Abstract Ganteng

Microfracture Statistical Analysis on Thin Section of Tonasa Formation, Enrekang Regency, South Sulawesi

M. Harifan, B. Priadi, and A. Priambodo

Department of Geological Engineering

Faculty of Earth Science and Technology

Institut Teknologi Bandung

Abstract – Sulawesi was formed during the Oligocene – Miocene collision between the Eurasian Plate and microcontental fragments detached from the Indian-Australian Plate. The research area is located in the west part of South Sulawesi, Enrekang Regency, at coordinate of 03°26’52” – 03°29’07” South Latitude and 119°45’46” – 119°48’03” East Longitude. The geological structures found in the study area are two dominant fault zones, those are Kasambi Strike-slip Fault and Bambapuang Strike-Slip Fault. The characteristic of the porosity in Tonasa Formation is included into secondary porosity type which dominantly formed a microfracture. Microfractures in the limestone of Tonasa Formation is affected by the fault zone due to relative force WNW-ESE. Thin section analysis of the samples show different amount of microfracture distibution among the samples taken from a different spots in the area, it possibly indicates different tectonic impact in Tonasa Formation.

Keywords: Enrekang, South Sulawesi, Bambapuang, Kasambi, microfracture

Sari – Daerah penelitian terletak di daerah Enrekang, Kecamatan Enrekang, Kabupaten Enrekang, Provinsi Sulawesi Selatan, tepatnya pada koordinat 03°26’52” – 03°29’07” Lintang Selatan dan 119°45’46” – 119°48’03” Bujur Timur. Geologi struktur yang ditemukan pada daerah pemetaan adalah dua zona sesar yaitu Sesar Mendatar Kasambi dan Sesar Mendatar. Karakteristik dari tipe porositas di Formasi Tonasa merupakan tipe porositas sekunder dengan dominansi berupa rekahan mikro. Rekahan mikro dalam Batugamping di Formasi Tonasa dipengaruhi oleh adanya zona sesar akibat tegasan gaya relatif berarah WNW – ESE. Analisis sayatan tipis dari sampel menunjukan perbedaan distibusi jumlah rekahan mikro diantara sampel-sampel tersebut yang diambil dari lokasi yang berbeda dalam area tersebut, hal tersebut menunjukan adanya perbedaan pengaruh tektonik di Formasi Tonasa

Kata kunci: Enrekang, Sulawesi Selatan, Bambapuang, Kasambi, rekahan mikro.

INTRODUCTION

Indonesia lies on a juction of three converging major plates, i.e the Eurasian, Indo-Ausralian, and Pacific Plates. Sulawesi was formed aling the Oligosen-Miocene collision between the Eurasian Plate and

micro-continental fragments derived from the Indian-Australian Plate. It can be devided into four arms, each is characterized by different tectonic provinces with particular reference to divergence of Kalimantan and Sulawesi through the Cainozoic (Hamilton, 1979).

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Geologists have discovered a reservoir potential in Enrekang, South Sulawesi. Spesifically in Sengkang Basin Tonasa Formation. Porosity occurences are found in Enrekang due to the tectonic impact that cause secondary porosity of microfracture on the Limestone in Tonasa Formation.

One factor causing the reservoir potential in South Sulawesi has not been maximum is because of the ignorance about micro analysis on the samples which also describe the condition in the subsurface. Therefore this study was conducted to determine the geology and tectonic setting upon the limestone formation, on the basis of detailed petrographical data analysis.

This study will investigate the microfracture potential which only performed on the surface and put it in a statistic analysis to determine the corelation of Limestone from Tonasa Formation’s location with Fault zone in the research area. Furthermore, the conditions of microfracture in the subsurface will be interpreted based on the analysis data dan appears on the surface outcrops.

LOCATION AND METODOLOGY

Reseach area is administratively located in the Enrekang regency, and geographically at coordinate 03°26’52” – 03°29’07” South Latitude and 119°45’46” – 119°48’03” East Longitude (Figure 1). It is also in the north of Enrekang City, precisely 30 minutes from Enrekang City, South Sulawesi province, which covers area 16 km2 (4 km x 4 km).

Outcrops which found in the study area has very diverse conditions, ranging from fresh to weathered outcrops.

Accomplishing the purpose of the study, specific geological investigation and laboratories were caried out. Then, the study

focused on the Tectonic and Microfracture analysis of the research area. Basically, outcrops from Tonasa Formation Limestone were sampled for a representative petrographical analysis in order to get the statistical percentage of the microfracture.

The methodology also included field observations and sampling, data calculation, thin section petrographical analysis conducted on five samples of thin sections from different area in Tonasa Formation. Microfracture statistical analysis result were evaluated based on the ocurrance of microfracture and precentaged in one thin section.

GEOLOGICAL SETTING

Regional Geology

According to the lithotectonic structure, Sulawesi and it’s island surrounding it is divided by four parts (Hall and Wilson, 2000), which is: West Mandala as a Magmatic path which is an east part of Paparan Sunda, Middle Mandala which contain metamorphic rocks, and East Mandala contains Ophiolite which is the segment of the Continental Plate that moved to west because of the Strike-Slip Faults from New Guinea (Figure 2).

The research area is included in the West Mandala. West Mandala is stretched from the Northern Arm until Southern Arm of Sulawesi. Van Leeuwen (1994) announce that West Mandala, as a Volcanic Arc, can be devided into two parts. The first part is the Northern part from Buol to Manado and the second part is the West part from Buol to Makasar.

There are so many Tectonic Models that have been offered to describe the Tectonic Evolution of Sulawesi Island. There are two

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major events happened in the West part of Sulawesi in Cenozoic. The first event is Oceanic Ridge was rifted in Paleogene that created space for clastic materials from Borneo to deposit. The second event is Compressional stage that began in Miocene. The Comprresion in affect by the Continent Collision from East, for example: Micro-Continet of Buton, Tukang Besi, and Banggai Sula. The compression produced a West Sulawesi Fold Belt and Thrust Fault in West Mandala of Sulawesi.

Geomorphology

The elevation of this area is 300 – 1200 meters above sea level, the landscape is dominated by a steep that elongated from North to South. The appearance of hills ad valley shows different resistancy level of rocks upon weathering. There river patern of the research area has a rectangular pattern (Figure 3) which controlled by structure. Genetically, rivers in the research area is divided into subsequent river, consequent river, and obsequent river.

According the classification of Lobeck (1939) which based on the dense contours, lithology, patterns of river flow, and their process which occur in the area, the research area is grouped into three geomorphological units. The geomorphological units are Lava Flow Unit, Fault Zone Hills Unit, Denudational Structur Valley Unit.

Regional Stratigraphy

The stratigraphic unit is determined by the physical appearance of the rocks and the relative stratigraphic position of the units observed in the field. The stratigraphy of the study area is sorted from old to young as follows (Table 1):

1) Sandstone Unit

The sandstone unit covered 35% of the study area and spread throughout the east of the research area. Based on the megascopic observation, this unit is generally fresh and some parts is weathered. It is dark brown in color, massive, fine-grained, fine to medium porosity, bad sorting, open fabric, containing feldspar and other rock fragments.

2) Sandstone – Siltstone UnitSandstone – Siltstone unit covered 22% of the study area. Based on the megascopic observation, this unit is generally appears in fresh condition. It is light brown in color, massive, fine-grained, bad sorting, carboniferous. The siltstone is observed as a dark colored rock, brittle, carboniferous, with paralel lamination structure.

3) Limestone UnitLimestone covered 31% of the study area. This unit appears on the center and elongate from north to south of the research area. Based on the megascopic observation, this unit is greyish white in color, bioclastic texture, contain sceletal fragments of phoraminifer, algae, and other sceletal fragments. Has a secondary porosity of vuggy and cavern.

4) Andesite Lava UnitAndesite Lava Unit covered 10% of the study area. This unit appears as a lava flowwhich is located in the south west of the research area. Based on the megascopic observation, this unit is light gray in color, hypocrystalline, showing porphyritic texture with plagioclase, hornblende, and biotite as phenocrysts within finer groundmass of glass.

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Structural Geology

The pattern of the existing structures at Enrekang interpreted WNW - ESE trending. Fault structures can be interpreted from the SRTM analysis, straight line pattern, and rock layer’s offset.

Kotu Thrust Fault has a Nort-South trending. Kasambi Fault and Bambapuang Fault are the strike slip faults that are trending northwest-southeast and located in the center part of the study area.

MICROFRACTURE ANALYSIS

Definition

Microfracture is a type of porosity that formed after the depositional process. Porosity that occur after the depositional process called secondary porosity. It has several factors that trigger rocks to have their secondary porosity. One of them is tectonic impact. The microfracture appears in the outcrop can be also used to determine the microfracture in subsurface. Microfracture’s occurence also shows reservoir potential on a rock.

Tectonic Evolution

The northern and southern arms of Sulawesi are here referred to as “Western Sulawesi” for simplicity. The east and southeast arms have an oceanic origin comprises Mesozoic and younger allochthonous metamorphic and ophiolitic rocks, which were obducted onto western Sulawesi during the Oligocene to Early Miocene. In addition, there are several small continental fragments which have collided with eastern Sulawesi, including the Banggai-Sula, Tukang Besi, and Buton Islands.

Geologically, southwest Sulawesi was formed during the Early Cretaceous time as a basement complex, and apparently includes an old continental crust of Australian origin. This is supported by the presence of recycled ancient zircons from Miocene igneous rocks, which on the basis of their chemical composition, they were originated from northern Aus- tralia. Possibly, these igneous rocks accreted onto the Sundaland during the Oligocene to Miocene collision event (Bergman et al., 1996; Priadi et al., 1993).During the Middle to Late Eocene, the west basement of the Walanae depression was overlain unconformably by volcanic and marginal marine coal-bearing deposits of the Malawa or Toraja Formations (Sukamto, 1982). Later, the deposition of Oligocene Tonasa Formation indicates the development of extensive areas of shal- low water carbonate platforms in southern Sulawesi.

Microfracture Statistics

The statistics of the Microfracture analysis on thin sections from 5 samples shows that the samples taken from outcrops located far away from fault zone shows less microfracture than samples taken near the fault zone.

CONCLUSIONS

Based on field observations, petrographic analysis, and literature reviews, it can be concluded that:

Stratigraphy of the study area, from old to young, i.e., Sandstone Unit, Sandstone - Siltstone Unit, Limestone Unit, and Andesite Lava Unit

Structural control is based on two dominant fault zones that are trending North West – South East.

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Microstructure statistical analysis shows that Microstructure percentage on samples taken on the faultzone has more amount of Microstructure rather than samples taken far from the fault zone

Tonasa Formation has a reservoir potential, but it categorized as a bad reservoir.

REFERENCES

Adams, C. G., 1970, A Reconsideration of East Indian Letter Classification of The Tertiary, Bulletin National History. (Geol), vol. 19, no. 3, p.85-137.

Dunham, R. J., 1962, Classification of Carbonate Rocks According to Depositional Texture, AAPG, Memoir 1.

Hall, R. & Wilson, M. E. J., 2000, Neogene sutures in eastern Indonesia. Journal of Asian Earth Sciences, 18, 781–808.

Harifan, M., 2010. Tugas Akhir A : Geologi Daerah Enrekang dan Sekitarnya, Kabupaten Enrekang, ITB, Bandung.

Pettijohn, F. J., 1987, Sedimentary Rock, Happer and Row Publisher Inc., New York.

Ratman, N. dan S. Atmawinata, 1993, Geologi Lembar Mamuju, Sulawesi, skala 1:250.000. Pusat Penelitian dan Pengembangan Geologi.

Rickard, M. J., 1972. Fault Clasification-Discussion. Geological Society of America Bulletin, vol. 83, p. 2545-2546.

Simandjuntak, T. O., E. Rusmana, Surono, dan J. B. Supandjono, 1991, Geology of the Malili Quadrangle, Sulawesi. Quad 2113, 1:250.000. Geol. Res. Dev. Centre, Bandung, 35 p.

Sukamto, R., 1975, Peta geologi Indonesia lembar Ujung Pandang (skala 1:1.000.000). Geological Survey of Indonesia (GSI), Bandung, Indonesia.

Van Leeuwen, T.M., 1994, 25 Years of Mineral Exploration and Discovery in Indonesia, Journal of Geochemical Exploration, 50, h.13-90.

Wilson, M.E.J., dan D.W.J. Bosence, 1996, The Tertiary evolution of South Sulawesi: a record in redeposited carbonates of the Tonasa Limestone Formation. In: R.Hall dan D.J. Blundell (eds.) Tectonic evolution of Southeast Asia. Geol. Soc., London, Spec. Publ.106, P. 365-389.

Yuwono, Y.S., Maury, R.C., Atmadja, R.S., dan Bellon, H., 1987, Tertiary and Quarternary Geodynamic Evolution of South Sulawesi: Constraints from the Study of Volcanic Units. Proceeding IAGI. Hal. 32-48.

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Figure 1. General Geological Map of Sulawesi, research area is marked by a red square (Hall and Wilson, 2000)

Table 1. Stratigraphic column of research area with non-formal naming of rocks unit (Harifan, 2012)

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Figure 3. Simplified and interpretive geology of southwestern Sumbawa, showing the location of the Batu Hijau district and mine. Schematic cross section illustrates the relationship between Early to Middle Miocene andesitic

basement rocks, Late Miocene to Pliocene dacitic cover rocks, and Pliocene felsic intrusions (Garwin, 2002).

Table 1. Stratigraphy of Batu Hijau (Puji, 2011)

Table 2. Alteration minerals that indicate temperature (Lawless et al., 1998)

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Figure 4. Hydrothermal alteration zoning pattern in the Lowell-Guilbert model of porphyry copper deposits (Lowell and Guilbert,1970 in Evans, 1993)

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Figure 5. Schematic diagram of the principal areas of mineralization in the Lowell-Guilbert model of porphyry copper deposits. Solid lines represent the boundaries of the alteration zones shown in Figure 4 (Lowell and

Guilbert,1970 in Evans, 1993)

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GEOLOGICAL MAP, ENREKANG REGENCY, SOUTH SULAWESI PROVINCE

DESCRIPTION:

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= ER 09102