Doctor of Philosophy
School of Geosciences
Pujobroto, Agus, Organic petrology and geochemistry of Bukit Asam coal, South Sumatra, Indonesia, Doctor of Philosophy thesis, School of Geosciences, University of Wollongong, 1997. http://ro.uow.edu.au/theses/1975
This study is a petrographic analysis of the coal in the Bukit Asam Coalfield which includes a complex of working coal mines located near the small town of Tanjung Enim, South Sumatra, Indonesia. The Bukit Asam coal seams are part of the Muara Enim Formation which is composed of shale, siltstone, claystone, sandstone and intercalated coal layers. Within the immediate Bukit Asam area, the Muara Enim Formation can be divided into two units - a lower unit, which consists mostly of coal, dark grey to black shale, brownish-grey claystone, sandy claystone and a few coarse-grained sandstone beds, and an upper unit which is composed of shale, coal, tuff, claystone and sandstone. Tectonically, the basin formed in a depression zone in a back-arc basin in the early Tertiary Sumatra subduction system. The Muara Enim Formation was deposited during a regressive phase in a transgressive-regressive cycle that controlled sedimentation in the South Sumatra Basin.
The lower part of the sequence was deposited in a freshwater lagoon with probably several marine incursions in the lower part and shallowing as indicated by the deposition of a prograding delta sequence preceding quieter water conditions, probably a tidal flat. Later deposition was in a deltaic environment. The Mangus Tuff (A2 Seam to A1 Seam interval) is a series of volcanic packages and represents a large increase in the volume of volcanic detritus entering the basin.
Bukit Asam coal is dominated by vitrinite with minor liptinite, inertinite and mineral matter (mostly clay minerals, quartz and rarely pyrite). Vitrinite is predominantly detrovitrinite with significant amounts of telovitrinite and minor gelinite. The detrovitrinite contents range from 7.8 to 76.1% (an average of 56.6%) whereas telovitrinite contents range from 3.0 to 62.6% with an average 21.1%; gelovitrinite contents range from 1.4 to 27.6% (an average of 9.2%). Liptinite averages 5.3%, but ranges from 0.2 to 25.4%. Resinite, sporinite, suberinite, cutinite, liptodetrinite are the dominant liptinite with minor fluorinite and exsudatinite. It is likely that much of the exsudatinite was produced by the influence of the intrusions. Inertinite constitutes from trace to 11.2% (average of 3.6%). The most common inertinite is semifusinite, followed by inertodetrinite, sclerotinite, fusinite and sparse macrinite. Samples from A2 seam have the highest inertinite content (>5%).
There is no significant interseam difference in maceral composition with all seams having high vitrinite contents (88.0 to 90.5%), and low liptinite (4.2 to 5.0%) and inertinite contents (4.1 to 5.5%). Intraseam variations in maceral composition are also relatively small.
The rank range from sub-bituminous (Rvmax ranges from 0.35 to 0.5% with most values falling in the range of 0.4 to 0.46%) to semi-anthracite (Rvmax 2 %) . The wide range of ranks is a result of igneous intrusions, of andesitic composition.
Fourpetrographic components, detrovitrinite content, sporinite-inertodetrinite-liptodetrinite content, telovitrinite content and mineral matter, were used to determine coal facies. The model for the formation of Bukit Asam coal is a raised peat bog, with gradual development from a topogenous peat at the base to a raised mire for the middle of the seam. The petrographic composition of Bukit Asam coal indicates that the mires in which the peat was deposited were mostly of a raised high moor type but these did not reach the mature stage of peat development, except for A1 seam, because of a lack of fibric peat at the top each peat episode. The mires of Bukit Asam were subjected to basement subsidence with the rate of subsidence similar to the rate of plant accumulation. A modern analogue is thought to be the Jambi peat deposit of Sumatra, Indonesia.
In Bukit Asam coal, the major oxide and trace element composition is closely linked to mineral content of the samples with an increase in quartz and calcite in the heated coal. Rb, Cs and K are most likely derived from detrital clay (K-rich clay or mixed clay) and have been leached from the claystone (tonstein) layers and redeposited in the coal. Y, Zr, Nb, Th and to a lesser extent Sb, Hf and U have also been mobilised from the tonsteins. Ti, and sometimes Ga, are immobile elements and are found in the tonsteins, and Ba, Sr and to a lesser extent Zn and Pb, are organically-bound elements.
The best use for Bukit Asam coal is as a steaming coal and as a feedstock for coal liquefaction. However, the economics of the liquefaction process in an Indonesian scenario, is still equivocal. Some medium-rank coal may be suitable as a blending component with high inertinite coal with the similar rank to produce strong coking coal.
Bukit Asam coal is likely to cause some fouling and slagging but selective mining probably can improve the performance of the coal.
The significant outcomes of this thesis are: documentation of the significant influence of volcanic activity, development of the model for the environment of deposition, demonstration that facies models for Permian and Carboniferous coals are not suitable for Indonesian Tertiary coals, showing that the quart-rich ply at the top of A2 is a direct result of the overlying volcanic ash deposit, and demonstration of the influence of the intrusions on the coal.