CORRELATION STUDY OF SOURCE ROCK AND OIL IN LIMAU GRABEN, SOUTH SUMATERA BASIN: SOURCE ROCK AND OIL CHARACTERIZATION

AND POTENCY OF LEMAT FORMATION AS HYDROCARBON SOURCE ROCKS

M. Syaifudin1,2,a, Eddy A. Subroto2, Dardji Noeradi2, Asep H.P.Kesumajana2

1Universitas Pembangunan Nasional “Veteran” Yogyakarta2Institut Teknologi Bandung

aSyaifudin_muhammad@yahoo.com

AbstractLemat Formation in Limau graben has been considered as syn-rift sediments until now, consist of fluvio-lacustrine sediments, creating source rock with fluvio-lacustrine characterization. While based on existing publications, showing that oil from Limau graben having fluvio-deltaic characterization and generated by source rock from Talangakar Formation with fluvio-deltaic characterization.

This paper emphasizes geochemistry methods. Source rock analysis consist of 26 samples for carbon isotope and 14 samples for biomarker, while oil analysis consist of 15 samples for carbon isotope and 19 samples for biomarker. Characterization has been based on qualitative and quantitative data. Qualitative data comprise evaluation based on chromatograms and mass-fragmentograms, whereas quantitative data consists of a series of cross-plots.

Based on geochemical analysis, source rocks of Lemat Formation in Limau Graben interpreted as source rock with fluvio-deltaic characterization, having terrestrial influence while Talangakar Formation in Limau Graben interpreted as source rock with deltaic characterization, having marine and terrestrial influence. Both, consist of humic kerogen. Whereas, oil samples in Limau Graben interpreted as oil which is generated by source rock with fluvio-deltaic characterization, having terrestrial influence, in anoxic-suboxic-oxic conditions, consisting of humic kerogen. Correlation result between source rocks and oils in Limau Graben, indicating that fluvio-deltaic oil families in Limau Graben are strongly correlate with not only the source rocks of Talangakar Formations but also with source rock of Lemat Formation.

Key words: biomarker, terrestrial, fluvio-deltaic

1. Introduction

South Sumatra Basin is a potential and mature basin for hydrocarbon kitchen. There are a

number of sub-basins which is potential as the hydrocarbon kitchen in this basin. Limau

Graben is one of sub-basin on the South Palembang Sub-basin, formed in Upper Cretaceous

to Lower Tertiary, in extensional phase, this is the beginning of sedimentation in this area.

Lemat Formation in Limau Graben has been considered as syn-rift sediments until now,

consist of terrestrial sediment (fluvio-lacustrine), creating source rock with fluvio-lacustrine

characterization. While based on existing publications, showing that oil from Limau Graben

having fluvio-deltaic characterization and generated by source rock from Talangakar

Formation with fluvio-deltaic characterization. This is interesting to study further, especially

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about possibility of source rock with fluvio-lacustrine characterization in research area. This

study emphasized to source rocks of Lemat and Talangakar Formation in Limau Graben,

South Palembang Sub-basin, South Sumatra Basin.

2. Research Area

The research area located in Limau Graben, South Palembang Sub Basin, South Sumatra

Basin.

3. Research Methods

This paper emphasizes geochemistry methods. Source rock analysis, consist of 26 samples

for carbon isotope and 14 samples for biomarker, while oil analysis, consist of 15 samples for

carbon isotope and 19 samples for biomarker. Characterization has been based on qualitative

and quantitative data. Qualitative data comprise evaluation based on chromatograms and

mass-fragmentograms, whereas quantitative data consists of a series of cross-plots, eg. cross

plot of carbon isotope δ13C saturates - aromatics, distribution of C27-C28-C29 sterane, Pr/nC17-

Ph/nC18, Pr/Ph-Pr/nC17, carbon isotope δ13C saturates-Pr/Ph, Pr/Ph-total hopane/total sterane,

and ratio of C26/C25 (tricyclic).

The results of this study expected could explain the character of source rocks and oil in the

Limau Graben, also to find out the possibility of lacustrine source rock existence and

determine the correlation between source rocks and oils in this area, so can be known whether

Lemat Formation source rocks also have contributed to produce oil in this area or not. In

addition, to provide a new opportunity in the exploration of hydrocarbons in the Limau

Graben which considered as a mature and potential basin for hydrocarbon.

4. Regional Structural Geology of South Sumatra Basin

Geological structures that control the regional of South Sumatra (Figure 1) were influenced

by three tectonic phases (Pulunggono et al., 1992):

Compression (Upper Jurassic – Lower Cretaceous)

Tension (Upper Cretaceous – Lower Tertiary)

Compression (Middle Miocene – Recent)

The fi r st phase : started in Upper Jurassic – Lower Cretaceous, characterized with the

subduction of India-Australia plate as a movement mechanism to yield primary stress to the

Sundaland trending N 30o W. This subduction resulted simple shear (N 300o E) as strike slip

fault that was actively moved laterally. This was assumed as the cause of linearity trending

N-S as antithetic fault which was inactive.

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The second phase: commenced during Upper Cretaceous-Lower Tertiary, characterized by

the change of the subduction trend of the India-Australia plate into N-S. This event resulted

in the formation of some geological structures (fractures) caused by tension force as linearity

with N-S direction. This phenomenon caused the formation of grabens and depressions, such

as Benakat Gulley. Initiation of graben filling with Tertiary sediments was started. In general

faults and grabens formed during this phase show N-S and WNW-ESE directions.

The third phase: commenced in the Middle Miocene-present, shown with, again, the change

of the subduction direction into N 6o E, causing rejuvenation and inversion processes on the

paleostructures (N 300o E/N-S) by Plio-Pleistocene (N 330o E) and the uplifting of the

Barisan Montains and also the formation of some thrust faults with the Lematang fault

pattern. During this phase, the Lematang fault pattern that initially acted as depocenter of the

Muara Enim Deep has been uplifted being anticlinorium series of Pendopo-Limau (Figure

2.5). Folding and thrust-faulting processes caused by compression force occurred in the back-

arc basinal and floured during Plio-Pleistocene.

5. Regional Stratigraphy of Sumatra Basin

Based on the tectonostratigraphy framework, Ryacudu (2008) divides Early Tertiary rock

units in the South Sumatra Basin as follows (Figure 2):

Pre-rift sequences

This sequence consists of volcanic rock of Kikim Formations and pre-Tertiary rocks. Kikim

Formations are the oldest Tertiary rocks in the South Sumatra Basin, consist of volcanic

rocks such as volcanic breccia, agglomerate, andesitic tuffs and igneous rocks (as intrusions

and lava flows). Age of Kikim Formation based on dating K-Ar is 54-30 Ma (Paleocene -

Lower Oligocene, Ryacudu, 2008). The oldest age and the contact with pre-Tertiary rocks are

unknown, while the relation with the Formation above is unconformity.

Syn-rift sequences

Syn-rift sequence consists of rock group of Lahat Group consisting of Lemat and Benakat

Formation with interfingering relations. The main constituent of Lemat Formation are coarse

clastic rocks (sandstone) with Tuff Member and conglomerate Member, while Benakat

Formations dominated by fine clastic rocks (shale). The group does not contain fossils, dating

is determined by palinomorf Meyeripollis naharkotensis in shale of Benakat Formations

indicating Upper Oligocene – Lower Early Miocene. The group has non-conformity

relationship with rock Formations above and below it. Sandstones of Lemat Formation

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deposited in fluvial environment, while conglomerate is interpreted as an alluvial fan

sediment. Shale of Benakat Formations interpreted as the result of deposition in the lake

system (lacustrine).

Post-rift sequences

This sequence consists of a rock from Telisa group consisting Tanjungbaru, Talangakar,

Baturaja, and Gumai Formation. Tanjungbaru Formation, originally considered a GRM

(Gritsand Member) formerly known as a member of the Talangakar Formation. This unit is

dominated by conglomeratic sandstone deposition system as a result of braided river.

Unconformity contact with Lahat Group below it. Member of the Formation Talangakar

commonly referred to as TRM (Transition Member) proposed a Talangakar Formation. This

Formation consists of alternating sandstones and shales, with thin coal interbedded, deposited

in the transition environment. Baturaja Formation, Early Miocene (N5-N6), composed of

limestone bioclastic, kalkarenit, bioclastic sandy limestones and reefal bioherm with

interbedded of calcareous shale, deposited on the carbonate platform. Gumai Formation,

Early Miocene to Middle Miocene, composed by calcareous mudstone that contains fossil

planktonic foraminifera Globigerina and shales napalan with glaukonitic quartz sandstones.

The deposition of Gumai Formation marked the peak transgression of the South Sumatra

Basin. Air Benakat Formation, Middle Miocene, composed by the dominance of shallow-

marine mudstone with sandstone interbedded which is thickening and dominating upward.

Sandstone at the top is a quartz sandstone, tufaan and glaukonitic. The presence of the tufa

material in the Formation marked the beginning of the influence of the source sediments from

the south or uplifting of the Bukit Barisan Mountains. Furthermore, the marine condition is

getting shallower so that it becomes transition environment, and then the Formation

Muaraenim deposited. Muara Enim Formation, Middle Miocene to Late Miocene. Consists of

mudstone, shale, and sandstone and coal interbedded deposited in the delta system or

transitional environment. Kasai Formation, Pliocene. Is a volcaniclastic sediment, consisting

of mudstone and sandstone's tufa interbedded deposited in fluviatil and terrestrial

environments.

6. Characterization of Source Rocks and oils in Limau Graben

Figure 3 shows location map of research area and data location of oil and source rocks in

Limau Graben. Figure 4 shows a cross plot Pr/nC17-Ph/nC18 and Pr/Ph – Pr/nC17, source rocks

of lemat and Talangakar Formations, and oils in Limau Graben. This image shows both

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source rocks of Lemat and Talangakar Formation and oils, consists of humic kerogen in

suboxic-anoxic until oxic conditions, but mostly in oxic conditions. Cross plot of carbon

isotope δ13C saturates - δ13C aromatics and carbon isotope δ13C saturates - Pr/Ph, source rocks

of Lemat and Talangakar Formations and oils in Limau Graben shown in Figure 5. This

figure shows source rocks of Lemat and Talangakar Formations and oils consists of terrestrial

and mixed material, in anoxic-suboxic to oxic conditions, but mostly in oxic conditions.

Figure 6 shows a cross plot of Pr/Ph-hopane/sterane and sterane distribution C27, C28, and C29,

source rocks of Lemat and Talangakar Formations and oils in Limau Graben. From this

picture it appears that the source rocks of Lemat and Talangakar Formations and oils affected

by terrestrial material in anoxic -suboxic until oxic conditions, but mostly in high oxic

conditions. Besides, it also looks like Lemat Formations derived from estuarine or shallow

lacustrine to terrestrial environments, whereas Talangakar Formation and oils derived from

marine or deep lacustrine, estuarine or shallow lacustrine, and terrestrial environments.

Figure 7 is a comparison of biomarker characterization qualitatively between source rocks of

Lemat and Talangakar Formation and oils in Limau Graben. From this picture it appears that

source rocks of Lemat and Talangakar Formations and oils, according to ten Haven and

Schiefelbein (1995), and Peters et al. (2005), is not lacustrine sediments because has C26/C25

(tricyclic) smaller than 1. Based on tricyclic data, according to Price et al. (1987), Lemat

Formation and oils show terrestrial pattern, whereas Talangakar Formations show marine and

terrestrial pattern. These data indicate Lemat Formation interpreted as fluvio-deltaic

sediment, whereas Talangakar Formation having more marine characterization than Lemat

Formation. Based on data of 29H and 30H (hopane) distribution, it appears that source rocks of

Lemat Formation and oils are marine clastic sediments because it shows a pattern 29H <30H,

while Talangakar Formation not only show 29H<30H but also show 29H>30H is evaporates-

carbonate sediment (Zumberge (1984); Connan et al. (1988); Price et al. (1987), all in Waples

and Machihara (1991). From data of homohopana distribution which decreased regularly

from C31 to C35, source rock of Lemat, Talangakar Formations, and oils in Limau Graben

interpreted as depositional environment which associated with clastic sediments (Waples and

Machihara, 1991) or more oxidizing conditions (Peters and Moldowan, 1993).

Based on these data, oil in the Limau Graben interpreted originated from fluvio-deltaic source

rocks and has a correlation with Lemat and Talangakar Formations in Limau Graben.

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Conclusion

Source rocks of Lemat and Talangakar Formations and oils in Limau Graben consists of

humic kerogen and terrestrial and mixed material. Source rocks of Lemat and Talangakar

Formations and oils in Limau Graben, is not derived from a lacustrine sediments, affected by

terrestrial material in anoxic -suboxic until oxic conditions, but mostly on high oxic

conditions. Besides, its also looks like Lemat Formations derived from estuarine or shallow

lacustrine to terrestrial environments, whereas Talangakar Formation and oils in Limau

Graben derived from marine or deep lacustrine, estuarine or shallow lacustrine, and terrestrial

environments. Based on tricyclic data, Lemat Formation and oils in Limau Graben show

terestrial pattern, whereas Talangakar Formations show marine and terrestrial pattern. These

data indicate Lemat Formation interpreted as fluvio-deltaic sediment, whereas Talangakar

Formation having more marine characteriztion than Lemat Formation. Oils in the Limau

Graben interpreted originated from fluvio-delta source rocks, has a correlation with Lemat

Formation and Talangakar Formation in Limau graben.

Acknowledgements

We would like to thank the management of Directorate General of Oil and Gas, Pertamina

EP, and Medco EP for their permission to publish this paper.

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Figure 1. Tectonic evolution of the South Sumatra Basin from Upper Jurassic-now (Pulunggono et al., 1992)

Figure 2. Regional stratigraphy of the South Sumatra Basin (modified Ryacudu, 2008).

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Figure 3. Location map of the study area and a summary of the source rock geochemical data in Limau graben.

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Figure 4. Cross plot of Pr/nC17-Ph/nC18 and Pr/Ph – Pr/nC17, source rocks of lemat and Talangakar Formation, and oils in Limau graben.

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Figure 5. Cross plot of carbon isotope δ13C saturates - δ13C aromatics and carbon isotope δ13C saturates - Pr/Ph, source rocks of lemat and

Talangakar Formation, and oils in Limau graben.

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Figure 6. Cross plot of Pr/Ph-hopane/sterane and sterane distribution C27, C28, and C29, source rocks of lemat and Talangakar Formation, and oils

in Limau graben.

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Figure 7. Comparison of biomarker characterization qualitatively between source rocks of lemat and Talangakar Formation, and oils in Limau graben

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