Intact microbial fossils in the Permian Lucaogou Formation oil shale, Junggar Basin, NW China

Abstract

The Permian Lucaogou Formation was deposited in a deep stratified lake with good preservation of the organic matter in the sediments likely due to anoxic bottom waters. The Lucaogou Formation oil shale contains excellent source rocks with high TOC contents (12.4–32.9wt.%) and high HI values (547–869mg HC/g TOC). The maceral compositions of the oil shales are dominated by lamalginite with some higher plant debris. Spherical structures about 75–150μm in diameter having yellow fluorescence were observed in the shale. EDX-SEM showed that these contained C, O, P and Ca. Two kinds of well preserved microbial fossils with different morphology, were found in these spherical structures using a confocal laser scanning microscope and SEM. Ellipsoidal cells (~1–1.5μm long and 0.5–1μm wide) occurred inside the spherical structure, whereas microspherical cells occur within the outer layers. Through excluding modern contaminants and endolithic microbes, we propose that these microbial fossils are indigenous and syngenetic with oil shale deposition. Their close physical association in the sediments suggests a symbiotic relationship. Hydrocarbon extracts of these oil shales have relatively depleted carbon isotopic signatures, which is suggestive of methane oxidation. This was confirmed by analysis of abundant C29 and C30 17α,21β(H)-hopanes which show distinctly negative δ13C values typical of those found in methanotrophs. Several possibilities are presented for the identity of these microorganisms, but it is not yet possible to state unequivocally that they are related to methane production and consumption.