Abstract: The Walloon coal measures in the Surat Basin, Australia, has a huge amount of coalbed methane (CBM) resources, but the spatial heterogeneity of CBM production capacity varies significantly. Variable characteristics of multiple thin coal seams in the Walloon coal measures, and rapid lateral changes in lithology limit further research on the lateral differences of coal reservoirs. Therefore, fine-scale stratigraphic framework delineation of the Walloon coal measures in the Surat Basin is fundamental for understanding the temporal and spatial differences of coal reservoirs. The S-4 well, which has a complete and continuous sedimentary formation, is used as the main drilling well. Time series analysis of its natural gamma (GR) logging data is used for the cyclostratigraphic study. The power spectrum analysis results show that the Walloon coal measures in the Surat Basin recorded 15 sedimentary cycles of 405 kyr periodicity, and orbital forcing had a significant impact on its depositional process. The long eccentricity, short eccentricity, obliquity, and precession are represented by sedimentary cycles with thickness of 23.9 m, 6.3-9.3 m, 2.4-2.7 m, and 1.2-1.4 m, respectively. Correlation coefficient analysis indicates that the Walloon coal measures was driven by astronomical cycles at >99% confidence level. Using the zircon CA-TIMS U-Pb age of 162.54±0.05 Ma from the S-4 well coal-bearing tuff as a constraint, an astronomical calibration was performed using a stable 405 kyr cycle to establish an astronomical time scale for the Walloon coal measures. The duration of this formation was determined to be 5.7-6.1 Myr, mainly deposited during the Middle Jurassic Callovian to Late Jurassic Oxfordian. Based on the consistency between time series analysis and identification of cyclic interfaces, combined with the identified sedimentary cycles of 405 kyr periodicity, the Walloon coal measures was divided into 5 third-order sequences and 15 fourth-order sequences, establishing a high-frequency sequence stratigraphic framework for the study area. The stratigraphic framework provides a reference for high-precision isochronous correlation of the Walloon coal measures in the Surat Basin.

Key words: time series analysis, high-frequency sequence stratigraphic framework, astronomical cycle, Walloon coal measures;
Surat Basin