Astronomical forcing of the Paleogene coal-bearing hydrocarbon source rocks of the East China Sea Shelf Basin

Abstract

The Paleogene coal-bearing deposits (Pinghu Formation) of the Xihu Depression form the most productive hydrocarbon source in the East China Sea Shelf Basin. Due to the lack of reliable magnetostratigraphic, paleontologic and geochemical/isotope data, these source rocks are still poorly dated. The absence of a robust time frame results in many disputes on the age and duration of the Pinghu Formation and hampers a thorough understanding of its sedimentary evolution. Here, we study the cyclic changes in the depositional environment that can be retrieved from seismic interpretation and sequence stratigraphic analyses of well log data (e.g., stacking patterns, logging, and lithofacies changes). We recognize one second-order sequence, three third-order sequences and 12 fourth-order parasequences. Next, we construct a floating astronomical time scale by using power spectra and evolutionary fast Fourier transformation analysis on the gamma-ray data. Wavelength variations in the gamma-ray spectra show similar ratios as the astronomically-forced Milankovitch cycles (i.e., eccentricity, obliquity and precession). The filtered long and short orbital eccentricity cycles show a good match with the La2010d astronomical target curve, allowing to create an independent time frame based on cyclostratigraphy. Our floating astronomical time scale provides new numerical age constraints for the Pinghu Formation, resulting in a total duration of ~4.95 Myr. A re-analysis of the available age data indicates that the sequence stratigraphic framework of the Pinghu Formation is best constrained to the early Oligocene, instead of the previously assumed late Eocene. Our study highlights that astronomical interpretation of sequence stratigraphic analyses is an excellent method to estimate the duration of sedimentary successions in boreholes and adds to a better understanding of the temporal and spatial sediment distribution patterns in the East China Sea Shelf Basin.