Strata of the Middle Miocene Balikpapan Formation from the Lower Kutai basin are well exposed in a section near the Samarinda city, East Kalimantan, Indonesia. The succession is characterized by thick sandstone bodies alternating with shales and coal beds. A 250？m thick composite section of exposed sediments (not including the soil interval) was measured, from which 25 coal samples were collected. Petrographic, microlithotype, and maceral analyses were performed in order to determine the depositional environment of the Samarinda coals. In order to assess the development of paleomires, coal facies diagrams were obtained from microlithotype and maceral composition. According to the organic petrologic results, the Samarinda coals represent a highly degraded humodetrinite-rich group deposited from terrestrial into telmatic condition of peat formation with vegetation characteristics of highly degraded woody forest type evolved under alternate oxic to anoxic moor conditions. These formed with intermittent moderate to high flooding as the paleopeat environment shifted from mesotrophic to ombrotrophic. 1. Introduction The Kutai basin is the largest Tertiary basin in Western Indonesia. Deltaic sedimentation has been continuous in the Kutai basin from Late Oligocene to the present day as represented by the modern Mahakam delta and upstream in the continent [1]. There is a structural deepening of formations in the basin to the east to the Mahakam area, where hydrocarbon fields are situated (Figure 1). Figure 1: (a) Location and regional geology of the studied area and (b) summary outcrop log and coal bearing interval of the studied interval. A composite log of 250？m (not including the soil interval) of exposed sediments of the Balikpapan formation was made on August 6 to 13, 2012. The sediments and coal were examined at 10 sites and the locations of which within the study area are shown in (Figure 1). This section has been previously investigated by Cibaj, 2010 and Cibaj et al., 2007 [1, 2]. They showed Middle Miocene Seravallian age (NN5 to NN11 nanoplankton zones). The purpose of the present study is to assess the organic petrology and the interpretation of coal facies and depositional environment of coals of the Samarinda area, Lower Kutai basin, Indonesia. In the study of coals, maceral analysis and microlithotype analysis are used to assess the environment of deposition when paleobotanical data are scarce or absent. This implementation of maceral ratios such as the Vegetation Index and Ground water Index tried to overcome this hurdle, but this approach has
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