Petrology and hydrocarbon potential of microalgal and macroalgal dominated oil shales from the Eocene Huadian Formation, NE China

Abstract

A series of oil shales of middle to late Eocene age occur in the Huadian area of Jinlin Province, northeast China. Analytical results from 22 rock samples collected from an underground mine in the Guanglangtou district in the Huadian Basin show that the total organic carbon (TOC) contents of the oil shale layers are very high (10.6wt.% to 39.6wt.%) with vitrinite reflectance (Ro) values of 0.41±0.02% and Rock-Eval hydrogen index (HI) values up to 887. Biomarker analyses show a high proportion of hopenes and hopanes with ββ-stereochemistry indicative of low maturity. Detailed organic petrologic analysis demonstrates that the organic matter is dominated by lamalginite and telalginite. The presence of Botryococcus in the telalginite indicates that the depositional environment of the oil shale is lacustrine. This is consistent with low Si and S contents and high C/S ratios. Fragments from a newly identified benthic macroalgal rhodophyte were found in some thin layers, raising the possibility that the depositional environment of some oil shale layers may have been influenced by marine flooding since Rhodophyta are uncommon in freshwater systems. Two oil shale samples with different algal compositions were selected for assessment of hydrocarbon generation potential: Sample HD-21 is dominated by phytoplanktic lamalginite with a high HI value of 869mg HC/g TOC, while sample HD-20 has a lower HI value (490mg HC/g TOC) and contains high contents of the benthic rhodophyte as well as some phytoplanktic lamalginite. HI values and thermal simulation experiments showed that the phytoplankton-dominated shale HD-21 has a much higher hydrocarbon generation potential than the rhodophyte-dominated oil shale HD-20 even though the TOC content of HD-20 was much higher, indicating that the benthic macroalgal macerals have a much lower capacity to generate oil when thermally mature.