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Deposition of Oil Shale in the Huadian Basin (Eocene, NE China)


The non-marine Huadian Basin is an Eocene strike-slip basin. The basin hosts 13 thin (<7 m) commercial oil shale layers (numbered from top to bottom). Oil shale occurs in the middle part of the sedimentary succession (Oil Shale Mb.). In contrast to the Huadian Basin, the nearby Fushun Basin hosts a single 300 m-thick oil shale layer. Aims of the study are to reconstruct the depositional environment of oil shale in the Huadian Basin and to compare the depositional environment with that in the coeval Fushun Basin. To reach the goal, 75 core samples from the Oil Shale Mb. in borehole Hd3 (202–371 m) were studied using bulk parameters. Biomarker and maceral analysis were restricted to oil shale layers 3, 4 and 11, 12. Vitrinite reflectance (0.33 %Rr) indicates that the entire member is immature. High TOC/S (2–55) and MTTC ratios between 0.24 and 0.57 indicate a freshwater environment, which is also supported by the absence of gammacerane. TOC contents of oil shale layers are up to 45 %. The HI decreases downwards from 600–750 (layers 3, 4) to 250–300 mgHC/gTOC (layers 11, 12), indicating the presence of kerogen type I-II and II-III, respectively. The different kerogen types are also reflected by the dominance of alginite in the upper and landplant-derived macerals in the lower unit. A strong input of terrestrial plants in the lower oil shale unit is also supported by high amounts of diterpenoids, sesquiterpenoids and triterpenoids. The angiosperm-derived triterpenoids are predominant. Significant amounts of benzohopanes in the lower unit result from wildfires. Hopanoids and aryl isoprenoids occur in low amounts. The results show that the Oil Shale Mb. was deposited in a relative shallow freshwater lake, which became deeper only during deposition of the upper part. Oxygen-reduced conditions result in high TOC contents, but permanent photic zone anoxia was not established. The lower unit is influenced by a high input of landplant material, whereas algae prevail in the upper unit. Thus the oil shale quality increases upwards. Compared to the Fushun Basin, the Oil Shale Mb. in the Huadian Basin is characterized by lower maturity and lower paleo-water depth. The latter is reflected by frequent clastic partings. Strong terrestrial input in the lower part of the oil shale unit is recorded in both basins, but angiosperms rather than gymnosperms prevail in the Huadian Basin. In contrast, bacterial biomass is more abundant in the Fushun Basin.