Pustaka Ilmiah Universitas Padjadjaran

Abstrak

Sajau Low Rank Coal Berau Basin Northeast Kalimantan Indonesia

The Berau basin which is located in East Kalimantan is one of the major coal basin in Indonesia are among of the sedimentary basin rich-coalbed methane with complicated geological structure conditions. The low rank Sajau coal formation is predominantly composed by lignite to sub bituminous C coal with a mean huminite reflectance of 0,32% indicated in the immature stage and have a thickness from 0,90 – 38 m. The main topics of this book are the tectonics, coal sequence stratigraphy, coal petrographic and organic, inorganic geochemical characterization of the low rank coal of Sajau Formation. To obtain optimal results in studies have been carried out various measurements both in the field and in the laboratory with various scales of molecular research (GC and GC-MS), micro (reflectedlight microscopy and scanning electron microscopy/SEM), meso (CT Scan Tomography) to macro levels. The stratigraphy succession in Berau Basin fromthe old to the young, such as ; Sembakung Formation,Tabalar Formation, Birang Formation, Latih Formation, Labanan Formation , Domaring Formation , and Sajau Formation. The Sajau formation consist of recurring intervals of sand, silt-, and mudstones with coals The Sajau low rank coal seam was rested on the top of regional unconformity in Early Pliocene sediments and formed in the graben and/or pull-apart basin. This basin type is characterized by very high subsidence rates, which control basin architecture and peat forming environments. The first stages of sedimentation of Sajau formation in Berau basin by filled with fluvial sediments. Because subsidence exceeded sediment input, the fluvial environment was replaced by a deltaic environment. The Sajau coal seam formed during the transition from the fluvial to the deltaic environment. The formations show four facies: braided river, meandering, and lower delta plain and upper delta plain. These facies are grouped to form three facies associations braided river, fluvial and delta based on vertical and lateral facies relationships. These facies and their associated environments to reflected the shifted landward and basinward as a response to base level changes resulting in a complex, but well-defined cyclic architecture within the Sajau Formation. The recurring architecture reveals a distinct deepening trend represented by successively more preferences marine sedimentation towards the top (fining upward). The high ash content in lower part of the Sajau coal seam (Seam A, B, and C) indicated there were a strong influenced by fluvial processes, while in the lowash coal seam (coal seam K, L, M) in the upper part has been mainly influenced by deltaic processes. The strong influenced of fluvial processes in lower part was also confirmed by the abundance of xylitic coal (XC) with a conifer trees in the lower part comparing in the upper part which dominated by detro-xylitic coal (DXC) rich in bush and reed vegetation. The macro organic petrography was studied from geological mapping of Sajau coals in the outcrops and borehole cores. The “10 % rule” is the quantitative rules was used by most scientist and also recommended by the ICCP (1993) in determining the low rank coal lithotypes; xylitic coal (forest swamp) facies characteristic for the lower part of the coal seam, to fine detrital (detro-xylitic coal) in upper part which has open-water surfaces with dwarf plant vegetation or fen facies predominating in the inner and the upper part of the seam. The xylitic coal is forming in a dry forest mire, whereas the detrital coal is forming in wetter swamp with herbaceous vegetation in the central and northern part of the basin. Based on the maceral assemblage, especially the huminite and mineral matter content, the Sajau Coal succession should be grouped into three coal facies i.e. (I) high humocolinite (more than 40%), categorized as limno-telmatic type, formed in the lower delta plain in wet conditions, with the herbaceous plant was a source of peat. (II) moderately humocolinite (20% – 40%) formed in the upper delta plain in the wet to dry condition is the most probable environment of this coal facies and wood was a major contributor of the peat; and (III) low humocolinite (less than 20%) was formed in fluviatile dominated influence environment with the conifer is a major source of peat. The organic geochemistry study of the low rank Sajau coals by the absence of saturated sesquiterpenoid and diterpenoid (pimarane, abietane, and phyllocladane – type diterpenoids of the C 19 to C 20 range); it is may likely result from the low maturity of organic matter and the dominance of an anoxic environment, where dehydrogenated and aromatic terpenoids (can be easily transformed and preserved through microbial activities. The predominance of hopanoid and non hopanoid terpenoid biomarkers, such as oleanane, lupane and ursane-type triterpenoids, indicate the predominance of angiosperm plants in the Sajau palaeomire, particularly Dipterocarpaceae. The low presence of aromatic sesquiterpenoid of cadelene-type (cadina triene, cadina tetraene) and A-ring opened isohexyl alkylaromatic; were confirmation that the gymnosperm/conifer trees should be possible contributor the organic material to the Sajau coals. The presence of such combustion-derived PAHs may reflect the burning of vegetation prior to deposition. The inorganic geochemical analysis of Sajau coal was performed throughout the DH-102 well profile. It was found that inorganic matter of the coal is characterized by the following composition: SiO2 (33.84–42.13%), Al2O3 (13.60–22.68%), Fe2O3 (18.10–38.82%), MgO (3.108.82%) and CaO (1.02–7.02%). The SiO2+ Al2O3 is increasing upward and following by the decreasing Fe2O3 and CaO. The kaolinite is a predominance clay mineral type in Sajau coals which indicated from strong correlation between SiO2 and Al2O3 (r = 0.92). The Pliocene lignite-sub-bituminous seams are characterized by the presence of natural discontinuities, so-called cleats. Most often they are opening-mode fractures, consisting of two orthogonal sets (face and butt), both almost perpendicular to the bedding. This paper determines distributions of cleat orientation, spacing, and aperture from the Sajau lignite seams. All observations and measurements were conducted at macroscopic, mesoscopic and microscopic scale. The butt cleats mean orientations are NE–SW, NW-SE; whereas mean orientation of face cleats NNE-SSW and NE–SE, dipping at a high angle N75°. The angle between strikes of cleat sets is nearly 90°. The spacing of macro face cleats is from 9.52 to 14.46 cm (averaging 11.61 cm) and the spacing of butt cleats is from 2.3 to 11.3 cm (averaging 5.35 cm), and the aperture has a mean 0, 54 cm On the other hand, the mean spacing of microcleats 1.58 mm and aperture measurements of these cleats range from 0.021 to 0.029 mm (averaging 0.026 mm), respecttively. The obtained results from outcrop and micro CT Scan Tomography clearly indicate that face cleat orientations NNE–SSW are strictly parallel to the elongation of the main tectonic structures in the study area. Their origin may be explained in at least there was relationship with local tectonic (the maximum principal stress, s1, was horizontal). The Partial Least Square analysis, of cleat and faults data in this area indicate that a power-law distribution exists between cleats characteristics (spacing, density and aperture) with the distance of faults ( R2 = 0.56). The cleat formation in Sajau Formation was mainly controlled by mechanical in response to tectonic. Based on SEM photography; the origin of cleats in Sajau Formation area endogenic process and tectonic activity which indicated by change the shape of the cleats; from the straight line cleats to curved shape and branching.