Dicky Muslim

R. Irvan Sophian

R.M. Riza Atmadibrata

Riyan Rizki Fauzi

Utsunomiya University Academic Information Repository

PHYSICAL AND GEOMECHANICAL CHARACTERISTICS OF 
KEUTAPANG FORMATION IN ARUN GAS FIELD, NORTH 
SUMATRA, INDONESIA 
Riyan Rizki Fauzi1∗, R.M. Riza Atmadibrata2, Dicky Muslim 3, R. Irvan Sophian4 
Laboratory of Engineering Geology, Faculty of Geological Engineering, Universitas 
Padjadjaran, Indonesia. 
*Corresponding author: riyanrizkifauzi.bpb3@gmail.com
Abstract 
Arun Field has been documented as one of the biggest gas field in the world. One of the unique 
formations in this giant field is the presence of Keutapang Formation which has an abnormal 
pressure zone. The presence of this zone is certainly correllated with Keutapang Formation 
characteristics. Well log data from three wells have been collected, processed, and analyzed to 
understand the physical and geomechanical characteristics of Keutapang Formation. The result of 
this study shows that Keutapang Formation consist interbedded three types of lithology (siltstone, 
shale, and sandstone). Sandstone has porosity 0.22 – 0.34, permeability 1 – 66.18 darcy, UCS 32.5-
56.42 Mpa, static and dynamic Young’s Moduly 2.48-4.03 GPa and 10.70-12.06 GPa. Interbedded 
siltstone and Shale has UCS 32.53-46.96 Mpa, static and dynamic Young’s Moduly 2.48-3.35 GPa, 
and 10.70-11.47 GPa. All of these characteristics proved that the occurrence of abnormal pressure 
zone is affected on the lithological characteristics of Keutapang Formation itself. 
Keywords: abnormal pressure, Arun Field, geomechanics, Keutapang Formation, petrophysics. 
1. INTRODUCTION
On several decades, hydrocarbon industries are always becomes the most important industries of 
Indonesian economic society. Years in the 2 decades have witnessed drastic decline in Indonesia’s 
national oil production. On the other hand, however, the same period is also characterized with 
encouraging development in natural gas production and reserves (Widarsono, 2013). 
One of the potential gas fields to be studied and developed is the Arun Field. The Arun field has 
recorded as one of the world's giant retrograde gas reservoirs (Affidick, 1994). Approximately 45 
years after production began, Arun gas field still has a lot of interesting phenomenon such as 
abnormal pressure and salt water sand zone in Keutapang Formation. The presence of this zone is 
certainly correllated with Keutapang Formation characteristics. 
This paper aims to understand the physical and geomechanical characteristics of Keutapang 
Formation including litological characteristics, petrophisical characteristics, and geomechancal 
characteristics. The research parameters are including type and litological description, porosity and 
permeability, and the value of Unconfined Compressive Strength (UCS) and young’s moduly. The 
results of this study are expected to be a reference in accordance of development in natural gas 
production and reserves. 
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2. LITERATURE REVIEW
Geology of Arun Field 
The Arun Gas field is one of the hydrocarbon field in the North Sumatra Basin, Indonesia. The 
North Sumatra Basin is a back-arc basin of Tertiary age bounded to the east by onlap onto the 
Malacca Platform and to the southwest by the Barisan Mountains uplift. The North Sumatra basin 
had been an area of active marine sedimentation for most of Tertiary times, representing a full 
marine depositional cycle (Soeparjadi, 1983). Sedimentation began with a transgressive sequence of 
coarse clastics that rests on Pre-Tertiary basement. The North Sumatra Basin was initially subject to 
Late Eocene rifting that formed the north-south horsts and grabens. A quiescent phase of basin sag, 
with widespread carbonate deposition and reef growth during the Late Oligocene and Early Miocene, 
followed the rifting (Barber, 2005). 
Figure 1. Regional Geology Map of Arun Gas Field 
During Oligocene time a thick transgressive sequence of black claystone (Bampo formation) was 
deposited, particularly in the topographic lows. The Lower Miocene was a time of maximum marine 
ingression. However, rapid subsidence had ceased and rate of deposition was minimal. Marls and 
very calcareous shales, rich in planktonic foraminifera, became the dominant sediments (Peutu 
formation). Environmental conditions were also ideal for development of reefs and other biogenic 
limestones (Arun Limestone). The period of quiescence continued through the Middle Miocene with 
the deposition of shale (Baong formation). This shale became the seal of the Arun reef and 
contributed most, if not all, hydrocarbons found in this area. The remainder of the Tertiary section 
comprises the regressive clastics of the Keutapang, Seurula, and Julu Rayeu formations in ascending 
orner, completing the depositional cycle (Soeparjadi, 1983). based on Arun-A1 well data, the 
stratigraphy of Arun Field from the oldest to the youngest as follows: 
SURFACE REGIONAL 
GEOLOGY MAP OF ARUN 
GAS FIELD 
 (Modification from Keats, 1981) 
Scale 1 : 100.000 
Created by 
Riyan Rizki Fauzi 
Universitas Padjadjaran 
Geological Engineering Faculty 
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Figure 2. Stratigraphy of Arun Gas Field (Modification from Atmadibrata, 1993) 
The object study of this research is Keutapang Formation. Keutapang Formation are were deposited 
in a rapidly shallowing basin, resulting from slowing sea level rise and the rapid influx of clastic 
sediment from the incipient Barisan uplift. The thickness of this Formation is about 404 – 1534 
meter. The sediments are predominantly deltaic and shallow marine sandstones and shales. These 
litology has a highly content of quarzt, pyrite, mica, and carbon.  
3. METHODOLOGY
The methodology of this research is analizing well log data. Well log in the term of oil and gas 
industris is a recording against depth of any characteristics of the rock formations traversed by a 
measuring apparatus in the well-bore. Well log constitute a fantastic source of information on 
subsurface geology with a vertical resolution that can even reach 1 cm. Well logging also has a 
represents cost generally between 5 and 10% of the drilling cost and that logs provide approximately 
90% of the information extracted from a well (Serra, 2003). 
The types of logging data that used in this research are gamma ray log, SP log (Spontanuous 
Potential), resistivity log, and sonic log. Gamma ray log responds to the natural gamma radiation in 
the formation. We shall first address the question of the origin of this natural radiation. The few 
isotopes which are responsible for it can be attributed to a small list of common minerals (Ellis, 
2007). The SP log is a record of direct current (DC) voltage differences between the naturally 
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occuring potential of a moveable electrode in the well bore, and the potential of a fixed electrode 
located at the surface (Doll, 1948 in Asquith, 1982). The SP log is used to identify impermeable 
zones such as shale, and permeable zones such as sand. 
Resistivity log is a measurement of a formation’s ressitivity, that is its resistance to the passage of an 
electric ccurrent. Electric log is used to determine hydrocarbon versus water bearing zones, indicate 
permeable zones, and determine resistivity porosity. The sonic log provides a formation’s interval 
transit time, designated (Δt). It is a of the formation capacity varies with lithology and rock texture, 
notably porosity (Rider 2002). 
McNally (1987) in Schon (2011) studied the correlation between the uniaxial compression strength 
(142 samples) and the compressional wave slowness (measured with a sonic logging tool) of fine to 
medium grained sandstone from the German Creek formation (Queensland, Australia). There is for 
this particular geological situation a good correlation that is probably controlled by variation of 
porosity and grain-contact quality. The resulting regression is originally as follows: 
𝜎𝜎𝑐𝑐 = 1277. exp (−0.0367.△ 𝑡𝑡)  R2 = 0.83 
𝜎𝜎𝑐𝑐= UCS value in MPa dan △t = interval transite time in μs ft-1.
Horsrud (2001) in Schon (2011) gives a detailed analysis and derived the empirical correlation 
between compressional wave velocity (dynamic) and static Young’s moduly Estat and static shear 
modulus μstat: 
………(for shale lithology) 
……..(for sand lithology) 
4. RESULT AND DISCUSSION
Physical Characteristics of Keutapang Formation 
Based on gamma ray log and SP log data from three wells in the research area, Keutapang 
Formation is composed of three types of lithology, there are sandstone, siltstone, and claystone. The 
result of electrofacies analysis shows that all of Keutapang Formation lithology makes a interbedded 
lithology which characterized by serrated cylinder shape of gamma ray log graphics. The 
sedimentation process in Keutapang Formation is influenced by sea water fluctuations (regression 
process). This regression is also characterized by serrated bell-shaped electrofacies which shows 
fining upward sequence. 
Figure 3. Gamma Ray Log Correlation From 3 
 
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The result of quantitative measurements shows that Keutapang Formation Sandstone has 
characteristics of gamma ray 20-35 API, shale volume Vsh 0.00-0.54, dan radioactive heat 
generation A 0.30-0.75 μWm-3. Keutapang Formation Siltstone has characteristics of gamma ray 35-
50 API, Vsh 0.54-0.67, dan A 0.75-0.78 μWm-3. Keutapang Formation Claystone has characteristics 
of gamma ray 50-100 API, Vsh 0.67-1.00, dan A 0.78-1.57 μWm-3. 
Table 1. Physical Characteristics From Gamma Ray Log 
Lithology 
Depth (ft) Thickness 
(feet) 
GR 
(API) Vsh 
A 
(μW/m3) Upper Lower 
Interbedded BLn & BLp 6850 6900 50 40-55 0.44-0.78 0.62-0.86 
Interbedded BLp & BLn 6900 7000 100 45-60 0.56-0.89 0.70-0.94 
Interbedded BLp & BLn 7000 7115 115 48-60 0.62-0.89 0.75-0.94 
Interbedded BLn & BLp 7115 7170 55 45-50 0.56-0.67 0.70-0.78 
Interbedded BLp & BLn 7170 7225 55 40-60 0.44-0.89 0.62-0.94 
Interbedded BLn & BLp 7225 7265 40 48-50 0.62-0.67 0.75-0.78 
Siltstone (BLn) 7265 7275 10 40 0.44 0.62 
Interbedded BLp & BLn 7275 7325 50 55 0.56-0.78 0.70-0.86 
Interbedded BLn & BLp 7325 7390 65 45-55 0.56-0.78 0.70-0.86 
Interbedded BLp & BLn 7390 7560 170 48-60 0.62-0.89 0.75-0.94 
Interbedded BLp & BLn 7560 7660 100 45-65 0.56-1.00 0.70-1.01 
Interbedded BLp & BLn 7660 7680 20 50-60 0.67-0.89 0.78-0.94 
Claystone (BLp) 7680 7725 45 45 0.56 0.7 
Interbedded BLp & BLn 7725 7750 25 30-40 0.22-0.44 0.46-0.62 
Interbedded BLn & BLp 7750 8000 250 40-50 0.44-0.67 0.62-0.78 
Sandstone 8000 8060 60 25-45 0.11-0.56 0.38-0.70 
Claystone (BLp) 8060 8150 90 45-35 0.56-0.33 0.70-0.54 
Sandstone 8150 8290 140 45-20 0.56-0.00 0.70-0.30 
Sandstone 8290 8340 50 20-40 0.00-0.44 0.30-0.62 
Interbedded BLp & BLn 8340 8355 15 25-35 0.11-0.33 0.38-0.54 
Interbedded BLn & BLp 8355 8450 95 30-40 0.22-0.44 0.46-0.62 
Interbedded BLp & BLn 8450 8555 105 40-50 0.44-0.67 0.62-0.78 
Interbedded BLn & BLp 8555 8580 25 30-35 0.22-0.33 0.46-0.54 
Interbedded BLp & BLn 8580 8600 20 40-50 0.44-0.67 0.62-0.78 
Sandstone 8600 8660 60 20-40 0.00-0.44 0.30-0.62 
Claystone (BLp) 8660 8680 20 30-45 0.22-0.56 0.46-0.70 
Interbedded BLp & BLn 8680 8710 30 30-40 0.22-0.44 0.46-0.62 
Claystone (BLp) 8710 8730 20 40 0.44 0.62 
Sandstone 8730 8760 30 25 0.11 0.38 
Based on sonic log data from well R3, The lower Keutapang Formation Sandstone has a good – 
excelent porosity (22% – 34%). This porosity value is used to determining the permeability value. 
The permeability of Lower Keutapang Formation Sandstone is k 1 – 66.18 darcy (Table 2). 
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Table 2. Porosity and Permeability of Keutapang Formation Sandstone 
Sandstone 
Depth (feet) △tlog φ Permeability (k) 
Upper Lower Average Average milidarcy darcy 
P1 7225 7750 95 30 1636.55 16.37 
P2 8000 8060 85 22 100.06 1.00 
P3 8200 8210 85 22 100.06 1.00 
P4 8220 8460 90 26 404.67 4.05 
P5 8610 8650 100 34 6618.40 66.18 
P6 8740 8760 100 34 6618.40 66.18 
Geomechanical Characteristics of Keutapang Formation 
The geomechanical characteristics of Keutapang Formation are explained by UCS and young 
moduly value. Keutapang Formation Sandstone has UCS 32.5-56.42 MPa, Estatic 2.48-4.03 GPa, and 
Edinamic 10.70-12.06 GPa. Siltstone and Claystone of Keutapang Formation has UCS 32.53-46.96 
MPa, Estatic 2.48-3.35 GPa, and Edinamic 10.70-11.47 GPa. Based on Carmichael UCS Clasification 
(1989), the strength of Keutapang Formation Sandstone is low – medium and the strength of 
Keutapang Formation Siltstone and Claystone is low. The rock strength (UCS) of Keutapang 
Formation is match with the cutting and side wall core description.  
Table 3. Geomechanical Characteristics of Keutapang Formation Sandstone and Interbedded 
Claystone-Siltstone 
Sandstone 
Depth (feet) △tlog σc Vp Young Moduly 
Upper Lower Average MPa m/s km/s Estatic (Gpa) 
Edinamic 
(Gpa) 
P1 7225 7750 95 39.09 3071.72 3.07 2.85 11.03 
P2 8000 8060 85 56.42 3418.34 3.42 4.03 12.06 
P3 8200 8210 85 56.42 3418.34 3.42 4.03 12.06 
P4 8220 8460 90 46.96 3229.17 3.23 3.35 11.47 
P5 8610 8650 100 32.53 2940.67 2.94 2.48 10.70 
P6 8740 8760 100 32.53 2940.67 2.94 2.48 10.70 
Lithology 
Depth (feet) 
(feet) △tlog σc Vp Young Moduly 
Upper Lower Average MPa m/s km/s 
Estatic 
(Gpa) 
Edinamic 
(Gpa) 
Interbedd
ed 
Claystone 
and 
Siltstone 
6840 6910 100 32.53 2940.67 2.94 2.48 10.70 
6910 7200 95 39.09 3071.72 3.07 2.85 11.03 
7200 7225 100 32.53 2940.67 2.94 2.48 10.70 
7225 7375 90 46.96 3229.17 3.23 3.35 11.47 
7375 7640 90 46.96 3229.17 3.23 3.35 11.47 
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5. CONCLUSION
Keutapang Formation consists of interbedded three types of lithology (siltstone, shale, and 
sandstone). Sandstone has porosity 0.22 – 0.34, permeability 1 – 66.18 darcy, UCS 32.5-56.42 Mpa, 
static and dynamic Young’s Moduly 2.48-4.03 GPa and 10.70-12.06 GPa. Interbedded siltstone and 
Shale has UCS 32.53-46.96 Mpa, static and dynamic Young’s Moduly 2.48-3.35 GPa, and 10.70-
11.47 GPa. All of these characteristics proved that the occurrence of abnormal pressure zone is 
effected on the lithological characteristics of Keutapang Formation itself. 
ACKNOWLEDGEMENT 
This study was supported by the Faculty of Geological Engineering, Universitas Padjadjaran, 
Indonesia. The authors are grateful to Mr Febriwan and Mr. Riza Atmadibrata for their kindly help 
during the field visits, the collection of data, and analizing the result of data. We are also wish to 
thank for Mr. Dicky and Mr. Irvan who guided us in writing and teach about geomechanical 
parameters. 
REFERENCES 
Asquith, G. B. & Gibson, C. R. 1982. Basic Well Log Analysis for Geologist. Tulsa: AAPG 
Afidick, Deddy et all. 1994. Production Performance of a Retrograde Gas Reservoir: A Case Study 
of the Arun Field. SPE-28749-MS 
Atmadibrata, R.M.R. 1993. Vertical-to-Horizontal Permeability Relationship: Arun Reservoir. SPE-
25362-MS 
Barber, A. J., dkk. 2005. Sumatra: Geology, Resources, and Tectonic Evolution. London: Geology 
Society Memoirs No.31. 
Keats et al. 1981. Geological Map of Lhokseumawe, Sumatra. Bandung: Pusat Penelitian dan 
Pengembangan Geologi. 
Ellis, Darwin V. 2007. Well Logging for Earth Scientist, 2nd Edition. Netherlands: Springer. 
Rider, Malcolm. 2002. The Geological Interpretation of Well Logs. 2nd Edition Revised. Caithless, 
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Schon, J.H. 2011. Physical Properties of Rocks: A Workbook. Handbook of Petroleum Exploration 
and Production. UK: Elsevier. 
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Soeparjadi, R. A. 1983.  Geology of the Arun Gas Field. SPE Paper No.10486. Presented at 
Offshore South East Asia Conference. Singapore. 21-24 February 1982. 
Widarsono, Bambang. 2013. National Gas Reserves and Production: An Analysis of 
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The 2nd Join Conference of Utsunomiya University and Universitas Padjadjaran, Nov.24,2017
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PHYSICAL AND GEOMECHANICAL CHARACTERISTICS OF KEUTAPANG FORMATION IN ARUN GAS FIELD, NORTH SUMATRA, INDONESIA

Fauzi, Riyan Rizki

Atmadibrata, R.M. Riza

Muslim, Dicky

Sophian, R. Irvan

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Physical and Geomechanical Characteristics of Keutapang Formation in Arun Gas Field, North Sumatra, Indonesia

PHYSICAL AND GEOMECHANICAL CHARACTERISTICS OF KEUTAPANG FORMATION IN ARUN GAS FIELD, NORTH SUMATRA, INDONESIA

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