Depositional Environment of the Sangkarewang Oil Shale, Ombilin Basin, Indonesia

Five samples from a 56 m long drill core that was recovered from the lacustrine Sangkarewang oil shale have been studied by means of petrography and organic geochemistry in order to investigate the organic matter composition and depositional environment of the shale. The organic matter consisted of abundant lamalginite (30%, v/v) and a very limited amount of vitrinite, suggesting an aquatic depositional environment with minor terrestrial influence. Organic geochemical analysis exhibited the dominance of pristane, phytane, and generally n-alkane compounds. These compounds may have originated mostly from aquatic photosynthetic organisms. The oil shale was likely deposited in an anoxic lake environment as suggested by the presence of framboidal pyrite (6%, v/v) and preserved organic matter with a total organic carbon (TOC) percentage of about 4.9%. The pristane/phytane ratio was relatively high (about 3.9) and thought to be source sensitive rather than redox sensitive. Hopanoid and aryl isoprenoid compounds were present in minor amounts. The latter compounds are interpreted to be derived from green sulfur bacteria dwelling in an anoxic environment with the presence of H2S in bottom water.