Indonesian Journal on Geoscience Vol. 4 No. 3 December 2017: 121-141

INDONESIAN JOURNAL ON GEOSCIENCE Geological Agency Ministry of Energy and Mineral Resources

Journal homepage: hp://ijog.geologi.esdm.go.id ISSN 2355-9314, e-ISSN 2355-9306

Fracture Characteristics of Mélange Complex Basement in Bantimala Area, South Sulawesi, Indonesia

Benyamin Sapiie, Muhamad Aziz Nugraha, Rizky Kurniawan Wardana, and Arif Rifiyanto

Geology Study Program, Faculty of Earth Sciences and Technology, Institut Teknologi Bandung Jln. Ganesha No. 10, Bandung 40132, Indonesia

Corresponding author: [email protected] Manuscript received: June 8, 2017; revised: June 22, 2017; approved: July 3, 2017; available online: August 1, 2017

Abstract - A detailed geological mapping and fracture characterization had been performed in Bantimala area, South Sulawesi, Indonesia. The geology of the studied area is composed of pre-Tertiary metamorphic, sedimentary, and igneous rocks which tectonically mixed forming a mélange complex. Located on the southeastern margin of Sundaland, the tectonic strongly influences the fracture occurrences in the studied area. A total of 3,841 fractures comprising shear fractures, extension fractures, veins, and joints have been measured and analyzed. The common fracture orientations are NW - SE, W - E, NNE - SSW, and ENE - WSW trends. Fractures developing in Bantimala have clearly been controlled by lithology and structure position (i.e. fault zones and fold hinge). The orientation of fractures in Bantimala area is different on each lithology, showing that the fracture system was complex. Fracture intensity in schist is higher compared to the other lithologies. The 3D fracture modeling through 3D geocellular modeling was generated using the result from field data measurements and analyses. Discrete Fracture Network (DFN) was built by fifty-one fracture sets that were analyzed from field measurement data. However, the estimation of average fracture porosity from modeling varies significantly depending on lithology. The value of fracture porosity is relatively small, varied from 0.0004 to 0.0029 %. A high fracture porosity number is observed in an area with a significant fracture intensity and most crosscutting of fracture which in turn is controlled by faults and lithology. A mélange complex can have high potential as a basement fractured reservoir target, where fracture distributions and their attributes will vary depending on the lithology as well as local deformation. Keywords: Bantimala Mélange Complex, basement fractured reservoir, Discrete Fracture Network, 3D Geocellular Modeling © IJOG - 2017. All right reserved

How to cite this article: Sapiie, B., Nugraha, M.A., Wardana, R.K., and Rifiyanto, A., 2017.Fracture Characteristics of Mélange Complex Basement in Bantimala Area, South Sulawesi, Indonesia. Indonesian Journal on Geoscience, 4 (3), p.121- 141. DOI: 10.17014/ijog.4.2.121-141

IJOGet al Introduction 1989; Nelson, 2001; Sapiie ., 2014). A recent oil and gas exploration in the offshore of the north- Naturally, a fractured reservoir is proven as ern Madura Island discovered gas within the base- a productive reservoir in numerous oil fields. A ment rocks. The basement lithology was made of basement is one of lithologies that performs as a various rocks which are interpreted as part of the naturally fractured reservoir. A number of frac- Cretaceous accretionary complex developed as tured basement reservoirs have been discovered mélange. Mélange complexes are widely distrib- in the world (Koning, 1985; Koning and Darmono, uted in the Indonesian archipelago but exposed in

AccreditedIJOG/JGI by:(Jurnal - LIPI, Geologi valid AugustIndonesia) 2016 - -Acredited August 2021 by LIPI No. 547/AU2/P2MI-LIPI/06/2013. valid 21 June 2013 - 21 June 2016 - RISTEKDIKTI, valid May 2016 - May 2021 121 Indonesian Journal on Geoscience, Vol. 4 No. 3 December 2017: 121-141 a few localities (Figure 1). However, until recent been understood. Bantimala area is one of well- years, fractured mélange basements have not well known places where the pre-Tertiary Mélange

106o E 110 o E 114 o E 118 o E 122 o E 126 o E 130 o E o N o 8 N Sulu Sea 8 N South China Sea

Cotabato Trench Sandakan Sabah Sulu Trench delta North West Borneodelta Trough 0 200 km SABAH Baram Philippines Trench Luconia Shoals delta

o o 4 N Balingian BRUNEI Celebes Sea 4 N Natuna delta Rajang river SARAWAK North Sulawesi Trench mouth Mangkalihat Peninsula Halmahera Semitau Ridge Muller Mts. Molucca Sea 0o Gorontalo 0o Mahakam Tomini Bay Bay SUNDALAND Schwaner Delta Mountains Sula Fault KALIMANTAN Karimata MahakamStraits Banggai-Sula Seram ‘Trough Block Straits Meratus SULAWESI Complex

o 4 S Bone Seram 4o S Patemoster Gulf Buru SUMATRA Platform Banda Sea

Bantimala Tukang Besi-Buton Block Luk Ulo Complex JS-1 RidgeSakala Fault Complex JAVA WMO

o 8 S 8o S

Sumba

Australian Continent Java Trench Timor Trough 114o E 118 o E 122 o E 126 o E 130 o E

Kalimantan West and North Sulawesi Volcano-Plotonic Arc East Sulawesi Ophiolite Belt Mesozoic/Early Tertiary deep marine sediments, Quaternary sediments Neogene and Quaternary sediments mafic igneous rocks and melange Mostly Mesozoic igneous, metamorphic, and Cenozoic volcanics and plutonic rocks Ophiolite sedimentary rocks with some Late Paleozoic and Early Cenozoic rocks Tertiary carbonates Banggai-Sula and Cretaceous granites and tonalites Tukang Besi Continental Fragments intruding Paleozoic metamorphic rocks Tertiary sediments Continental basement and cover Mesozoic or younger metamorphic and Early Cretaceous volcanic arc and ophiolite sequence ultramafic basement complex Continental basement below sea level Ophiolite basic/ultrabasic units Central Sulawesi Methamophic Belt Cretaceous granites Ophiolite Melange Continental and microcontinental HP metamorphic rocks (Pompangeo schists) West Madura Offshore

Tectonic Element Major Structure Subduction zone - Surface trace of Benioff-Wadati Seismic Zone. Fault and Fracture Zone a dipping surface ofIJOG earthquake foci believed as surface of infiltrating lithosphere, Undifferentiated serration morphology show the top of crust Normal fault (dip slip) Transform fault - The principal fault at strike-slip plate boundaries; Lateral fault (strike-slip, wrench fault) which pulled down underneath the plate boundary that covered by crust Thrust fault (low angle)

Spriding axis or ridge Reverse fault (high angle)

Figure 1. Tectonic and location map of studied area at Bantimala Complex, South Sulawesi, showing structural pattern and tectonic units. West Madura Offshore is an oil and gas field where well penetrates basement with similar rock types exposed at Bantimala area (modified from various sources).

122 Fracture Characteristics of Mélange Complex Basement in Bantimala Area, South Sulawesi, Indonesia (B. Sapiie et al.)

Complex basement is exposed along river beds. fracture characteristics of this kind of litho- The area lies in the south arm of Sulawesi in the logy, the study involves fieldwork activity and southeastern margin of Sundaland. The Bantimala subsurface modeling as well. Several stages of Mélange Complex is excellent for understanding this research are as follows. a mélange complex basement reservoir since it is well exposed in a river in Pangkep-Bone-Barru Geological Mapping Regency, South Sulawesi. The geological mapping covered an area of The study was concentrated in Bantimala and about 560 km2 (20 km x 28 km) in the Bantimala surrounding areas. Administratively, the area area. The objectives of the geological mapping are includes Pangkajene and Kepulauan (Pangkep), to convey information of the structure and stra- Bone, and Barru Regencies in South Sulawesi tigraphy within the area. It is also to determine Province, Indonesia. In the UTM coordinate sys- where the most suitable areas are for conducting tem, the studied area is located in the zone of 50S the scan line sampling in the studied area. At this 9460700 - 9488700 mN and 789000 - 809000 mE. stage, the primary data were collected from the The Bantimala Complex is a pre-Tertiary mélange field observation while satellite images and litera- tectonic complex, and also performs as basement tures from previous researchers were also used rocks in the south arm of Sulawesi. According as secondary data. Laboratory activities such as to its location that has strongly been affected by petrographic and micro-paleontology analyses younger tectonics, the basement is well predicted had also been conducted to support the geologi- to be the home of high intensive fractures. The cal interpretation. The analysis of fracture data is main purpose of this study is to understand the needed to know the characteristics of the fracture fracture distributions and characteristics in the system in the studied area. The interpretation basement outcrop of mélange complex rocks. In was conducted to investigate factors controlling addition, this study will develop a fracture model the fracture distribution in the mélange complex and characterize the relationship between lithology basement (e.g. lithology, structure). and fracture distribution in the mélange complex basement. Scan Line Sampling for Fracture Character- Furthermore, the result of the study hopefully istics can be used as an analog model for basement frac- The scan line sampling was employed in the tured reservoir exploration activity within the oil Bantimala Mélange Complex and some Creta- field in West Madura Offshore (WMO) at the East ceous to Tertiary rocks. There were forty-seven Java Basin (Figure 1). The regional reconstruction locations of scan line which was successfully shows that both Bantimala and WMO areas are conducted during the study. Fracture data were located on the southeastern margin of Sundaland collected by the 1D scan line method. The data and share a similar geological setting, particularly that were observed using the scan line sampling during the Cretaceous time. Considering this idea, are fracture types, orientation, length, aperture, the fracture interestingly shares similar character- spacing, and filling material (Figure 2). Fracture istics. Thus, the result of this study in Bantimala data collected from the field observation were can be a preliminaryIJOG reference for further and then processed into the tables, diagrams, graphs, advanced basement exploration in WMO. and were analyzed in order to understand their characteristics. The result of this step will then be used as a guidance in the next fracture mod- Materials and Methods eling step.

This study presents a comprehensive method Fracture Modeling regarding the fracture distribution in the mélange The fracture modeling was constructed to complex basement. In order to understand the perform conceptual visualization using DFN

123 Indonesian Journal on Geoscience, Vol. 4 No. 3 December 2017: 121-141

AN 1916 (Hall and Smyth, 2008). Tectonic evolution of the southwestern part of Sulawesi is related to the col- lision between Eurasia and East Java Microplate during Late Cretaceous forming the last bound- ary of Sundaland margin (Figure 3). Sulawesi and its surrounding islands can tectonically be divided into four belts. Each belt

S shows characteristic lithological association and tectonic environments. Those are Western Su- lawesi Volcano-Plutonic Belt, Central Sulawesi Metamorphic Belt, Eastern Sulawesi Ophiolite Belt, and Banggai-Sula Microcontinent Platform Z’ Z (for detail see Figure 1). The studied area is located in the Western Sulawesi Volcano-Plutonic Belt. This belt can be divided into two north - south trending mountain

L A chains, the Western Divide Mountains and the Bone Mountains. The Walanae River runs in Figure 2. Scan line sampling method showing measure- a graben-like structure, known as the Walanae ment of fracture attributes such as fracture length, aperture, depression, which occupies the area between and spacing. Z-Z’ is the length of scan-line measurement; A is the aperture size of the fracture; S is the spacing between these two mountain chains. This structure forms adjacent fractures; and L is the trace length of the fracture. a major north to northwest trending fault zone (Walanae Fault Zone). This depression appears (Discrete Fracture Network) algorithm (Winberg to structurally separate South - West Sulawesi et al., 2003). The DFN approach is a modeling from the rest of the western arc. The structural methodology that aims to describe the fractures and tectonic pattern of this region is strongly in statistical calculation by generating a series of influenced by the Walanae Fault Zone, which is discrete fracture objects based on field observa- manifested as a major N - S low bounded on the tions of fracture properties, such as size, orien- east and west by major sinistral wrench faults tation, and intensity. Reliable data are needed (Figure 4). to make a good fracture model. Data such as The Walanae Fault Zone divided the South Su- aperture, length, and type of fracture will guide lawesi into two parts. To the west of the fault zone the distribution of fracture. Furthermore, these it is an uplifted area, dominated by a mountain- data will be used to determine other properties, ous belt of Late Tertiary volcanic rocks known such as fracture porosity. as Camba Volcanics. The Bantimala Complex and the overlying Cretaceous sediments were intensively faulted during the Plio-Pleistocene RegionalIJOG Geology times as a result of the northward movement of the Banda Sea Microplate with respect to western Tectonic Setting Indonesia (Berry and Grady, 1987). Sulawesi is located within a critical position in the Indonesian archipelago. Tectonically, it is Stratigraphic Setting situated between a number of major lithospheric Regional stratigraphy in this study referred to plates; i.e. the northward moving Indo-Australian the stratigraphy of Western South Sulawesi by Plate, the westward-moving Pacific Plate, and previous researchers and compared to this study the south - southeastward moving Eurasian Plate as shown in Figure 5.

124 Fracture Characteristics of Mélange Complex Basement in Bantimala Area, South Sulawesi, Indonesia (B. Sapiie et al.)

Before 70 M.A. a 70 - 35 M.A. b N N

South China Sea

Pasific Plate

Sunda Micro Plate

SFZ Lupar Subduction

Bangka SFZ Bangka Shear Shear Early Cretaceous Jurassic-Cretaceous Subduction Magmatic Arc Meratus Belt Bantimala Cretaceous Eocene Melange Complex Magmatic Arc Magmatic Arc Paternoster Paternoster Paleocene-Eocene Subduction Indian-Australian Jatibarang Plate Luk Ulo 100-80 Ma Melange Complex South Sulawesi

Magmatic Arc Magmatic Arc Suture Zone Suture Zone

35 - 20 M.A. c 20 - 5 M.A. d N N

Rifting-Spreading

Bantimala Oligocene Sakala Fault Complex Magmatic Arc Zone Oligocene-Miocene Cimandiri Subduction Java Trench Fault Citanduy Fault Outer Ridge INDIAN-AUSTRALIAN Magmatic Arc PLATE Magmatic Arc INDIAN-AUSTRALIAN Suture Zone IJOGSuture Zone PLATE Figure 3. Tectonic evolution of Eastern Sundaland margin during Cretaceous–Pliocene showing evolution of Bantimala Mélange Complex (studied area) as part of the collision between Eurasia and East Java Microplate (modified from Sribudi- yani et al., 2003).

Bantimala Tectonic Complex of the Bantimala Complex are sandstone, shale, The Bantimala Complex is a tectonic assem- conglomerate, chert, siliceous shale, basalt, ul- blage of various lithologies ranging in age from tramafic rocks, schist, and metamorphic breccia. Jurassic to Cretaceous. The main components The stratigraphic relationships among the com-

125 Indonesian Journal on Geoscience, Vol. 4 No. 3 December 2017: 121-141

Tempe Depression Key to Lithologies 4.5o S 4.5o S Alluvial Deposits Pare-pare

Lake Tempe Pleistocene Volcanics (Lompobatang)

Walanae Formation

Barru Sengkang Tacipi Limestone Member of Walanae Fm.

Pliocene Volcanics (Camba Fm.)

o o 5 S 5 S Miocene Trachyte Intrusion

Oligo-Miocene Carbonates (Tonasa Fm.) Walanae Depression Watampone Eocene-Oligocene Clastics (Malawa Fm.) Makassar Strait

Paleocene Volcanics (Bua/Langi)

Upper Cretaceous Clastics (Balangbaru Fm.)

5.5o S 5.5o S Pre-Tertiary Basement

Major Fault/Lineaments MAKASSAR Sinjai

Studied Area

Lompobatang N 6.5o S Volcano 6.5o S

0 10 20 30 40 119.5o E 120o E Location Map km

Figure 4. Regional geology and structure map of Southwest Sulawesi region modified from various sources showing rock units and major structures as well as location of the studied area pre-Tertiary basement known as Bantimala complex (red box). ponents are mostly unclear because of their fault Paleocene Volcanic Rocks contacts with each other. All the components are They consist of andesitic lava and pyroclastic surrounded by sheared claystone matrix. deposits with trachyandesite composition and have intercalations of limestones and shales. Paleocene Balangbaru Formation volcanic rocks in the Biru area are called Langi Vol- Balangbaru Formation consists of flysch canics, while in the southeast Bantimala, are known sequences that are deposited in a deep-water as Bua Volcanics (Yuwono et al., 1988). Sukamto environment (bathyal - abyssal). The Balang- (1982) called the unit as a prophylitized volcanic baru Formation is widelyIJOG distributed in northeast rock because of its strong alteration. In a more de- and north of Bantimala area. These Cretaceous tailed mapping, volcanic rocks in eastern Bantimala flysch sediments are predominantly interbed- are called Alla Formation (Sukamto, 1986). These ded sandstone and silty shale with subsidiary Paleocene volcanic rocks were unconformably conglomerates, pebbly-sandstone, and conglom- deposits out at the top of Balangbaru Formation. eratic-breccia. The sediment was unconformably deposited on the Bantimala subduction complex Malawa Formation and unconformably overlain by Eocene sediments The Malawa Formation consists of sandstone (Coffield et al., 1997). (arkose), siltstone, claystone, marl, and conglomerate

126 Fracture Characteristics of Mélange Complex Basement in Bantimala Area, South Sulawesi, Indonesia (B. Sapiie et al.)

Age Sukamtono and Supriatna Garrard et al. (1988) Hasan (1991) Hall and Smyth (2008) This Study Ma Period (1982) Tacipi Upper Camba Fm. Walanae Upper Camba Fm. Erasa Molasse Trachyte 5 Lower Panettekang Camba Fm. Lower Diorite

Miocene Camba Fm. Camba Fm. 11 Breccia Unit Early Middle Late Early Middle

14

Tonasa Fm. Tonasa Fm. Soppeng Volcanic Tonasa Fm. 22.5 Oligocene Tonasa Fm. 37.5 Tonasa Fm. Limestone Unit Early Middle Late Early 43 Malawa Fm. Malawa Fm.

Eocene Langi 49 Volcanic

Sandstone Unit Langi Volcanic Langi

Early Middle Malawa Fm.

53.5 Salo Kalupang Pasorang Bua Dacite Bua 60 Volcanic Langi Volcanic Malawa Fm. Langko Volcanic Paleocene Early Late Early Diorite 65 Bua Member

Balangbaru Fm. Balangbaru Fm. Panggalungan Member Balangbaru Fm. Claystone-Sandstone Unit

Balangbaru Fm. Allup Member 70 Cretaceous

Basement Complex Basement Basement Accretionary Basement Complex Basement Melange Complex Middle Late Complex

Figure 5. Regional stratigraphy of South Sulawesi area showing comparison of several previous study and this study (see text for detail). interbedded with layers of coals and limestone. Camba Formation This formation is thought to be deposited from The Camba Formation is located in the west- terrestrial to shallow marine environment. This ern part of South Sulawesi, and it comprises Eocene transitional sequence is located in the volcanic breccia, conglomerate, lava, and tuff, western part of South Sulawesi and is generally interbedded with marine sedimentary rocks. dominated by quartz-rich sandstones. In addition, The formation is alkaline due to upper mantle this formation has unconformably overlain the partial melting that has incompatible elements- Balangbaru Formation and interfingering with rich with metasomatism. This may be related to the Paleocene volcanics. the previous subduction in the Early Miocene in the context of intraplate extension. Alkaline Tonasa Formation nature is thought to be caused by an excessive The Tonasa FormationIJOG consists of limestone, assimilation of melted old limestone and joining marl, sandstone, siltstone, and claystone which the continental material into the volcanic arc were deposited in Late Eocene-Miocene (van subduction (Yuwono et al., 1988). Leeuwen, 1981). The formation was deposited conformably over Toraja Formation (Malawa Formation) and Langi Volcanics. This forma- Geology of The Studied Area tion is widespread in the western part of South Sulawesi, where this formation is not exposed in A detailed geological mapping during the the eastern part of the Walanae Valley. fieldwork activity in Bantimala area, resulting

127 Indonesian Journal on Geoscience, Vol. 4 No. 3 December 2017: 121-141 in a detailed geology map. This was done not Bantimala Mélange Complex Unit only to understand the geological setting of the The Mélange Complex Unit is the oldest rock studied area, but also with the aim to determine in the studied area and composed of a tectonic where the most suitable locations for conduct- assemblage of slabs and blocks consisting of ing scan line sampling method as well. Figure 6 sandstone, schist, ultramafic rocks, metamorphic shows a detailed geological map of the studied breccia, chert, basalt, and conglomeratic sand- area. stone (Figure 7). The tectonic slabs are oriented Mélange Complex Unit, Claystone-Sandstone NW - SE and dip steeply to the east. Stratigraphic Unit, Andesite Lava-Tuff Unit, Pasorang Dacite and structural relationships among the compo- Intrusion Unit, Langko Diorite Intrusion Unit, nents are unclear. Each block is bounded by fault Sandstone Unit, Limestone Unit, Breccia Unit, which is a kind of a structure boundary. Erasa Syenite Intrusion Unit, Panettekang Dio- rite Intrusion Unit, Collovial Deposit Unit, and Matrix Mélange Alluvial Deposit Unit underlie the Bantimala The matrix of the Mélange Complex Unit is area. sheared claystone. The matrix is widely distri-

Explanation Alluvial Deposit Colluvial Deposit Erasa Syenite Intrusion Panettekang Diorite Intrusion Breccia (Camba Volcanics) Limestone (Tonasa Fm.) Sandstone (Malawa Fm.) Pasorang Dacite Intrusion Langko Diorite Intrusion Andesite Lava - Tuff (Bua Volcanics) Claystonee - Sandstone (Balangbaru Fm.)

Bantimala Melange Complex Matrix Chert Sandstone Basalt Schist Ultramafic Metamorphic Breccia

Stratigraphic Contact Reverse Fault Normal Fault Strike-slip Fault

Syncline

Anticline N IJOG

0 2 4 km 9462000 9466000 9470000 9474000 9478000 9482000 9486000

790000 794000 798000 802000 806000

Figure 6. A detailed geological map of Bantimala complex resulting from fieldworks showing various rock units and major structural boundaries. The equivalent formation name refers to regional stratigraphic nomenclatures as presented in previ- ous figure.

128 Fracture Characteristics of Mélange Complex Basement in Bantimala Area, South Sulawesi, Indonesia (B. Sapiie et al.)

a An0910//NIKOL An0910 XNIKOL

b Bn0307//NIKOL Bn0307 XNIKOL

c An1202//NIKOL An0307 XNIKOL

d An0634//NIKOL An0634 XNIKOL

e An0605//NIKOL An0605XNIKOL

f An0908//NIKOL An0908 XNIKOL

g An0650 //NIKOL An0650 XNIKOL

h IJOGAn1502 //NIKOL An1502 XNIKOL

Figure 7. Some field photos of various mélange blocks found in the studied area showing their lithology characteristic and structures. The third and fourth columns are microphotographs thin section of the blocks of the first and second columns. (a) claystone of matrix mélange; (b) blueschist; (c) ultramafic rocks; (d) greywacke sandstone; (e) basalt; (f) chert; (g) breccia; (h) conglomeratic sandstone blocks.

129 Indonesian Journal on Geoscience, Vol. 4 No. 3 December 2017: 121-141 buted in the Mélange Complex Unit. In the matrix, posed of thinly bedded sandstone and shale. Some there would be tectonic clasts but in small sizes. sedimentary structures such as parallel lamina- The most distinct outcrops of the matrix and tion and cross lamination can still be recognized small size blocks occur near the junction of the in this unit. The sandstone in this block is grey, Pangkajene River, Pateteyang River, and Cem- medium to fine-grained, subrounded - subangu- paga River. Also, the major clasts in this mélange lar grain shapes, well sorted, and compact. This are metamorphics, sandstone, chert, ultramafics, sandstone beds have a range of dip angles varying and basalt. The clast sizes range from several from 16° to 48°, striking N - S to NW - SE. The millimeters to several hundred meters. The clasts petrographic analysis shows that the sandstone in are locally in boudinage structures, sometimes this unit could be classified as lithic-wacke. Based showing a chaotic orientation (Figure 7a). on lithology characteristics, the sandstone unit is equivalent to Paremba Sandstone (Sukamto, Schist Block 1986). Sukamto and Westermann (1993) reported The Schist Blocks are located in the central that sandstone in Bantimala Mélange Complex part of the Mélange Complex Unit. The schist yielded Early to Middle Jurassic age based on the blocks are mainly composed of green schist, fossil presence, which were Ammonites (e.g. Li- blue schist, phyllite, locally eclogite quartzite, assic fuciniceras), gastropods, and brachiopods. and gneiss. The schist blocks well outcrop in Fractures in this unit commonly present as shear Pateteyang, Bontorio, and Cempaga River. The fractures, which are dominantly to be oriented in blueschist block is locally brecciated. This block NNW - SSE direction. is commonly foliated northwest-southeast by the trend and 34° to 48° by the dip angle. The result Basalt Block of petrographic analysis shows the lithology in The Basalt Block is distinctly exposed in Den- schist block is glauchophane-mica-garnet schist, geng-dengeng River and Paremba River, as can quartz-mica schist, and glaucophane-mica schist be seen in Figure 7e. The basalt is very dark grey (Figure 7b). K-Ar radiometry has been conducted to black and massive structure. Petrographically, from mica components of schist in Bantimala it shows that these basalts have an intergranular with an age range of 132 to 113 Ma (Wakita, texture and consist of pyroxene and plagioclase. 2000). Chert Block Ultramafic Block The Chert Block is located in the southeastern Ultramafic rocks of the Mélange Complex of the Mélange Complex Unit. The block is well Unit are mainly distributed in the northern and exposed in the Baruttung River. The chert layers eastern part of the complex. The ultramafic rocks range from 1 - 20 cm thick, and are sometimes are mostly serpentinized peridotite and serpenti- intercalated with thin shale. The bedded chert is nite, generally dense, pale green to bluish green mostly red or reddish brown. The chert is mainly rocks, with a waxy luster and tend to be slicken- composed of radiolarian skeletons and fragments. sided in the hand specimen. The rocks are com- Quartz and calcite veins usually fill the extension monly found disrupted,IJOG fractured, and sheared fractures in the chert unit (Figure 7f). Fractures as shown in Figure 7c. The petrographic analysis in this unit commonly present as shear fractures shows that this block has the sea and island tex- that have an E - W orientation. ture constructed by olivine and serpentine. Breccia Block Sandstone Block Breccia is a sedimentary rock consisting The Sandstone Block is located in the central mainly of metamorphic rock fragments. The part of the Mélange Complex Unit, well outcrops block is mostly distinct in the southwestern of in Paremba River (Figure 7d). This block is com- the Mélange Complex Unit in Baruttung River,

130 Fracture Characteristics of Mélange Complex Basement in Bantimala Area, South Sulawesi, Indonesia (B. Sapiie et al.) while the distribution also occurs in the north- The Camba Formation is composed of marine western of the Mélange Complex Unit. The main volcanic and sedimentary rocks (Bergman et al., component of this breccia is schist with 5 - 125 1996). In the Bantimala area, the Camba Forma- cm sized, angular - subangular, bad sorted, and tion Unit could be mapped into detailed units as coarse sand matrix. Quartzite also composes as Breccia Unit, Erasa Trachyte Intrusion Unit, and fragments in this breccia (Figure 7g). Panettekang Diorite Intrusion Unit. Quaternary alluvial deposits are mostly situ- Conglomeratic Sandstone Block ated in the western part of the Bantimala area. The Conglomeratic Sandstone Block is situ- This unit consists of various unconsolidated ated in the eastern part of the Mélange Complex material sizes ranging from clay to gravel. The Unit. The block is distinctly exposed in Kam- material originated from the oldest rock, which botti River. The sandstone colour ranges from widely spread at the upstream side in the eastern grey to dark grey, pebbly - coarse sand grain part of the studied area. size, subangular - subrounded, medium sorted, and polymict. The fragments are schist, basalt, marble, quartz, and sandstone (Figure 7h). Fracture Characterization

Fracture is a general term for material failure Pre-Tertiary and Tertiary Rocks on brittle condition (Koestler et al., 1995). In The Claystone-Sandstone Unit (Balangbaru this research, the definition of fracture is more Fm.) lies unconformably on the Bantimala Mé- underlined for naturally formed fractures. The lange Complex Unit. This unit was deposited other definition is by Nelson (2001), that reservoir in a marine fan environment during the Late fracture is natural macroscopic discontinuity Cretaceous (Hasan, 1991). The volcanic activity plane on rocks, which is formed by deformation occurred during Paleocene resulting in Andesite or physical diagenesis. Lava-Tuff Unit (Bua Volcanics) in the Bantimala According to the direction of movements, area. This unit has unconformable stratigraphic fractures in rock are classified as extension and relationship with the Claystone-Sandstone Unit. shear fractures. Shear fracture involves parallel The other volcanic products during Paleocene movements of a fracture plane, while the move- are Pasorang Dacite Intrusion Unit and Langko ment of extension fracture is perpendicular to Diorite Intrusion Unit. Both of them intrude the the fracture plane. Fossen (2010) illustrates vari- Mélange Complex Unit. ous types of fracture as seen in Figure 8a. The During Eocene, Makassar Strait rifting took fracture type that was observed in the Bantimala place and caused the deposition of Sandstone area consisted of fault/shear, gash/vein, extension Unit (Malawa Fm.). This unit was deposited in fracture, and joint (Figure 8b). the fluvial - transition environment, recognized Fracture characterization in this study is con- by coal seam and limestone particularly. Fol- ducted using surface fieldwork and subsurface lowed the rifting events, Limestone Unit (Tonasa modeling. Geological mapping and scan line sam- Fm.) was deposited IJOGduring Late Eocene - Middle pling are the first fieldwork activities to collect Oligocene where sagging tectonic events began. geological and fracture data from the Bantimala Tonasa Limestone was widely spread as karst and surrounding area. Thus, those fracture data hill landscape in the south arm of Sulawesi. In are sorted in order to divide the fracture system Bantimala area, this limestone has Late Eocene into several fracture set and to obtain fault-related - Middle Oligocene in age. fractures. After all, the subsurface model will The volcanic activity became active for the be generated using data observed from the field. second time in Late Miocene. This volcanic Fracture types, distribution including orienta- product is well known as Camba Formation. tion, density and intensity vary in each lithology

131 Indonesian Journal on Geoscience, Vol. 4 No. 3 December 2017: 121-141

Shear fracture Joint a b Extension fracture

Shear fracture Joint

s3 Stylolite Vein Extension fracture

Vein

s2 Joint

Figure 8.Types of geological fractures observed and mapped in the studied area. a) shear fractures (SF) and extension fractures (EF) including joint (JT) and vein (VE) (modified from Fossen, 2010). b) Types of fractures observed. within mélange blocks in the Bantimala complex for helping to develop fracture modelings in (Figure 9). A total of 3,841 fractures is measured subsurfaces. One of the problems in exploring in the studied area from various locations and natural fracture reservoirs is to distribute frac- lithology, particularly concentrated in different tures in 3D rock volume. The fracture modelings mélange blocks. Strike and dip orientation are were directed to build a discrete fracture network different in each lithology, especially in mélange from field data, and furthermore to determine complex block units (Figure 10). Based on sta- the fracture porosity. Fault geometry, lithology tistical analyses using both lower hemisphere distribution, and geological cross section are stereographic projections and rose diagram of the main components used for constructing a fracture orientations it indicates various fracture geometrical model of the studied area. Frac- trends. The main orientations are relatively NW tures modeling (3D Geocellular modeling) in - SE, N - S, and NE - SW trendings (Figure 11). this paper was build using Petrel 2016 software The fracture intensity is defined as the number from Schlumberger with workflow presented in of fracture per length unit, and it becomes an Figure 12. essential attribute of a fracture pattern. The field observation and measurement indicated Grid and 3D Cube that fracture intensity in schist is the most in- Due to the complexity of the relationship tensive fractured spot in The Bantimala Mélange with the mélange unit, structural gridding was Complex. This fractureIJOG intensity that has been used in this modeling (without pillar gridding). measured from the field is used for subsurface The three-dimensional cube was generated by modeling in the further step. combining the SRTM (Shuttle Radar Topo- graphic Mission) image and surface geology with constant depth of 1,000 m below sea level. Fracture Modeling The complex structure of the mélange complex causes problems for common geometry model- The main purpose of this study is to under- ing. Stair-step faulting is used for the modeling stand fracture distribution) in basement rocks of complex fault relationship. The complex rela-

132 Fracture Characteristics of Mélange Complex Basement in Bantimala Area, South Sulawesi, Indonesia (B. Sapiie et al.)

a b

c d

e f

Figure 9. Outcrop photographs showing types of fractures observed in the studied area: (a) quartz VE observed in interbedded chert and red limestone; (b) SF in schist; (c) SF in ultramafic rocks; (d) SF and JT in clay matrix; (e) SF in sandstone; (f) SF in mudstone. VE= vein; SF= shear fractures; JT= joint. tionship of each block in the mélange complex Fracture Guide Model unit is properly modeled using stair-step faulting Several parameters such as distance to the process. Stair-step faults are created based on fault, distance to the surface, and lithology are fault geometries, which have been defined in the measured to ascertain the structure control of structural framework. Stair-step faulting would fracture distribution. These three properties were convert a fault modelIJOG into stair-step faults mak- created on geometrical modeling, with some cal- ing them into stair-step form. The lithology was culation to approach the real number. Distance to distributed using facies modeling according to a the surface is a linear function, while the distance polygon and point that was created based on the to fault was calculated using equation between geological map and cross section. The rock units distance and fracture intensity. Distance to the are vertically dipping with the aim to make model surface also plays a role in the fracture distribu- simpler (Figure 13). The grid size (in meter) used tion, as well as lithology. Distance to a fault, in the model is 100x100x65 with the total cell of distance to the surface, and lithology are used as a 1,288,000. fracture intensity guide to distribute the fracture

133 Indonesian Journal on Geoscience, Vol. 4 No. 3 December 2017: 121-141

N N N

Breccia Sandstone (Camba Volcanic) (Malawa Fm.)

N N

Claystone Ultramafic Rocks (Malawa Fm.)

N N

Basalt Diorite

N N

Claystone Sandstone Block (Balangbaru Fm.)

N N

0 2 4 km Sandstone

Schist 9462000 9466000 9470000 9474000 9478000 9482000 9486000 (Balangbaru Fm.)

790000 794000 798000 802000 806000 N N N N N

Trachyte Limestone Claystone Matrix Breccia Block Chert

Figure 10. Distributions of fracture orientation of each lithology (mélange blocks) in Bantimala Area showing various orienta- tions with three major trends: NW–SE, N–S, and NE–SW. This trending variation is interpreted due to different ages, deforma- tions history, and environmentIJOG of formations of the blocks (see text for detail). intensity. Basically, modeling can be conducted and lithology controls are set up as parameters with a limited number of good data that assume to build a fracture intensity guide. The fracture from the scan line traverse. The Bantimala Mé- intensity guide will be used in distributing frac- lange Complex is composed of various tectonic ture intensity throughout the studied area. blocks which come from different ages and en- vironments. Therefore, each block has different Fracture Intensity Model characteristics and indicates that lithology also The Fracture Intensity Model is the intensity controls the fracture distribution. All structure of fracture that is inferred from scan line data

134 Fracture Characteristics of Mélange Complex Basement in Bantimala Area, South Sulawesi, Indonesia (B. Sapiie et al.)

N N Color Density Concentration 0.00 - 0.30 0.30 - 0.60 0.60 - 0.90 200 0.90 - 1.20 1.20 - 1.50 1.50 - 1.80 150 1.80 - 2.10 2.10 - 2.40 2.40 - 2.70 100 2.70 - 3.00 n = 3841

W 200 150 150 200 E W E

100

150

200

S S

Figure 11. Total of 3841 fractures are delineated from field observation. Fracture orientation is commonly NW-SE, W-E, NNE- SSW, and ENE-WSW. This trending variation is interpreted due to different ages, deformations history, and environment of formations of the blocks (see text for detail).

Structural Frameworks (Based on Field Data)

Structural Gridding

Simple Gridding Make Horizon Layering Stair-step Fault

3D Structural Model

Fracture Intensity Guide Fracture Intensity Distance to surface (Kriging Method) Distance to fault DFN Parameters Lithology Geometry: fracture length Orientation: Fracture Modeling a. main fracture dip azimuth (Discrete Fracture Network) Stochastic Method b. main fracture dip angle c. concentration Aperture size ODA Method

Fracture Porosity Figure 12. 3D fracture modelingIJOG work flow used in this study using PETREL. and scaled up into grid model. The fracture in- distance to the fault, distance to the surface, and tensity model was generated from the guidance their intensity. of fracture guide that was generated beforehand. The modeling result shows that the most popu- Discrete Fracture Network lous fracture will occur on the red coloured zone Discrete Fracture Network (DFN) is a and the less fracture appeared on the turquoise method that can describe the fracture network coloured zone (Figure 14). The main controls in the field as a model (Dershowitzet al., 2002). on this distribution of intensity model are the The main idea to make this concept is by using

135 Indonesian Journal on Geoscience, Vol. 4 No. 3 December 2017: 121-141

9488000 792000 9484000 796000 X-axis Y-axis 9480000 800000

9476000 804000

9472000 808000

9468000

9464000 0 Z-axis

9488000 1000

9484000 Z-axis 0

-1000 9480000

792000

9476000 Lithology Units 796000 Undefined Alluvial Y-axis 9472000 Collovial Camba Breccia X-axis Tonasa Limestone 800000 Matawa Sandstone Balangbaru Claystone Langko Diorite 9468000 Pasorang Dacite Erasa Trachyte Panettekang Diorite 804000 Bua Volcanic Matrix Schist Block 9464000 Ultramafic Block Sandstone Block Chert Block Basalt Block 808000 Breccia Block Conglomerate Block

Figure 13. 3D geological model of Bantimala Complex showing lithology distributions and structural relationship. The model was generated based on field mapping (see text for detail).

9485000 792000

9480000 796000 X-axis Y-axis 800000 9475000 804000 9470000 808000 9465000

0 Z-axis

9488000 0 Z-axis 9484000

792000 9480000

796000 9476000 Intensity Intensity Critical (U) Y-axis 9472000 7.00 800000IJOG6.00 X-axis 5.00 4.00 9468000 3.00 804000 2.00 1.00 9464000 0.00 -1.00 808000 -2.00 -3.00

Figure 14. 3D fracture intensity model that is built and guided from both structure and lithology control showing high intensity fracture area is strongly controlled by faults.

136 Fracture Characteristics of Mélange Complex Basement in Bantimala Area, South Sulawesi, Indonesia (B. Sapiie et al.) some limited data, such as a fracture from a scan be parameters for generating DFN. The length line mapping and distribute it by using some of fractures is distributed using the power law. fracture guidance and several parameters. In a The aperture of fractures has a range from 0 - 1 3D Geocellular DFN model (Petrel software), mm, and is also distributed using the power law. the fractures and faults, and their properties All parameters are generated to produce a dis- are generated directly from that cell-based in- crete fracture network model as seen in Figure formation. The guidance consisted of distance 15. As an example, a zoomed-in fracture set in to the fault, distance to the surface, and lithol- claystone matrix shows domination of two sets ogy intensity. The orientation of fracture in of fracture system (Figure 16). Bantimala area is different on each lithology, showing that the fracture system was complex. Fracture Porosity The distribution of fracture orientation is guided The secondary porosity or fracture porosity by lithology. There are fifty-one fracture sets, was playing an important role in rocks, because which is distributed in the discrete fracture it could provide an economic value. Scaling up network model using the stochastic method. the DFN model using scale up fracture network All of the fracture set orientation were gener- ated by the Fisher method (Fisher et al., 1987; properties generates a fracture porosity model Allmendinger et al., 2012). The Fisher method (Oda, 1985). The value of porosity varies from describes the distribution of fracture angles in 0.0004 to 0.0029 %. The high fracture porosity much the same way as for continuous variable. number is found in an area with a significant The Fisher model method gives more isotropic fracture intensity and most crosscutting of frac- data for fracture deviation. The length and ap- ture which is controlled by faults and lithology erture are both fracture geometry, which would (Figure 17).

9485000 792000 Discrete Fracture Network (dip azimuth) X-axis 796000 Y-axis 9480000 800000 9475000 804000 9470000 808000

9465000

0 Z-axis

9488000 00

Z-axis 9484000

792000 9480000

796000 9476000

Dip azimuth (deg) Y-axis 9472000 350.00 800000 325.00 IJOG300.00 275.00 250.00 9468000 Figure 15 225.00 200.00 804000 175.00 150.00 125.00 9464000 100.00 75.00 50.00 808000 25.00 0.00

Figure 15. Discrete Fracture Networks (DFN) modeling of the Bantimala Mélange Complex (Dip Azimuth) using 51 fracture sets resulted from field measurements.

137 Indonesian Journal on Geoscience, Vol. 4 No. 3 December 2017: 121-141

Dip azimuth (deg) 350.00 325.00 300.00 275.00 250.00 225.00 200.00 175.00 150.00 125.00 100.00 75.00 50.00 25.00 0.00

Lithology: Claystone (matrix) Fracture sets: 2

N219o E/78 o NW

N352o E/61 o NE

Figure 16. Example of zoomed-in fracture sets in claystone matrix from Discrete Fracture Networks (DFN) model as pre- sented in Figure 15.

Analysis and Discussion The field evidence indicated the Bontorio Meta- morphic Rocks and Jurassic Paremba Sandstone The Bantimala Mélange Complex consists blocks have an intensive fracture since they are of an assemblage of metamorphic, igneous, and the most brittle unit among others. The structure sedimentary rocks, which were tectonically position in a particular fault also has the control in mixed. Each block came from different ages and fracture distribution, where the fracture intensity environments. Considering the various natural increases towards the structure boundary. forming of each block, they will have different The result of this modeling can be used for characteristics in any circumstances as well. This conducting basement fractured reservoirs in the study has shown that fractures in the Bantimala West Madura Offshore (WMO), in particular Mélange Complex areIJOG strongly controlled by designing drilling targets. Using this model- lithology and fault zones. A fractal analysis is ing result, exploration wells should be drilled the first important step in order to determine slightly incline toward the SE direction in order to the fracture distribution. The fracture intensity penetrate a maximum fractured area. Certainly, shows a comparative frequency of fracture pres- detailed reservoir geomechanic works need to ence on a certain distance interval. In the Ban- be done in WMO field to have a better under- timala area, the fracture intensity is dominantly standing concerning subsurface stress condition. controlled by lithology. The number of fracture Applying results from geomechanic to fracture intensity is increasing to the brittleness of rocks. distributions, exploration well can be directed

138 Fracture Characteristics of Mélange Complex Basement in Bantimala Area, South Sulawesi, Indonesia (B. Sapiie et al.)

9485000 792000 Porosity Model X-axis 796000 Y-axis 9480000 800000 9475000 804000 9470000 808000 9465000

0 Z-axis

9488000 0

9484000

792000 9480000

796000 9476000

9472000 800000

9468000 804000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 36 9464000 33.6 32 808000 28 26.5

24 23.1 Porosity (m3 /m 3 ) ceitical fracture_porosity 20 0.0011 0.0010 16 0.0009 0.0008 12 0.0007 10.2 0.0006 8 0.0005 0.0004 4.2 0.0003 4 Porosity (%) 1.7 0.0002 Min 4.11 x 10-12 0.5 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0001 0 Max 0.0029 0 0.0006 0.0012 0.0018 0.0024 0.003 0.0000 Average 0.0004 ceitical fracture porosity

Figure 17. Fracture porosity distribution model showing high porosity area is controlled by fault, fracture intensity, and litho- logy (see text for detail). to cut critically stress fractures which will have 1. The fracture type that was observed in the better permeability. Bantimala area consists of fault, shear fracture, gash fractures (vein), extension fractures, and IJOGjoints. Conclusions 2. A total of 3,841 fractures are measured from various different mélange blocks. Strike and Structural factors as well as lithology types dip orientation are different in each lithology played a critical role in the development of where the main orientations are relatively NW basement fractured reservoirs in the Bantimala - SE, N - S, and NE - SW trendings. Mélange Complex. Detailed structural mapping 3. The orientation of fracture in Bantimala area in the area revealed the following points: is different on each lithology, showing that the

139 Indonesian Journal on Geoscience, Vol. 4 No. 3 December 2017: 121-141

fracture system was complex. The fracture in- Cambridge University Press, 289pp. DOI: tensity in schist is higher compared to the other 10.1017/CBO9780511920202 lithology in the Bantimala Mélange Complex. Bergman, S.C., Coffield, D.Q., Talbot, J.P., and 4. Discrete Fracture Network (DFN) model was Garrard, R.A., 1996. Tertiary Tectonic and generated using fifty-one fracture sets from magmatic evolution of western Sulawesi and field measurements, which are distributed the Makassar Strait, Indonesia: evidence for using the stochastic method. a Miocene continent-continent collision. In: 5. The value of the fracture porosity is relatively Hall, R. and Blundell, D. (eds.), Tectonic Evo- small, varied from 0.0004 to 0.0029 %. The lution of Southeast Asia, Geological Society high fracture porosity number is observed in Special Publication No. 106, p.391-430. DOI: an area with a significant fracture intensity 10.1144/GSL.SP.1996.106.01.25 and most crosscutting of fracture which in Berry, R.F. and Grady, A.E., 1987. Mesoscopic turn is controlled by faults and lithology. structures produced by Plio-Pleistocene wrench faulting in South Sulawesi, Indonesia. The conclusion is that the Mélange Complex Journal of Structural Geology, 9, p.563-571. can have high potential as a basement fractured DOI: 10.1016/0191-8141(87)90141-6 reservoir target, where fracture distributions and Coffield, D.Q., Guritno, N., Wilson, M.E.J., and their attributes vary depending on the lithology as Ascaria, N.A., 1997. Petroleum Systems of well as local deformation. It is believed that the South Sulawesi, Indonesia. In: Howes, J.V.C. recognition of fracture styles, distribution, and and Noble, R.A. (eds.), Proceedings of the intensity in each mélange block is an essential Conference on Petroleum Systems of SE Asia factor in exploring their potential, particularly and Australasia, p.1001-1010. for planning exploration well location. Integrated Dershowitz, W., Klise, K., Fox, A., Takeuchi, S., fieldwork and 3D geocellular modeling results and Uchida, M., 2002. Channel Network and are proven as a powerfull approach to help un- Discrete Fracture Network Modeling of Hy- derstanding the fracture distributions in space. draulic Interference and Tracer Experiments and the Prediction of Phase C Experiments. SKB Report IPR-02 - 29. Stockholm, SKB. Acknowledgements Fisher, N.I., Lewis, T., and Embleton, B.J.J., 1987. Statistical Analysis of Spherical Data. This research is part of ongoing evaluation of Naturally Fractured Reservoir potential in Cambridge University Press, 329pp. DOI: Western Indonesia at the Geodynamic Research 10.1017/CBO9780511623059 Structural Geology Group ITB. The authors thank students and Fossen, H., 2010. . Cambridge staff of Geological Engineering Study Program, University Press, 463pp. DOI: 10.1017/ Faculty of Earth Science and Technology ITB in CBO9780511777806 supporting this research. Thanks are expressed to Garrard, R.A., Supandjono, J.B., and Surono, Pertamina WMO geologists for their contribution 1988. The Geology of The Banggai-Sula and knowledge sharing of their project in base- Microcontinent, Eastern Indonesia.Proceed- IJOG th ment fractured reservoir in the mélange complex. ings, Indonesian Petroleum Association, 17 Annual Convention, p.23-52. Hall, R. and Smyth, H.R., 2008. Cenozoic arc References processes in Indonesia: identification of the key influences on the stratigraphic record in Allmendinger, R.W., Cardozo, N., and Fisher, active volcanic arcs. In: Draut, A.E., Clift, D.M., 2012. Structural Geology Algorithms: P.D., and Scholl, D.W. (eds.), Lessons from the Vectors and Tensors. Cambridge, England, Stratigraphic Record in Arc Collision Zones,

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