Mineralogy and organic petrology of oil shales in the Sangkarewang Formation, Ombilin Basin, West Sumatra, Indonesia

Abstract

The Ombilin Basin is filled by late Eocene to early Oligocene marginal fan deposits (Brani Formation) and lacustrine shales (Sangkarewang Formation), unconformably overlain by a late Oligocene to early Miocene fluvial sequence (Sawahlunto and Sawahtambang Formations) and capped by an early to mid-Miocene marine sequence (Ombilin Formation). Significant oil shale deposits occur in the Sangkarewang Formation, intercalated with thin laminated greenish-grey calcareous sandstones. X-ray diffraction shows that the sediments consist mainly of quartz, feldspar, carbonates and a range of clay minerals, together in some cases with minor proportions of sulphides, evaporites and zeolites. Feldspar and non-kaolinite clay minerals decrease up the sequence, relative to kaolinite, suggesting a changing sediment source as the basin was filled. Calcite, thought to be mainly of authigenic origin, is also more abundant in the middle and upper parts of the sequence. 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.

Paleoenvironmental conditions in the late Paleogene, Sumatra, Indonesia

“…The Sumatran Fault System resulted from the oblique subduction of the Indo-Australian plate under the Eurasian plate (Situmorang et al, 1991). Tensional tectonics that resulted from the strike-slip movement of this collision produced the graben-like, pull-apart structure of the Ombilin Basin (Koning, 1985;Fatimah and Ward, 2009). The present study area is located within the Talawi syncline or Talawi Sub-basin in the northwestern section of the Ombilin Basin (Howells, 1997).…”

“…These shale successions, interpreted as freshwater lacustrine, are overlain by latest Eocene to early Miocene fluvial sediments of the Sawahlunto and Sawahtambang formations. The Sawahlunto Formation is composed of coal, shale, siltstone, and quartz-rich sandstone that occur primarily as migrating point bar channel fills (Fatimah and Ward, 2009). The Sawahtambang Formation overlies the Sawahlunto and is interpreted as a braided river deposit primarily consisting of cross-bedded quartz sandstone and conglomerate (Koesoemadinata and Matasak, 1981).…”

“…This study focuses on a small part of the late Paleogene Sawahlunto Formation. The oldest sediment in the Ombilin Basin, the Brani Formation, represents Eocene marginal fan deposition and consists of a sequence of breccias and polymictic pebble to cobble conglomerate in a muddy to sandy matrix that interfinger laterally and basinward with the Sangkarewang Formation (Fatimah and Ward, 2009;Whateley and Jordan, 1989). The Sangkarewang Formation is composed of dark bluish gray to black laminated shale that is commonly intercalated with greenish-gray feldspathic turbidite sandstone.…”

“…The basin's Cenozoic fill overlies pre-Cenozoic basement rocks and includes in ascending order; the Brani Formation, the Sangkarewang Formation, the Sawahlunto Formation, the Sawahtambang Formation, and the Ombilin Formation (Fatimah and Ward, 2009;Koesoemadinata and Matasak, 1981;Whateley and Jordan, 1989). This study focuses on a small part of the late Paleogene Sawahlunto Formation.…”

Geochemical and palynological investigation of the Shengli River marine oil shale (China): Implications for paleoenvironment and paleoclimate

“…This interpretation is consistent with the low Pr/Ph ratios as discussed above and is also supported by abundant syngenetic pyrite (Fu et al, 2007). Syngenetic pyrite in coal and sedimentary rocks usually represents a reducing environment (Dai et al, 2002(Dai et al, , 2003Fatimah and Ward, 2009;Hackley et al, 2009;Jelonek et al, 2007;Nowak, 2007).…”