02 Heri Hermiyanto

Jurnal Geologi dan Sumberdaya Mineral Vol.19. No.2 Mei 2018 hal 73 - 82 Geo-Resource

Jurnal Geologi dan Sumberdaya Mineral

Center for Geological Survey, Geological Agency, Ministry of Energy and Mineral Resources G e o l o g i Journal homepage: http://jgsm.geologi.esdm.go.id ISSN 0853 - 9634, e-ISSN 2549 - 4759

Rock-Eval Pyrolysis of the Oligocene Fine-grained Sedimentary Rocks from the Pamaluan Formation, Gunung Bayan Area, West Kutai Basin, East Kalimantan : Implication for Hydrocarbon Source Rock Potential Rock-Eval Pyrolisis pada Batuan Sedimen Berbutir Halus Berumur Oligosen dari Formasi Pamaluan, Daerah Gunung Bayan, Cekungan Kutai Barat, Kalimantan Timur : Implikasinya untuk Potensi Sebagai Batuan Induk

Moh. Heri Hermiyanto Zajuli and Joko Wahyudiono

Centre for Geological Survey, Geological Agency, Ministry of Energy and Mineral Resources Jalan. Diponegoro no. 57 Bandung

email : [email protected]

Naskah diterima : 29 Februari 2018, Revisi terakhir : 12 Mei 2018, Disetujui : 12 Mei 2018, Online : 07 Juni 2018 DOI: http://dx.doi.org/10.33332/jgsm.geologi.19.2.73-82

Abstract - In this study, we perform organic geochemistry Abstrak - Dalam penelitian ini, kami melakukan analisis analysis for evaluating source rocks in Gunung Bayan geokimia untuk mengevaluasi batuan induk di daerah area, West Kutai Basin. The Oligosen fine-grained Gunung Bayan, Cekungan Kutai Barat. Batuan sedimen sedimentary rock of Pamaluan Formation consists of berbutir halus yang berumur Oligosen yaitu Formasi shale, siltstone and claystone. The organic geochemistry Pamaluan, tersusun atas batuan serpih, lanau, dan batu data includes pyrolysis data as total organic carbon lempung. Data geokimia organik yang tersedia terdiri atas (TOC%), generating source potential (S2), production total organic carbon (TOC %), potensi batuan induk (S2), index (PI), oxygen and hydrogen indices(OI, HI) and indeks produksi (PI), indeks oksigen dan hidrogen (OI dan

(Tmax). The results show that the Oligocene source rocks HI), dan suhu maksimum (Tmax). Hasil dari analisis have poor into good quality with type III kerogen and have tersebut menunjukkan bahwa batuan induk berumur true capability to generate gas. The source rocks candidate Oligosen mempunyai kualitas mulai dari buruk hingga is characterized by HI 5 - 115 (mg/g), TOC from 0.19 to baik dengan tipe kerogen III yang mengindikasikan 1.78 wt%, S1 from 0.01 to 0.09 (mg/g) and S2 from 0.05 to sebagai penghasil gas. Kandidat batuan induk dicirikan 1.74 (mg/g) that indicating poor to fair source rocks with dengan HI antara 5-115 (mg/g), TOC mulai dari 0.19 type III kerogen and capable to generate gas. The hingga 1.78 wt%, S1 dari 0.01 hingga 0.09 (mg/g), dan S2 maturity parameter of the fine-grained sedimentary rocks mulai 0.05 hingga 1.74 (mg/g) yang mengindikasikan tend to indicate immature to mature stage. Overall fine- kualitas batuan induk buruk hingga baik dengan tipe grained sedimentary rocks of the Pamaluan Formation has kerogen III yang berkemampuan menghasilkan gas. Hasil a capability to produce gas with poor to fair quality. dari analisis kematangan batuan induk menunjukkan tahap belum matang – matang. Secara keseluruhan, Keyword : Oligocene, organic geochemistry, source rocks, Kutai Basin. batuan berbutir halus Formasi Pamaluan mempunyai kemampuan menghasilkan gas dengan kualitas buruk – baik. Kata Kunci : Oligosen, geokimia organik, batuan induk, Cekungan Kutai

Jurnal Geologi dan Sumberdaya Mineral - Terakreditasi KEMENRISTEKDIKTI No. 21/E/KPT/2018 Berlaku sejak Volume 17 Nomor 1 Tahun 2016 sampai Volume 21 Nomor 4 Tahun 2020 Jurnal Geologi dan Sumberdaya Mineral Vol.19. No.2 Mei 2018 hal 73 - 82

INTRODUCTION regional tectonic activities influenced the formation of Kutai Basin. Background Like most basins in western Indonesia, Kutai Basin began This research is located in the Gunung Bayan area, West to emerge in Middle Eocene. According to Van der Weerd Kutai Basin, East Kalimantan (Figure 1). This study & Armin (1992), Kutai Basin formed as an extensional discusses about organic geochemistry for evaluating basin which appeared in the Middle Eocene. This is source rocks of fine-grained sedimentary rocks. Organic slightly different from the Bachtiar (2004) opinion which geochemistry is used as fundamental approach to state that the Kutai Basin is an aulacogen derived from understand the properties of source rocks and to failures in the expansion phase of the continent (rifting) at determine the productive and non-productive zones as Middle Eocene. Subduction accretion formation occured well as oil migration and development of oil fields. The during Jurassic and Cretaceous. The west and northwest term source rock refers to an organic-rich fine-grained Borneo basement is an accretion prism containing sedimentary rock which can produce hydrocarbons due metasediment, metavolcanic rocks, and material of to thermal maturation. Source rock is one of the main magmatic and amphibole. elements of a hydrocarbon system. The objective of this research is to assess hydrocarbon source potential of The Kutai Basin is the largest (165,000 km) and deepest fine-grained sedimentary rocks of Pamaluan Formation (12.000 to 14.000 meters) Tertiary sedimentary basin in in Gunung Bayan areas. The Pamaluan Formation has Indonesia. This basin is bounded by Mangkaliat High to thick flakes that are suitable for shale hydrocarbon. the north, hinges of Adang-Flexure (Adang-paternoster Furthermore, the formation was deposited in Oligocene fault) to the south, Kuching High - part of Borneo Central that maturity has a sufficient level of maturity. Range to the west, and Makassar Strait to the east (Darman and Sidi, 2000). GEOLOGICAL SETTING The Kutai Basin consists of Paleocene alluvial Kutai Basin is located at the southeast of Sundaland and sediments of the Haloq Kiham Formation, close to the influenced by three world major active tectonic plates: western boundary (Figure 2). Subsidence occurred in

the Eurasian, Indo-Australian, and Pacific. This various late Paleocene – Oligocene due to basement rifting and 111° E 114° E 117° E 120° E ▲ 123° E

( SULU SEA ▲ ▲ Sabah 6° N Sabah Crocker Sandakan ▲ SOUTH CHINA Group Cotabato SEA ▲ Baram Dent-Samarinda High RANGES T rench East Natuna ▲ SULAWESI SEA AN

Balingian High 3° N Tarakan

Berau

KALIMANT ▲

West

North Sulawesi▲ T

Natuna Embaluh Sikatak rench ▲

Group ▲ Sangihe Lupar Line Mangkaliat ▲

CENTRAL Arc

Kuching T

Ketungau High rench ▲

0° Melawi Kutei anticlinorium North Makassar Adang Fault ADANG ▲ Samarinda ▲ Schwaner Trench ▲ Core ▲ Buntok LUP Palu-Koro Fault Batul AR MEGA

Barito SHEAR Pembuang Sula-Sorong Fault

3° S Paternosfer ▲ W micro- alanae Fault

SUNDALAND continent ▲ ▲

Asem - Asem South

Research areas Makassar ▲

JAVA SEA ▲ Basin

Source : Modified from Pertamina BPPKA, (1999) Figure 1. Research area on Gunung Bayan, West Kutai Basin, East Kalimantan.

Rock-Eval Pyrolysis of Oligocene Fine-grained Sedimentary.... ( M.H.H. Zajuli & J. Wahyudiono)

Source : Satyana et al., (1999). Figure 2. Stratigraphic coloumn of the Kutai Basin, East Kalimantan deposition of Mangkupa Shale in the edge until open composed by local interbedded calcareous mudstone, marine environment. Coarse siliciclastic sand in clay quartz sandstones and tuffaceous siltstone, and sequence indicate an interruption by lifting in this area. intercalated by fossilized limestone. In the northern part The basin subsidence continued by sagging phase that of Kutai Basin, the Wahau Formation consists of produced clay and carbonate deposit of Kedango Atan interbedded of mudstone, sandstone, and silty Formation (Satyana and Biantoro, 1995). sandstones. The Lembak Formation comprised interbedded of marl and limestone that deposited in the Tectonic uplift occurred during subsidence in Late eastern part of Cape Mangkaliat. The Mau Formation Oligocene associated with Sembulu volcanic sediment consists of siltstone, claystone and sandstone deposited in in eastern part of the basin. The next phase inversion the southern part of Kutai Basin. In the southern bank of began with the lifting during Early Miocene. Series of Kutai Basin, there was deposited of the Oligo-Miocene alluvial deposition and a very comprehensive deltaic Pamaluan Formation which consist of sandstones with Pamaluan, Pulubalang, Balikpapan, and Kampung Baru mudstone, shale, siltstone, coal and limestone. Interclast Formation prograded to the east on early Miocene to distribution of this formation is very widespread along the Pleistocene. The deposition of the delta has continued southern to the northern edge of Kutai Basin. eastwards to the Kutai Basin offshore until this day The Bebulu Formation consists of limestone reefs (Satyana and Biantoro, 1995). deposited in the Early Miocene. The Bebulu Formation West Kutai Sub-Basin comprised the Oligo-Miocene interfingers with the top Pamaluan and Maau Formations. Purukcahu Formation which consist of claystone with The Pulaubalang Formation was deposited with thin sandstones and coal inserts intercalations. This sufficient distribution area ranging from the south edge of formation is interfingering with Keramuan Formation central part of the Kutai Basin in the Middle Miocene. Jurnal Geologi dan Sumberdaya Mineral Vol.19. No.2 Mei 2018 hal 73 - 82

The formation is composed by quartz sandstones and focuses on the stratigraphic analysis of the Pamaluan greywacke, claystone with interbedding of limestone, Formation by measuring section (MS) method using tuff, and coal. The Maluwi Formation consists of geological compass and global positioning system limestone, marl, and sandstone with intercalation on the (GPS). Each location (Figure 3) was selected bottom; claystone and sandstone, with intercalation of considered the most representative section, which was marl, carbonaceous shales and limestones deposited in supported by rock samples collecting for laboratory the upper part in the north border of Tarakan Basin during analysis such as total organic carbon (TOC) and rock- Middle Miocene. eval pyrolysis. Rock-eval pyrolysis and total organic carbon (TOC) were performed at Lemigas Laboratories The Pulaubalang Formation is composed by interbedding by following the standard procedures. of quartz sandstone, mudstone and shale, with inserts marl, limestone and coal. It was conformably deposited with the Balikpapan Formation. The Menumbar RESULT Formation consists of calcareous mudstone with Stratigraphic limestone at the bottom and massive sandstone at the top containing glauconite and cross-bedding structure The Pamaluan Formation in Gunung Bayan area is deposited in the northern part during the Late Miocene. composed by sandstone, shale, claystone, coal, and siltstone (Figure 4 and 5). Sedimentary rock in the The volcanic activity in the Miocene resulted the research area is interpreted as a lower part of the Meragoh Formation volcanic rocks consisting of lava, Pamaluan Formation. tuff, volcanic breccia, and agglomerates where the composition is structured basalt until andesite. The Source Richness volcanic rock source is estimated derived from the The results of TOC analysis on thirty samples of fine- Meragoh Mountain and distributed on the western edge grained sedimentary rocks from Pemaluan Formation of Kutai Basin. indicate that the shale, siltstone, and claystone have The Kampung Baru Formations was deposited in TOC range 0.38% - 1.78 %, 0.19% - 0.77%, and 0.57% Pliocene, on the southern edge of Kutai Basin. The - 0.82%, respectively (Table 1). Kampung Baru Formation is non- conformably deposited above the 115° 56' 0" E 115° 58' 0" E 116° 0' 0" E 116° 2' 0" E Balikpapan Formation which consist of sandy mudstone, quartz sandstone, siltstone * 15MH07 with intercalation of coal, marl, limestone 15MF01 and lignite. Interbedding of coal and lignite 0 0.5 1 2 3 4 Kilometers have a thickness of less than 3 m. At the same 15MF04 time in the northern edge of Kutai Basin, there was unconformable deposition of the Golok Formations above the Menumbar Formation. Furthermore, the Machete Formation consists of an insert marl clay, marly limestone, molluscs and coal material 15MF02 of yellowish gray to brown colour. Deposition event in the Kutai Basin ended 15MF08 by the volcanic activity in Pliocene resulting in the Mentulang volcanic rock units which consist of andesite, basalt, lava, lava breccia, tuff, agglomerate, breccia lava with andesite to basalt composition. 0° 36' 0" S 34' 32' 30' 0° 36' 0" S 34' 32' 30'

METHODS

Specific field geological investigation and 115° 56' 0" E 115° 58' 0" E 116° 0' 0" E 116° 2' 0" E laboratory analysis were carried out to achieve the aims of the study. The study Figure 3. Locality of sampling for laboratory analysis in Gunung Bayan area. Rock-Eval Pyrolysis of Oligocene Fine-grained Sedimentary.... ( M.H.H. Zajuli & J. Wahyudiono)

15 MH 07 15 MH 07

TION TION NOTES NOTES AGE OTHERS AGE OTHERS MUD SAND GRAVEL MUD SAND GRAVEL

SCALE (m)

LITHOLOGY

SCALE (m)

LITHOLOGY

FORMA vf m vc

FORMA vf m vc gran pebb cobb bout clay silt gran pebb cobb bout clay silt f c f c

Sandstone, grain size is fine to - Coal, black, banded, strike dip BB BX 0 - medium, hard, medium sorted, open , N 2410 E /44. subangular to sub 20 - rounded grains bedding structure, BV 1 - comprising silica - mineral, quartz, felsdpar, silica BU Shale, grey, bedding, carbon streak, BA - cementation, thickness of 1,8 meters 21 - - Ferri Oxide. 2 - BT - 22 - - 3 Shale, brownish black,hard, clay grain size,bedding, flake structure, - Shale, grey, clay grain size, - total of thickness 4 meters, carbon bedding, carbon streak, lamination AZ streak. 23 - in the middle part. - BS 4 - Sandstone ligth grey,very fine - grain size consist of quartz, feldspar 24 - - BR 5 - - AY 25 - Siltstone, grey, clay grain size, - AX - 6 - 26 - - Sandstone, reddish grey, grain size is fine to medium, hard, medium - 7 sorted, open , subangular to sub Sandstone reddish ligth grey, fine - grain size, laminaton consist of AW rounded, intercalated by siltstone 27 - and ferri oxide layer, comprising quartz, feldspar - silica mineral, quartz, felsdpar,silica cementation, thickness of 1,8 m. - 8 - 28 - - - BQ 9 - 29 - Sandstone dark grey,fine grain Shale, grey, clay grain size, AV - BP carbon streak Coal black,dull banded,in the lower BO - consist of coally shale. BN 10 - BM Coaly shale, blackish brown, papery AU 30 - Sandstone very fine grain size BL - consist of volcanic material BK Shale, grey, clay grain size, AT - carbon streak - 11 31 - Coaly shale, blackish brown, papery AS - - Coal brigth banded, black, layered BJ contains resin, hard, thickness of AR - 1.25 m. strike/dip N 247 o E/ 42o 12 32 - Shale, ligth grey,hard, clay grain - AQ size,bedding, flake structure, - Shale, grey, clay grain size, total of thickness 5.5 meters, carbon carbon streak filling. - streak. BI 13 - - 14 BH - - 15 - BG Sandstone reddish grey,very fine BF - grain size consist of oxide BE 16 - - B D2

17 Claystone, grey, clay grain size, - bedding, carbon streak, lamination in the middle part. - Samples for laboratory analysis B D1

18 - Coally shale, blackish grey. BC

19 Figure 4. Stratigraphic section of the upper part Pamaluan Formation In Gunung Bayan area. Jurnal Geologi dan Sumberdaya Mineral Vol.19. No.2 Mei 2018 hal 73 - 82

15 MH 07 15 MH 07

TION NOTES TION NOTES AGE OTHERS AGE OTHERS MUD SAND GRAVEL MUD SAND GRAVEL

SCALE (m) SCALE (m)

LITHOLOGY LITHOLOGY

FORMA FORMA vf m vc vf m vc gran pebb cobb bout gran pebb cobb bout clay silt clay silt f c f c

45 - Sandstone reddish ligth grey, medium grain size, consist of - Shale, grey, clay to siltstone grain AB - quartz, feldspar size,carbon streak . 63 - 46 - AA - - 64 - Z Sandstone reddish ligth grey, 47 - AP coarse grain size, laminaton - consist of quartz, feldspar - Y 65 - 48 - - x - 66 - W 49 - - - V 67 - 50 - Shale, grey, clay to siltstone grain - size,carbon streak . U - Sandstone reddish ligth grey, AO 68 - 51 - coarse grain size, laminaton T consist of quartz, feldspar - - 69 - S 52 - - - R 70 - 53 - Q - - 71 - P 54 - - Shale, grey, clay to siltstone grain O - size,carbon streak . 72 - 55 - N Sandstone reddish ligth grey, AN coarse grain size, laminaton - - consist of quartz, feldspar M 73 - Ferri oxide. 56 - Sandstone very fine grain size, - carbon streak, quartz, feldspar. - ferri oxide AM Shale, grey, clay to siltstone grain L AL 74 - size,carbon streak . 57 - Alternating of sandstone and shale AK - - sandstone grey, medium grain size, l K shale grey carbon streak AJ 75 - Coal, black, banded, consist of resin 58 - AI - J - AH Sally coal, black, papery 76 - Sandstone ligth grey, medium to 59 - coarse grain size, cross laminaton Shale, grey, clay to siltstone grain consist of quartz, feldspar - I - size, lamination carbon streak . 77 - 60 - AG - H Sandstone ligth grey, fine to medium AF - grain size, cross laminaton consist G of quartz, feldspar, carbon streak 78 - Shale, grey, clay to siltstone grain size,lamination, carbon streak . 61 - AE Alternating of shale and sandstone - - ligth grey, fine grain size, consist E of quartz, feldspar, carbon streak AD 79 - AC Coal, black, brigth banded, consist D 62 - of resin - sally coal C Coal, black, brigth banded, consist 80 - B Samples for laboratory analysis of resin, - sally coal A

Figure 5. Stratigraphic section of the lower part of Pamaluan Formation in Gunung Bayan area Rock-Eval Pyrolysis of Oligocene Fine-grained Sedimentary.... ( M.H.H. Zajuli & J. Wahyudiono)

Table 1. Geochemical data the from fine-grained sedimentary rocks of Pamaluan Formation

TOC S1 S2 S3 PY Tmax No. Sample ID Sample Type General Lithology Description PI PC HI OI (%) mg/g (oC) 1 15MH7E OC Shale, lt grey,soft, oxidated, Non Calc 0.72 0.04 0.34 0.19 0.38 436 0.11 0.03 47 26 2 15MH7G OC Shale, lt grey,hard, oxidated, Non Calc 0.78 0.02 0.27 0.17 0.29 434 0.07 0.02 35 22 3 15MH7H OC Shale, lt grey,hard, oxidated, Non Calc 1.09 0.04 0.46 3.31 0.50 439 0.08 0.04 42 303 4 15MH7O OC Shale, lt grey,hard, oxidated, Non Calc 0.85 0.05 0.46 1.05 0.51 440 0.10 0.04 54 124 5 15MH7R OC Shale, lt grey,hard, oxidated, Non Calc 0.93 0.01 0.05 0.06 542 0.17 0.00 5 6 15MH7X OC Shale, lt grey, soft, oxidated, Non Calc 1.05 0.07 0.62 0.55 0.69 437 0.10 0.06 59 52 7 15MH7AB OC Shale, brownish grey, soft, Non Calc 0.64 0.05 0.42 1.41 0.47 435 0.11 0.04 66 220 8 15MH7AU OC Shale, lt grey, soft, oxidated, Non Calc 1.78 0.05 1.74 0.24 1.79 434 0.03 0.15 98 13 9 15MH7AZ OC Shale, lt grey, soft, oxidated, Non Calc 0.84 0.02 0.41 0.89 0.43 436 0.05 0.04 49 106 10 15MH07 AQ OC Shale, lt grey, soft, Non Calc 0.40 0.02 0.17 0.14 0.19 438 0.11 0.02 42 35 11 15MH7BD2 OC Claystone, lt grey, soft, oxidated, Non Calc 0.66 0.02 0.35 0.62 0.37 438 0.05 0.03 53 94 12 15MH7BH OC Claystone, lt grey, soft, oxidated, Non Calc 0.57 0.03 0.28 1.85 0.31 436 0.10 0.03 49 323 13 15MH7BW OC Shale, lt grey, soft, oxidated, Non Calc 0.72 0.09 0.84 0.21 0.93 429 0.10 0.08 116 29 14 15MF2B OC Shale, dk grey, soft, oxidated, Non Calc 0.75 0.07 0.53 0.54 0.60 445 0.12 0.05 71 72 15 15MF2J OC Shale, dk grey, soft, oxidated, Non Calc 0.52 0.03 0.26 0.18 0.29 446 0.10 0.02 50 35 16 15MF2T OC Shale, lt grey, soft, oxidated, Non Calc 0.93 0.07 0.58 0.55 0.65 445 0.11 0.05 62 59 17 15MF2W OC Shale, lt grey, soft, oxidated, Non Calc 1.15 0.08 0.57 0.15 0.65 434 0.12 0.05 49 13 18 15MF2Y OC Shale, lt grey, soft, oxidated, Non Calc 1.17 0.09 0.55 0.18 0.64 436 0.14 0.05 47 15 19 15MF2AB OC Shale, dk grey, soft, oxidated, Non Calc 0.62 0.05 0.38 0.23 0.43 441 0.12 0.04 61 37 20 15MF2AS OC Siltstone, lt grey, soft, oxidated, Non Calc 0.77 0.06 0.37 0.09 0.43 441 0.14 0.04 48 12 21 15MH02 OC Shale, lt grey, soft, oxidated, Non Calc 1.03 0.02 0.37 0.26 0.39 437 0.05 0.03 36 25 23 15MF08 OC Shale, brownish grey, soft, oxidated, Calc 0.62 0.02 0.34 0.57 0.36 441 0.06 0.03 55 93 24 15MH06 OC Shale, lt grey, soft, oxidated, weathered, Non Calc 1.55 0.05 0.80 0.79 0.85 424 0.06 0.07 51 51 25 15MH04 OC Shale, lt grey, soft, oxidated, Non Calc 1.06 0.08 0.93 0.39 1.01 437 0.08 0.08 88 37 26 15MH01 OC Shale, dk grey, soft, oxidated, Non Calc 1.25 0.02 0.63 0.26 0.65 435 0.03 0.05 50 21 27 15MF01 OC Claystone, lt grey, soft, oxidated, Non Calc 0.82 0.07 0.41 0.13 0.48 446 0.15 0.04 50 16 28 15MF04 OC Shale, lt grey, soft, oxidated, Non Calc 0.72 0.08 0.27 0.08 0.35 458 0.23 0.03 37 11

Remarks : HI : Hydrogen Index = (S2/TOC) x 100 PC : Pyrolysable Carbon

S1 : Amount of free hydrocarbon PY : Amount of Total Hydrocarbons = (S1 + S2) OC : Outcrops Tmax : Maximum Temperature ( oC) at the top of S peak S2 : Amount of Hydrocarbon released from kerogen PI : Production Index = (S1/ S1 + S2) TOC Total Organic Carbon OI : Oxygen Index = (S 3/TOC) x 100 S3 : Organic Carbondioxide :

Based on the TOC analysis result, fine-grained Type of Kerogen sedimentary rocks tend to indicate a poor to good The initial genetic type of organic matter of a particular quality source rock. source rock is essential for predicting oil or gas production potential. Waples (1985) used the hydrogen Maturity index values (HI) to differentiate types of organic matter. Hydrogen index value <150 mg/g indicate a potential Thermal maturity can be indicated by Tmax from source for generating gas (mainly Type III kerogen). pyrolysis data. Maximum temperature (Tmax) of the shale, siltstone, and claystone shows varying values 424 Source rock with hydrogen index values between 150 - 542C, 441C, and 436 - 446C, respectively. The shale and 300 mg/g defined more as a Type III kerogen (gas tends to indicate as an immature until mature stage, generation) instead the type II kerogen (oil generation). while one sample indicates over mature (15 MH 07R). Source rock with hydrogen index value >300 mg/g The siltstone and claystone tend to indicate early mature contains a substantial amount of Type II organic matter until mature stage. and thus is considered to have a good potential for generation of oil and minor gas. Kerogen with hydrogen The hydrogen index (HI) for shale, siltstone, and index value >600 mg/g usually consist Type I or Type II claystone shows result 5-115, 48, 49-53, respectively. kerogen; it has excellent potential to generate oil.

Based on the HI / Tmax plot (Figure 6), the maturity level suggested as immature until mature categories with Based on TOC versus S2 plot, kerogen in the present minor over mature (Figure 6). study is suggested as Type III (Figure 7). The diagram also reflects that all of the samples tend to indicate as a According to Figure 6, the shale, siltstone, and kerogen Types III. Therefore the studied fine-grained claystone tend to indicate gas prone source rock sedimentary rocks from Pamaluan Formation have the especially condensate to wet gas zone. opportunity to produce gas. Jurnal Geologi dan Sumberdaya Mineral Vol.19. No.2 Mei 2018 hal 73 - 82

1000 0,55% Ro also a non-marine especially the lacustrine facies which Shale Claystone occurred more frequently (Zhang et al., 2008 in Ju et al., WINDOW 900 HIGHLY OIL Siltstone 2014). OIL PRONE According to Widayat (2013), the availability of carbon 800 matter is strongly influenced by water fluctuations. Water level fluctuations can induce the water conditions 700 become oxic, anoxic, or sub-oxic. Anoxic condition is a

OC) T condition which organic materials are formed more 600 OIL PRONE abundant rather than the oxic condition. For example

500 only one – third shale gas resources in North America generated from marine environment shale, while the

400 TURE other two-third derived from transition and terrestrial IMMA environments, including lacustrine (Zhang et al., 2012). 300 On the other hand, according to El Nady et al, (2015) the MIXED WINDOW

Hydrogen Index (mg oil/g OIL AND GAS OIL plot of S1 versus TOC can be used to differrentiate the

200 GAS ZONE non indigenous (allochthonous) with indigenous

TE-WET hydrocarbons (autochthonous). In conclusion, the PRIMARILY 1,00% Ro 100 GAS PRONE studied fine grained sedimentary rocks tend to indicate CONDENSA 1,40%Ro Y GAS ZONE DR the autochthonous (indigenous hydrocarbons) (Figure 8). 0 330 380 430 480 530 580 Tmax ( oC) Hydrocarbon Potential

Figure 6. Hydrogen Index (HI) versus Tmax diagram shows The generation potential of a source rock is identified kerogen type of sedimentary rock from the Pamaluan using the results of pyrolysis analysis. The generation Formation.

40 Shale Kerogen Type I Kerogen Type II Kerogen Type II/ III Claystone 35 (Oil prone/Lacustrine) (Oil prone/marine) (Oil/gas prone/marine) Siltstone

25 Kerogen Type III (Gas prone) 20

10 lean organic

Generation potential (S2 mg/g) (S2 potential Generation

5 Dry gas generation

0 0 2 4 6 8 10 12 14 16 18 20 Total organic carbon (TOC, wt.%)

Figure 7. TOC vs S2 diagram shows the kerogen type and possibility to produce oil or gas.

Origin of Organic Matter potential (GP) is the sum values of S1 and S2. According The kerogen type of the shale, siltstone, and claystone to Hunt (1996), source rocks with GP <2, 2 to 5, 5 to 10 samples shows that the organic material is composed by and >10 are considered to have a poor, fair, good, and, very a terrestrial environment that lack in fatty or waxy good generation potential, respectively. The relationship component (Waples, 1985). The depositional between (S1+S2) and TOC can be found in Figure 9. environment of Kutai Basin is generally terrestrial to As seen in Figure 10, the plot of TOC (wt%) versus HI transitional (deltaic). An organic matter richness on mg/g reflects that the shale tends to be potentially as a shale is not only derived from marine environment, but gas producer or less oil producer. Rock-Eval Pyrolysis of Oligocene Fine-grained Sedimentary.... ( M.H.H. Zajuli & J. Wahyudiono)

100 Pamaluan Formation : 10000 Pamaluan Formation : Shale Shale Claystone Claystone Siltstone Siltstone

Very good Oil 10

OC) Good Oil 1000

Allochthonous Fair Oil

Gas or oil source S1 (mg/g) S1

100 Autochthonous Gas source 0.1

Hydrogen Index (mg oil/g T Non Source Non

0.01 10 0.1 1 10 100 0.1 1 10 100 TOC (wt%) TOC (wt%) Source : modified from El Nady.et.al., (2015). Source : modified from El Nady.et.al., (2015) Figure 8. Origin of organic matter in fine-grained sedimentary Figure 10. Plot of TOC versus HI that showing the producing rocks formation. type of fine grained sedimentary rocks of Pamaluan Formation.

1000 Pamaluan Formation : Shale TOC and rock–eval pyrolysis data, such as S1 and S2. Claystone Siltstone Excellent Based on the organic matter richness and depositional

100 environmental of the fine-grained sedimentary rocks of ield (PY) ield

Y the Pamaluan Formation, there is a relationship between depositional environment with an organic matter Very Good 10 richness in the research areas. Deltaic system of fine Good grained sedimentary rocks is defined as a transitional

Fair environment that correlate to the kerogen Type III in the research area.

1 S1 + S2 (mg/g) /Potential /Potential (mg/g) S2 + S1 Poor The plot of S1 versus TOC shows that the sedimentary rocks of Pamaluan Formation can be determined as an

0.1 indigenous hydrocarbons (autochthonous). Source rock 0.1 1 10 100 potential rocks from the Pamaluan Formation have a TOC (wt%) poor to fair quality, so that the majority produced gas Source : modified from El Nady.et.al., (2015). from Oligocene source rocks sample assumed not Figure 9. Plot of TOC versus PY that showing the potential of migrated from another source rock. fine-grained sedimentary rocks of Pamaluan Formation. Figure 9 shows a positive correlation between Potential Yield (PY) and Total Organic Carbon (TOC). The higher TOC content results the higher value of PY (S1 + Subsequently, the siltstone and claystone concluded as a S2). Water level change during depositional event have gas producer. Therefore, the fine-grained sedimentary influenced the fine grained sedimentary rocks of rocks of Pamaluan Formation tend to indicate as gas Pamaluan Formation quality for hydrocarbon resources source rocks and one sample as fair oil source rocks. in Gunung Bayan Areas.

DISCUSSION CONCLUSIONS The organic carbon richness of the rock samples (TOC Based on the maturity level, source rocks in the %), is important in evaluating sediments as a source for Pamaluan Formation have potential for generating gas petroleum generation. Tissot and Welte (1984), Peters with a poor to fair quality. Moreover, all samples are and Cassa (1994), and Peters (1988) presented a scale for assumed to have potential as source rocks for the assessment of source rocks potential, based on the % Jurnal Geologi dan Sumberdaya Mineral Vol.19. No.2 Mei 2018 hal 73 - 82

hydrocarbon especially the shale. In general, the prone. This condition is also presumed to indicate as a kerogen type is dominated by Type III. low gas potential. The majority of rocks in the Pamaluan Formation are located at gas prone zone specifically poor to fair level. ACKNOWLEDGEMENTS The fine-grained sedimentary rocks of Pamaluan The authors express their gratitude to PT Gunung Bayan Formation, analyzed from the shale, siltstone, and who have allowed author to attain the data and stay on claystone, appear to be more gas prone rather than oil coal site.

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