Samples of coal from a stratigraphic section of Balikpapan Formation, near Samarinda , Lower Kutai Basin were studied for their mineralogy and distribution of pyrite using plasma low-temperature ashing plus X-ray diffraction, petrographic examination and SEM analysis. The results show that the inorganic matters of the coal are composed of quartz, clay minerals, calcite, pyrite, chlorite, feldspar and gypsum. Both epigenetic and syngenetic pyrite occurred in this coals and influence of marine conditions is more favored in the lower portion of the studied stratigraphic section. Epigenetic pyrite and minerals might originate from the erosion of Early Tertiary marine sediments of the Central Kalimantan ridge.
