Abstract:

There are different scales of cyclicity in the Earth’s evolution, from one day to several hundred million years. Presently, the scale of the astronomical cycles controlled by the Earth’s orbit is one of the most suitable timescales for high-precision stratigraphic comparison. Astronomical signals are more likely present in the continuous succession of organic-rich shale, where the “precise periodicity” of Milankovitch cycles makes it feasible to achieve stratigraphic isochronal correlation therefore to allow high-precision comparison of organic-rich shale. In this paper, the main source rocks of the Shahejie Formation in Dongying sag, Bohai Bay Basin, eastern China are studied, and the multi-scale isochronal correlation of continental organic-rich shale is achieved by using the astronomical cycles, which demonstrates that the sixth-order sequence division and organic-rich shale comparison can be performed with an error of less than 40000 years by using the obliquity cycle as the basic unit. According to the results, the high-production shale formation in the Dongying sag is located at 42.85-41.85 Ma near the E2-E4 long eccentricity cycles and coinciding with a high-amplitude obliquity cycle. It was hypothesized that the high-production shale is likely related to the obliquity-driven laminated shale that has high mobility and oil-bearing property. This result further demonstrates that the high-precision isochronal comparison of continental organic-rich shale using astronomical cycles can provide a high-precision chronological framework for the prediction of high-quality hydrocarbon source rocks.

Key words: cyclostratigraphy, shale oil, fine-grained sedimentary rocks, high-frequency sequence, Milankovitch theory, astronomical timescale