Microfacies and Depositional Environment of the Eocene Sawahlunto Coal, Ombilin Basin, Indonesia
Ombilin Basin is well-known as one of the largest coal-producing basins in Indonesia. There is no comprehensive study conducted regarding the relationship between petrographical characteristics and coal properties in the area. The results of the petrographic study on Ombilin coal are dominated by vitrinite (45.82-72.21 col.%), liptinite (14.91-42.55 vol.%), inertinite (15.19-25.68 vol.%), and mineral matter (0.36-11.45 vol.%) which is mostly dominated by pyrite. Based on the maceral association, Ombilin coal is distinguished into five groups; telovitrinite-rich group, detrovitrinite-rich group, telovitrinite-liptinite-rich group, telovitrinite-inertinite-rich group, and inertinite-rich group. A similar correlation is found between maceral and maceral-precursor abundances in modern tropical peat in Indonesia. The basal section of Ombilin coal is interpreted to be composed of sapric and hemic peat, represented by the detrovitrinite-rich and telovitrinite-liptinite-rich groups, middle section is distinct due to the high content of the telovitrinite-rich and inertinite-rich groups, these groups are derived from fine hemic and hemic peat. The top section is dominated by inertinite-rich and detrovitrinite-rich groups and is typically comprised of fibric peat. Correlation between vertical section and maceral abundances represent the evolution changes from topogenous mire to ombrogenous mire. Coal facies and distribution of critical macerals suggest that the coal is derived from degraded woody plant tissues and marginal aquatic or herbaceous plants. The depositional environment of the precursor plants was deposited in wet forest swamp conditions and exhibited transition from limnic to limno- telmatic environment.