Doctor of Philosophy
Department of Geology
Nas, Chairul, Spatial variations in the thickness and coal quality of the Sangatta Seam, Kutei Basin, Kalimantan, Indonesia, Doctor of Philosophy thesis, Department of Geology, University of Wollongong, 1994. https://ro.uow.edu.au/theses/1409
The Sangatta coal seam, which contains high volatile bituminous coal and is the most important seam in the Sangatta Coalfield, was deposited within a Late Middle Miocene fluvial system that occupied the northern Kutei Basin, Indonesia. A study of clastic sedimentology, coal petrology of standard and etched coal samples, thickness and coal quality parameters, integrated with a geostatistical analysis, identified the depositional environment of the seam. This study indicates that the coal seam was deposited within the floodplain of a mixed-load fluvio-deltaic system with the rivers flowing southeastward. This sedimentary system is believed to have been a major control on the development of the Sangatta peat mire.
Local changes in sedimentary facies below and above the Sangatta seam caused variations in the local rates of subsidence and compaction which in turn controlled the peat swamp morphology and coalification pattern. The morphological variations governed hydrologic conditions in the swamp which, in turn, influenced peat accumulation and subsequent geological processes acting on, and within, the peat. These factors also influenced seam thickness, maceral composition and coal quality parameters.
The Sangatta seam has an average thickness of 6 m. The coals are characterised by a high vitrinite (average of 9%), low liptinite (average of 3%), low inertinite (average of 3%) , very low mineral matter (average of 2%) and low sulphur (average of 0.4%). The seam formed as a raised peat bog and, at some stages, was confined by river channels with vegetated levees. The general climate was very humid.
The Sangatta seam shows a bimodal normal thickness distribution with a high variability, a low degree of regularity (R = 0.36) and a low degree of spatial continuity, with a range of influence of 420 m. The seam formed under a complex process which involved an initial stage of peat generation followed by thickness modification attributed to geological process such as erosion (washout) and faulting. The spatial distribution of the thickness is anisotropic with the greatest continuity of thickness in a southeast direction (135°) which parallels the direction of clastic sedimentation in the coalfield. There is a negative statistical correlation between the local mean of the thickness and the thickness variability, whereby the thickest parts of the seam have a less variable thickness than the thinner parts.
The Sangatta seam was deposited in four geographic zones; the zones also correspond to differing geological processes within the zones. Within each zone the statistical parameters are zone specific. In the western zone the seam is thickest, does not split, has unimodal thickness and sulphur populations, the smallest thickness variability, greatest thickness continuity (range = 700 m ) , greatest thickness isotropy, abundant structured vitrinite, smallest number of clastic partings and a low ash content. The central zone has a bimodal population and a high variability for the thickness data, low thickness continuity (range = 300 m ) , low anisotropic thickness distribution, low sulphur content, little woody tissue and mostly degraded plant tissues. The eastern zone shows the greatest thickness variability with a relatively high ash yield, lowest thickness regularity (R = 0.23) and a strong thickness anisotropy. The northern zone has statistical parameters that tend to have intermediate values.
During peat accumulation the four zones had different depositional histories. The central zone formed as the central part of a raised bog, but immediately subsided, resulting in retardation of peat development. The western zone experienced the most stable development pattern of any zone. In the eastern zone, although subsidence was not as great as in the central zone, syn-depositional structures and subsequent sedimentary processes acted on the coal seam. The northern zone is the least known part of the seam due to a limited drilling program, although preliminary data indicate it experienced a similar history to the eastern zone.
The depositional model formulated should improve the geological certainty in reserve estimation, mine planning and utilisation of the Sangatta coal. The zoned nature of the thickness and quality data necessitates "zoned kriging" for coal reserve estimation and in mine planning. In "zoned kriging", the depositional model for the Sangatta seam should control the steps in geostatistical reserve estimation. More specifically, reserve estimation should be undertaken separately for each zone.
This study can be used as a predictive model for coal exploration and basin analysis assessment in other Indonesian coalfields where geological conditions are similar.