Paleoenvironmental, paleoecological and thermal metamorphism implications on the organic petrography and organic geochemistry of Tertiary Tanjung Enim coal, South Sumatra Basin, Indonesia

Organic petrography and organic geochemistry studies have been conducted on Tertiary age coals from Tanjung Enim, South Sumatra Basin, Indonesia. The coals are of subbituminous rank (vitrinite/huminite reflectance, VRr, = 0.35 - 0.46%). In this area, the coals were also locally intruded by andesitic sills. The thermally metamorphosed coals are of medium-volatile bituminous to meta-anthracite rank (high vitrinite reflectance, VRr up to 5.18%). The studied low rank coals are dominated by huminite. Less abundant are liptinite and inertinite. Minerals are found only in small amounts; mostly as iron sulfide. The high rank coals are dominated by thermally altered vitrinite. Mosaic structures can be recognized as groundmass and crack fillings. The most common minerals found are carbonates, pyrite or marcasite and clay minerals. The latter consist of kaolinite in low rank coal and illite and rectorite in high rank coal. Based on maceral assemblages, the low rank coals can be grouped into five classes: (1) humotelinite-rich group, (2) humodetrinite-rich group, (3) humocollinite-rich group, (4) inertinite-rich group and (5) humodetrinite-liptinite-rich group. The basal section of the studied coal seams is represented by the humodetrinite-liptinite-rich group. The middle section of the seams is characterized by humotelinite-rich and humocollinite-rich groups. The top section of the coal seams is typically represented by the humodetrinite-rich or inertinite-rich group. The sequence of maceral assemblages thus represents the change of topogenous to ombrogenous peat and the development of a raised peat bog. The coalification path of each maceral shows that vitrinite, liptinite and inertinite reflectance converge in a transition zone at VRmax of around 1.5%. Significant decrease of volatile matter occurs in the zone between 0.5 - 2.0% VRmax. A sharp bend occurs at VRmax between 2.0% and 2.5%. Above 2.5%, the volatile matter decreases only very slightly. Between VRr = 0.5% and 2.0%, the carbon content of the coals is ascending drastically. Above 2.5% VRr, the carbon content becomes relatively stable (around 95 wt.%, daf).Using Rock-Eval pyrolysis, it is known that the Oxygen Index values are low (<5 mg CO2/g TOC) and the high rank coals have a lower Hydrogen Index (< 130 mg HC/g TOC) than the low rank coals (~300 mg HC/g TOC). The Tmax increases with maturity (420 - 440ºC for low rank coals and 475 - 551ºC for high rank coals). Based on some petrographical and chemical properties, it was calculated that the temperature of contact metamorphism reached 700 - 750ºC in the most metamorphosed coal. GC/GC-MS analysis indicates that the n-alkane distribution as well as the relative amount of phytane and pristane correlate with the organic facies of the studied coals and their maturity. Some identified terpenoid biomarkers, most of all cadinane- and eudesmane-type sesquiterpenoids and oleanane- and ursane-type triterpenoids, indicate the predominance of angiosperm plants in the paleomire. Reversals in the trends of molecular maturity parameters based on aliphatic hydrocarbons (n-alkane distribution and pristane/phytane ratio) and aromatic hydrocarbons (methylphenanthrene) with coal rank are observed. This trend is also reflected by the ratio of aromatic C-H to aliphatic C-H absorbances of coals based on FTIR analysis