Influencing factor of Chang 7 oil shale of Triassic Yanchang Formation in Ordos Basin: Constraint from hydrothermal fluid

The Chang 7 oil shales of the Triassic Yanchang Formation in the southern margin of the Ordos Basin has been a concern of researchers because of the extensive development of lacustrine hydrothermal sedimentary rocks in it. Here, we studied the petrological and mineralogical characteristics of hydrothermal sedimentary rocks through ordinary thin sections, scanning electron microscope and electron probe observations, and evidences about hydrothermal activity were identified from the oil shales. On this basis, we further complete their geochemical analysis. The results show that the Cs, U, Th, Pb, and Mn of the Chang 7 oil shale have significant positive anomalies and that the Eu and Ta elements have negative anomalies. The TOC value of Chang 7 oil shale ranges from 0.18 wt% to 18.15 wt%, with an average value of 3.85 wt%. The average value of the Chang7-3 section is 5.33 wt%, the average value of the Chang7-2 section is 2.15 wt%, and the average value of the Chang7-1 is 3.05 wt%. In addition, the hydrothermal index of Chang 7 source rock shows: The correlation coefficient between the hydrothermal index and the TOC content is as high as 0.86 and 0.85. There is the most hydrothermal input in Chang7-3 deposition period, it was somewhat weakened in the Chang 7-2 deposition period, and it was enhanced in the Chang 7-1 deposition period. Hydrothermal deposition is involved in the hydrocarbon generation of oil shales. The greater the hydrothermal input intensity is, the greater the TOC content of the oil shale is.

Automated summary

The Chang 7 oil shales in the Triassic Yanchang Formation show significant positive anomalies in Cs, U, Th, Pb, and Mn, and negative anomalies in Eu and Ta. The TOC values vary across different sections, and there is a strong correlation between hydrothermal input intensity and TOC content.