This paper was first published in October 2023.
Abstract
Low rank coal is not as stable as sub-bituminous and bituminous coal. This makes the exploration and measurement of these deposits require specific techniques to ensure results are representative of the actual low rank coal in the ground. The physical structure of low rank coal can quickly breakdown and change, soon after extraction, so samples must be handled carefully to preserve the original condition of the coal.
Regional Studies
Ideally, exploration for these deposits should begin with regional studies. In Indonesia, these types of deposits are relatively easy to find. They can occur anywhere, where Neogene sediments have accumulated. Generally, the exploitation of these resources is very sensitive to the logistics of transportation and handling so targeting places where these challenges can be minimized will give an immediate economic advantage to a low rank coal project. For example mine-mouth power stations are particularly suited to lignite utilization.
Mapping
Often low rank coal deposits in Indonesia are thick and flat lying in areas of relatively flat topography. This does not create ideal conditions for mapping. Often exposures of coal in the field take the form of an eroded coal surface exposed in a stream with no indication of thickness or inclination. Where thick seams are exposed faulting of the coal should be suspected and recorded. Sampling from exposed outcrops can also produce oxidized samples not representative of the true quality of the coal deposit.
Figure 1 mapping a low rank coal deposit, East Kalimantan
Drilling
The best type of drilling for these deposits is rotary drilling using water for circulation. Drill muds can be used but care must be taken that drill mud, grease oils or other potential contaminants don’t affect the coal samples. Coal should be cored for sampling purposes and N size (76mm) is usually adequate for initial exploration. Larger sample diameters such as H and P size core can be beneficial in core recovery and potentially provide better samples for laboratory testing mainly because they expose less of the coal surface area to potential oxidation during handling and transfer to a laboratory.
Drilling these deposits is best on a grid basis to ensure an even spread of points of observation throughout the deposit. In some areas where ash, partings and wash-outs are observed more closely spaced drilling is recommended. Geostatistics should be used to ensure adequate drill spacing to sample coal deposits representatively based on the statistical variability of the coal seam thickness and quality.
Figure 2 a typical gridded drill pattern for a low rank coal deposit in Indonesia
Downhole Geophysical Logging
Gamma ray, density (long and short) measurements are essential to accurately measure the coal seam depth, thickness and purity. It is crucial the geophysical logs are used to reconcile the coal thickness as lignite tends to expand on extraction from the core barrel which can lead to erroneous measurements of core lengths. Correlation of coal seams from one location to another can also be confirmed by geophysical logs giving confidence to geological interpretation. Geophysical logs can also help to recognize lithotypes and cyclothems, that represent variations in environmental conditions such as seasons, that occurred during the original accumulation of the peat.
Figure 3 a down hole logging operation
Facies Modeling
Facies is a term used in geology that means the result of deposition in a particular environment. As low rank coals in Indonesia are relatively young the depositional environments tend to be readily recognized and these can be used to direct exploration and drilling to optimize results. It is important to recognize the depositional environment of any low rank coal deposit to be able to map out likely zones of prospective ground as opposed to areas where discontinuities and contamination might occur. Typically, in Indonesia, low rank coal deposits are the result of deposition in a fluvial (river) or deltaic environment. In these environments overbank swamp areas, is where coal seams are thickest. These areas must be distinguished from the location of stream channels where coal can be high in ash or completely eroded and lost due to washouts. Understanding the original depositional environment for any low rank coal deposit is the key to understanding the deposit and optimizing the exploitation process.
Figure 4 peat swamp environment determines the shape of the low rank coal deposit
Sampling
The best samples for low rank coal testing are fresh cores. On extraction from the core barrel these samples should be immediately described by the geologist, photographed and divided into sample intervals. In most cases low rank coal deposits are banded. These are called lithotypes and they represent variations in the original vegetation in the peat as it accumulated and or different degrees of breakdown (oxidation) of the original organic matter as the deposit was formed. Ideally low rank coal should be sampled according to these lithotype boundaries and not necessarily on purely meter intervals. Different lithotypes will have different coal properties. Most low rank coals are heterogeneous in this way.
Figure 5 coal lithotypes displayed as lighter and darker layers
Moisture testing is particularly important for low rank coal. High, porosity can make moisture content of the coal vary dramatically within only very short periods of exposure to the atmosphere. In sampling low rank coal deposits, it is important to minimize any oxidation of the coal on extraction from the core barrel. The best way to achieve this is to immediately immerse the coal core into PVC tubing full of water after the coal core has been logged by a geologist. This method preserves the coal from oxidation while transported to the laboratory. Samples with clay or non-coal partings are not suited to this sampling technique as the partings may be dissolved and lost during transfer to the lab. Another technique is to vacuum pack the coal sample cores into a suitably sized core sock. Cores should be transferred to the lab in this way as soon as possible and any storage should be done under low light, refrigerated or at least cooled conditions.
Figure 6 low rank coal cores stored in PVC pipes containing water
Figure 7 low rank coal cores sealed in plastic core sock
The low rank coal insitu moisture content is crucial and difficult to determine. Total Moisture, although a good measure of coal bed moisture in higher rank coal, may vary significantly from the moisture holding capacity of low rank coal and Equilibrium Moisture is recommended as a more representative way of determination of low rank coal moisture levels insitu. Once the insitu bed moisture is established the Preston & Sanders equation for the calculation of relative density (RD) can be used. If this is not done correctly it can easily impact negatively by causing an over-estimation of coal Reserves (Fletcher et al April 2012).
Figure 8 Preston & Sanders Formula
CONCLUSION
Low rank coal is different to bituminous coal and requires specialized exploration methods particularly in relation to sampling and coal quality analyses. Low rank coal is not as stable as bituminous coal and will start to change physically and chemically as soon as its exposed to the atmosphere. To measure coal quality in low rank coal accurately, samples need to be quickly stored so that oxidation is minimized and the original condition of the coal is preserved. Equilibrium Moisture, which measures the moisture holding capacity of the coal as well as a density adjustment using the Preston Sander’s Formula are crucial to avoid over estimation of coal tonnage in estimating low rank coal Resources and Reserves.
References
Cook A.C. 1982, The origin and petrology of organic matter in coals, oil shales and petroleum source rocks. University of Wollongong, Australia
Durie R.A. 1991, The science of Victorian brown coal: structure, properties and consequences for utilization. CSIRO, Division of Coal and Energy Technology, Butterworth Heinemann
Preston K.P. & Sanders R.H. Estimating the insitu Relative Density of coal. Australian Coal Geology Volume 9 14 July 2013