coal rank type maceral Ombilin depositional environment petrographic analysis utilization Asem-Asem coal Bayah Bojongmanik coals Bukit Asam CBM Cigudeg coals Coal Jambi Subbasin South Sumatera Spontaneous combustion Tertiary coal UCG coal characteristic coal quality comparative petrography crossing point temperature differential thermal analysis exploration geologic aspects geology inertinite intrusion effect low rank coal mineral matter moisture content organic petrology petrographic analyses petrographic characteristics petrography type and rank of coal upgraded brown coal utilisation