SEDIMENTOLOGY AND ORGANIC PETROLOGY OF COAL AND COAL-BEARING SEQUENCES IN THE SAMARINDA AREA, LOWER KUTAI BASIN, INDONESIA

CHAW THUZAR WIN, Dr. Sugeng Sapto Surjono

2015 | Disertasi | S3 Teknik Geologi

Penelitian ini dilakukan pada batubara dan sikuen yang mengandung batubara di wilayah Samarinda, Cekungan Kutai Bagian Bawah, Indonesia. Penelitian ini dilatarbelakangi oleh ketidakjelasan informasi mengenai petrologi organik dan hubungan stratigrafi detil di antara lapisan batubara. Penelitian ini bertujuan mengintegrasikan antara karakterisasi fasies dengan sikuen stratigrafi menggunakan petrologi organik dan mineralogi batubara untuk interpretasi lingkungan pengendapan batubara di daerah penelitian. Metodologi yang digunakan dalam penelitian ini meliputi tinjauan pustaka, studi lapangan, sedimentologi, petrologi organik, mineralogi batubara, pengolahan dan interpretasi data. Pengukuran detil jalur stratigrafi, pengamatan mikroskopis dan fluoresensi sinar-X (XRF) untuk geokimia batuan dan asal batuan dilakukan untuk studi sedimentologi. Pengamatan petrografi, analisis microlithotype, analisis maseral dan reflektansi vitrinit dilakukan untuk keperluan petrologi organik. Analisis mineralogi batubara meliputi pengabuan suhu rendah (LTA), fluoresensi sinar-X (XRF) dan Scanning Electron Microscopy SEM-EDX. Urutan batuan yang diteliti dicirikan oleh batupasir tebal berselingan dengan serpih dan lapisan batubara. Batuan sedimen tersebut diteliti dan diukur pada 10 titik. Studi jalur stratigrafi komposit menunjukkan lapisan sedimen vertikal setebal 250 m (tidak termasuk interval tanah) yang tersusun atas tiga litologi utama: (1) tubuh pasir tebal (0,5-14 m), (2) interval serpih (0,5-6 m) dan lapisan batubara (0,5 sampai 1,5 m). Batupasir diinterpretasikan sebagai saluran fluvial (fluvial channels) dan asosiasi lingkungannya yaitu crevasse splays. Serpih diinterpretasikan sebagai tatanan dataran banjir. Lapisan batubara diinterpretasikan sebagai lingkungan rawa daratan. Berdasarkan pengamatan singkapan, total tujuh fasies yang berbeda dapat dibedakan berdasarkan karakteristik fisik yang dominan. Fasies-fasies tersebut adalah: fasies konglomerat dan batu pasir (Cst); fasies batupasir sedang-kasar dengan struktur silang siur trough (St); fasies batu pasir halus dengan struktur gelembur gelombang (FSR); fasies perselingan batulanau tipis dan batulumpur (FSC), fasies mire (batubara) (M), fasies batulumpur (Ms) dan fasies karbonat (Cc). Pada bagian bawah jalur parasikuen laut dangkal ditemukan adanya prograding shelf sand lobes yang ditutupi oleh terumbu karbonat transgresif. Parasikuen laut dangkal diikuti oleh parasikuen fluvial. Suksesi fluvial dicirikan oleh tubuh batupasir tebal berselingan dengan serpih dan lapisan batubara. Bagian bawah sedimen di daerah penelitian menunjukkan pergeseran fasies yang sangat cepat dari paparan laut terbuka menuju lingkungan fluvial. Material sedimen diangkut ke daerah tersebut dari sumber vulkanik dan plutonik. Batupasir di daerah tersebut merupakan sublithic arenite dan lithic arenite. Unsur-unsur utama dari batupasir di fasies ini didominasi oleh SiO2. Horizon batubara produktif berkaitan dengan periode perulangan setelah naiknya muka air laut selama progradasi sedimen di bawah deposisi regresif normal selama Low Stand Sistem Tract dan pembentukan gambut terjadi selama transgresi subsikuen. Dalam transgresi subsikuen lingkungan dataran banjir interfluvial, mires secara bertahap berkembang menjadi raised mires. Batubara yang dihasilkan didominasi oleh ombotropik dan autochothous, mengindikasikan bahwa batubara tersebut terbentuk dalam rawa gambut (peat bogs) selama iklim tropis di Miosen Tengah. Menurut hasil petrologi organik, batubara Samarinda diwakili oleh kelompok kaya humodetrinite sangat terdegradasi yang diendapkan dari darat ke kondisi telmatic. Pembentukan gambut ini memiliki karakteristik vegetasi hutan kayu yang sangat terdegradasi, berevolusi di bawah perubahan kondisi oxic ke anoxic moor dengan penggenangan menengah sampai tinggi di mana lingkungan gambut purba bergeser dari mesotropik ke ombotropik. Mineral yang dominan dalam batubara Samarinda adalah kuarsa, kelompok mineral lempung (kaolinit, dan ilit), hydra argillite, kalsit, pirit, feldspar dan gipsum. Berdasarkan komposisi dan cara terjadinya bahan mineral, dapat disimpulkan bahwa meskipun batubara terendapkan di darat dalam kondisi asam di dalam cekungan pengendapan, batubara tersebut kadang-kadang dipengaruhi oleh laut intermiten dari peristiwa banjir.

This study was carried out on the coal and coal-bearing sequences in the Samarinda area, Lower Kutai Basin, Indonesia. It was initiated against the background of indistinct information on organic petrology and detail stratigraphic relationship between the coal-bearing strata. The study aimed at integrating investigations of characterization of the facies in combination with a coal sequence stratigraphic interpretation with the aid of organic petrology and mineralogy of coal which may lead to interpretation of the depositional environment of the coal in the study area. The methodologies used in this study include literature review, field visits, sedimentology, organic petrology, coal mineralogy, data processing and interpretation. Measuring detail stratigraphic columnar sections, microscopy and X-ray Fluorescence (XRF) for rock geochemistry and provenance were done in order to study sedimentology. The Petrographic, microlithotype analysis, maceral analysis, Vitrinite Reflectance Measurement were conducted for organic petrology. The coal mineralogy analyses include Low Temperature Ashing (LTA), X-ray Fluorescence (XRF) and Scanning Electron Microscopy SEM-EDX. The studying succession is characterized by thick sandstone bodies alternating with shales and coal beds. The sedimentary rocks were examined and measured at 10 sites. The study composite stratigraphic section shows 250 m thick (not included soil interval) of vertically stacked sediments constituted by three main lithologies: (1) thick sand bodies (0.5 to 14 m), (2) shale intervals (0.5 to 6 m thick) and coal layers (0.5 to 1.5 m thick). The sandstones are interpreted as fluvial channels and their associated crevasse splays environment. The shales are interpreted as a flood plain setting. The coal layers are interpreted as a terrestrial swamp environment. From the outcrop observation, a total of seven facies can be discriminated base on their predominant physical characteristics. They are conglomerate and sandstone facies (Cst), coarse to medium-grained sandstone with trough cross-bedding facies (St), fine-grained sandstone with current ripples facies (Fsr), thinly interbedded siltstone and mudstone facies (Fsc), mire (coal) facies (M), Mudstone facies (Ms) and Carbonate facies (Cc). At the bottom of the section shallow marine parasequences are observed containing prograding shelf sand lobes capped by transgressive reef carbonates. After the shallow marine parasequence, terrestrial fluvial parasequence is followed. The fluvial succession of strata is characterized by thick sandstone bodies alternating with shales and coal beds. The sediment in the research area shows at its bottom a very rapid shift of facies from open marine shelf to terrestrial fluvial environment. The material was transported to region from both volcanic and plutonic sources. The sandstones are sublithic arinite and lithic arenite. The major elements of sandstones in these facies are dominated by SiO2. The productive coal horizons are related to recurrent periods after sea-level high-stand during sediment progradation under normal regressive deposition during Low Stand System Tract and peat formation occurred during subsequence transgression. In this subsequence transgression of interfluvial flood plain environment, the mires gradually developed into raised mires and the coal is mostly ombrotropic and autochothous coal indicating that it was formed in peat bogs during tropical climate in Middle Miocene. According to the organic petrological results, Samarinda coals are represented by the highly degraded humodetrinite-rich group deposited from terrestrial into telmatic condition of peat formation with vegetation characteristics of highly degraded woody forest type evolved under alternate oxic to anoxic moor condition with intermittent moderate to high flooding and the paleo-peat environment shifted mesotrophic to ombrotrophic. The predominance of mineral matter in the coals of Samarinda area is quartz, clay group minerals (kaolinite and illite), hydra argillite, calcite, pyrite and feldspar and gypsum. Based on the composition and mode of occurrence of mineral matter, it can be said that although the coals are deposited landward under acidic condition within a depositional basin, they are sometimes influenced by intermittent marine influence from flooding event.

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