slope break is related to the underlying

extensional fault system and related tectonic



subsidence that was still active during deposition of

the Oligocene carbonate platform.

TOP SEAL

Hydrocarbon top seal is affected by the age of the

clastic section deposited on the crest of each

individual buildup. The overall stratigraphy of the

Lower to Middle Miocene section at Cepu is

generally progradational. Lower Miocene age,

deepwater marine mudstones seal the older buildups

which tend to contain tall gas columns. The

youngest buildups are sealed by middle Miocene

clastics that were deposited in shallower water

depths and which often contain beds of sandstone

and siltstone. Buildups sealed by these younger

clastics tend to contain smaller gas columns, but

quite thick oil columns. Variable lithology,depositional environment, and geological age result

in differences in capillary entry-pressure properties

of the Cepu topseals. Differences in topseal can

explain much of the variation in oil and gas column

height from one buildup to another.

CONCLUSION

Three-dimensional seismic data has been used to

study the morphology of several carbonate build-

ups in the Cepu Block. These have diversemorphology ranging from steep-sided, narrow

pinnacles to broad, low relief deposits. The

buildups developed on a single larger isolated

platform that began to be deposited during the

Lower Oligocene. Carbonate deposition ceased over

significant areas of this platform during the Upper

Oligocene and Lower Miocene while other areas

continued to grow as carbonate deposition kept up

with relative sea level rise. This process eventually

resulted in several separate and isolated carbonate

buildups that drowned at different times and have

distinct morphologies, mostly related to theunderlying extensional faults and relative

subsidence rates across the platform.

Several buildups drown in Lower Miocene time and

are covered by Middle Miocene clastics that are low

quality seals. However, these sometimes result in

large oil accumulations because the trapped gas is

held in the overlying clastic reservoirs or is leaked

due to a top seal with low capillary-entry pressure.

ACKNOWLEDGEMENTS

The authors would like to acknowledge

ExxonMobil, Mobil Cepu Ltd, Pertamina and Migas

for permission to publish this material. Steve Buck

and Mark Thomsen are thanked for editorial

comments. Technical discussions and contributions

provided by the Cepu Exploration Team (including

John Thompson, Rick Vaught, R. Yudantoro and

Andrew Pierce) were instrumental to our

understanding of the Cepu Block.

REFERENCES CITED

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Fulthorpe, C. S., and S. O. Schlanger, 1989, Paleo-

oceanographic and tectonic settingsd af early

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F

i g u r e 1

- L o c a t i o n m a p o f t h e C e p

u B l o c k a n d O l i g o - M i o c e n e c a r b o n a t e b u i l d u p s .



F i g u r e 2

- C e p u 3 D s e

i s m i c i m a g e o f t o p O l i g o - M i o c e n

e c a r b o n a t e r e s e r v o i r ( T W T ) . N o

r m a l f a u l t s , r e l a t e d t o e a r l i e r r i f t i n g ,

d e f i n e l o c a t i o n o f p l a t f o r m a n d i n f l u e n c e e v o l u t i o n o f b u i l d u p m o r p h o l o g y .



F

i g u r e 3

- C o m p a r i s o n o f t h r e e

C e p u O l i g o - M i o c e n e c a r b o n a t e

b u i l d u p s . T o p c a r b o n a t e r e s e r v o

i r s e i s m i c i m a g e s i n t w o - w a y t i

m e . E a c h

b u i l d u p t e n d s t o f o l l o w t h e n o r m a l f a u l t s t r i k e d i r e c t i o n . C e n d a n a h a s t h r e e d i f f e r e n t p e

a k s w i t h c e n t r a l p i n a c l e ( 3 A ) , J a m b a r a n i s

l o n g a n d n a r r o w

b u t

a c o n t i n u o u s r i d g e ( 3 B ) , a n d A l a s T u a e x h i b i t s f l a t t e r g e o m e t r y w i t h d e e p m u d - f i l l e d s e a w a y s c u t t i n g

p e r p e n d i c u l a r t o t h e n o r m a l f a u l t s ( 3 C ) .

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Cepu 3D Variations in Carbonate Build Up Morphology