Astronomical forcing in Eocene coal‐bearing series: A case study from the Pinghu Formation in Xihu Sag, East China Sea Shelf Basin

Abstract

The coal‐bearing series of the Eocene Pinghu Formation is one of the focused reservoir exploration in the Xihu Sag. This study conducted a cyclostratigraphy time series analysis using natural gamma‐ray (GR) logging data from four boreholes of the Xihu Sag. The spectral analysis shows a good match with the La2004 orbital solution. The sedimentation duration calculated by the astronomical age models spans at least 4.31–5.94 Myr, with an average sedimentation rate of 13.47–18.57 cm/kyr. The Pinghu Formation is subdivided into 1 second‐order, 3 third‐order and 14 fourth‐order sequences within seismic, well logging and 405 kyr‐long cycles constraints. Based on available age‐control data, the Pinghu Formation is corresponding to the middle‐late Eocene. The floating astronomical timescale of XH‐3 borehole now provides a new age constraint for the Pinghu Formation. There are four shorter‐term (<2 Myr) regional sea‐level cycles in the Pinghu Formation, which are similar to the calibrated global sea‐level fluctuations. Inconsistencies between reconstructed sea‐level curve by sedimentary noise modelling and true sea level inferred from lithology and sequence reveal various sources of noise in the sedimentary record. Variations in the distribution and thickness of coal seams in the Pinghu Formation appear to be a stratigraphic response to astronomical and non‐astronomical forcing such as regional tectonicssm and facies changes.