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