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Abstract
The Ombilin Basin, which lies in Sumatra Island, is one of the Tertiary basins in Indonesia. This basin contains a wide variety of rock units, from pre-Tertiary to Quaternary in age. Significant oil shale deposits occur in the Sangkarewang Formation which was deposited during Paleocene-Eocene time.
Several analyses have been carried out namely, XRD, XRF, carbon and sulphur determination, thin section petrology, polished section petrology as well as Fischer assay. These were intended to determine the inorganic and organic constituents of the Sangkarewang oil shale.
Inorganic constituents of the Sangkarewang oil shale consist mainly of quartz, feldspar, carbonates and a range of clay minerals, together in some cases with minor proportions of sulphides, evaporites and zeolites.
The organic matter in the oil shales of the sequence is dominated by liptinite macerals, particularly alginite (mainly lamalginite) and sporinite. Cutinite also occurs in some samples, along with resinite and traces of bituminite. The dominance of lamalginite in the liptinite components suggests that the material can be described as a lamosite. Samples from the Sangkarewang Formation have vitrinite reflectance values ranging between 0.37% and 0.55%. These are markedly lower than the vitrinite reflectance for coal from the overlying Sawahlunto Formation (0.68%), possibly due to suppression associated with the abundant liptinite in the oil shales.
Fischer assay data on outcrop samples indicate that the oil yield is related to the organic carbon content. Correlations with XRD data show that, with one exception, the oil yield and organic carbon can also be correlated directly to the abundance of carbonate (calcite) and inversely to the abundance of quartz plus feldspar. This suggests that the abundance of algal material in the lake sediments was preferentially associated with carbonate deposition. High yields of oil are noted in some samples, as a percentage of the organic carbon content. This may indicate that partial generation of hydrocarbons from the material has already taken place, in association with thermal maturation of the Sangkarewang succession.