DIVERSE SEDIMENTARY CONDITIONS DURING DEPOSITION OF COAL AND OIL SHALE FROM THE MEIHE BASIN (EOCENE, NE CHINA)

摘要

The Meihe Basin in northeastern China is a normal-fault-controlled basin filled with Eocene nonmarine deposits, the sediments assigned to the Meihe Formation. Sediment grain-sizes, core study, well-log analysis and micropetrographic study combined with sedimentary facies analysis indicate that the main sedimentary facies of the Meihe Basin represent fan-delta, lake, and turbidite deposits. Thick coal seams that are mainly distributed in the eastern part of the studied area were deposited in the swamp facies of the Lower Coal-Bearing Member. Oil shale was mainly deposited in the deep lake represented by the Mudstone Member, and is mainly distributed in the southern part of the studied area. In the Lower Coal-Bearing Member, organic-rich sediments are characterized by TOC content between 0.6 wt. and 43.1 wt. and contain Type II and III kerogen. The thick coal seams are the result of high land-plant productivity and good preservation conditions. Thin coal seams in the fan-delta plain resulted from the migration and erosion of the fan-delta channel. In the shallow lake and fan-delta front, limited productivity coupled with dilution by inorganic matter resulted in lower organic-matter content in the Mudstone Member. Oil shale and mudstone rich in organic matter were deposited in a deep lake with an average TOC content of 3.4 wt.. The origin of the organic matter was mainly algae and aquatic plants mixed with land plants. The improved preservation conditions and high bioproductivity in this environment resulted in three oil-shale layers in the lower part of this sedimentary succession.