266 / 2025-04-21 10:05:01

Indicative significance of bacterial hopane carbon isotopes in sedimentary organic matter for oil source correlation, palaeoclimatic change, and palaeoecological restoration

bacteria, hopane, carbon isotopes, sorce rocks, indicative significance

Theme 1: Novel Tools and Techniques in Geobiology > Session 1e: Frontiers in Biomarkers and Application of Paleoecosystem Reconstruction

The carbon isotopes of hopane inherit the isotopic signature of the carbon sources used by the microorganisms that form the hopane precursors. The broader geochemical significance of the carbon isotopes of hopane remains to be explored. Nine source rocks from different depositional environments were selected for this study. A hopanes fraction was separated from organic extracts of the rocks by neutral alumina column chromatography, and the carbon isotope values of individual hopanes were then determined by gas chromatography-isotope ratio mass spectrometry (GC-IRMS). A schematic diagram of the carbon isotope distributions of hopanes in different depositional environments was constructed. GC-MS, optical microscopy, and EA-IRMS analytical techniques were also used to reveal the molecular geochemistry and organic petrology of the four lacustrine source rocks. The results showed that the carbon isotope values of the hopanes in the source rocks vary considerably from one sedimentary environment to another. The general carbon isotope distribution characteristics of hopane in source rocks of salt lakes, marine, coal seam and freshwater lake was δ¹³C _{salt lakes} > δ¹³C _coal > δ¹³C _marine > δ¹³C _{freshwater lake}. Traditional geochemical parameters for the Kongdian and Lucaogou Formations source rocks, such as biomarkers, microscopic components, and component carbon isotopes, were highly similar. However, there were significant differences in the carbon isotope values of the hopanes. The Lucaogou Formation shale from Permain had δ¹³C values of hopanes between -41‰~-63‰ and decreased with increasing carbon number. The hopanes in the source rocks of the Kongdian Formation were apparently depleted in ¹³C, with carbon isotope ratios ranging from -61‰ to 72‰. As the number of carbon atoms increased, the hopane isotope value tended to become lighter first and then heavier. Variations in the range of carbon isotope values of hopanes with different carbon numbers and configurations in the same source rock was sometimes more than ±15‰, which indicated multiple bacteria contributions as sources of these compounds. The source rocks of the Kongdian Formation were characterized by the coexistence of carbon isotope-deficient hopanes (-72‰) and carbon isotope-enriched gammacerane (-28‰). These differences may be related to the different palaeoclimatic conditions. The present study is a new attempt in the field of carbon isotope application of hopane, and will help to promote the widespread use of hopane carbon isotopes in oil and gas geochemistry and environmental geochemistry.