AbstrakKabupaten Nunukan termasuk wilayah yang signifikan menyumbang cadangan batubara di Indonesia dengan cadangan hipotetik mencapai sepuluh juta ton. Salah satu data yang dibutuhkan dalam tahapan eksplorasi lanjut batubara adalah informasi lingkungan pengendapan. Penelitian ini bertujuan untuk menentukan lingkungan pengendapan batubara Miosen di Kabupaten Nunukan, Kalimantan Utara. Metode yang dilakukan dalam penelitian yaitu analisis petrografi mencakup analisis maseral dan mineral, pengukuran vitrinite reflectance, plotting Tissue Preservation Index (TPI) terhadap Gelification Index (GI). Komposisi batubara di daerah penelitian tersusun dari dominasi vitrinit (68%-91%) juga dijumpai mineral pirit (0,9%-6,6%). Berdasarkan hasil analisis, batubara di daerah penelitian adalah lignit (Ro= 0,39%-0,44%). Plot Tissue Preservation Index (TPI) dan Gelification Index (GI) menunjukkan bahwa pada saat mengendapkan gambut, daerah penelitian berada pada lingkungan lower delta plain, lahan gambut wet forest swamp dan clastic marsh, serta pada kondisi lingkungan pengendapan telmatic dan limno-telmatic. Kehadiran pirit framboidal mengindikasikan formasi pembawa batubara mengalami reduksi sulfat, yang berasal dari lingkungan anoxic, dengan ion besi pada saat pembatubaraan. Kata kunci :Maseral; Nunukan; Reflektansi Vitrinit; TPI-GI. AbstractNunukan Regency, North Borneo, significantly contributes coal reserves in Indonesia with hypothetical reserves reaching ten million tons. One of the data required in the next stage of coal exploration is information of depositional environment. This study aims to determine the Miocene coal depositional environment in Nunukan Regency. The method used in this research are petrographic analysis including observation of minerals and minerals, measurement of vitrinite reflectance, plotting tissue preservation index (TPI) of Gelification Index (GI). The composition of coal in the study area composed of vitrinite dominance (68% -91%), pyrite minerals are also identified (0.9% -6.6%). Based on the analysis, coal in the study area is lignite (Ro = 0.39% -0.44%). Tissue Preservation Index (TPI) and Gelification Index (GI) plot shows that the study area is in the lower delta plain environment, wet forest swamp and clastic marsh peatlands, and in the telmatic and limno-telmatic depositional environment during peat accumulation. The presence of ramboidal pyrite indicates the coal bearing formation experienced reduction of sulfates, from anoxic environment, with iron ions during coalification.