Astronomical influence of the development of Paleogene thin coal seam groups in offshore Lacustrine basins: A case study of the Zhu I Depression’s Enping Formation located in the northern South China Sea

The development of the Paleogene coal seams in China’s offshore basin areas generally had the characteristics of coal measures with large thicknesses, large numbers of coal seams, thin single coal seams, poor stability, scattered vertical distribution, and a wide distribution range. This study selected the Enping Formation of the Zhu I Depression in the northern section of the South China Sea as an example to determine the macro-control factors of the development of the Paleogene coal seam groups. An analysis was carried out on the influencing effects and patterns of the astronomical cycles related to the development of the thin coal seam groups in the region. A floating astronomical time scale of the Enping Formation was established, and the sedimentary time limit of the Enping Formation was determined to be approximately 6.15 Ma±. In addition, the cyclostratigraphy analysis results of the natural gamma-ray data of Well XJ in the Enping Formation of the Xijiang Sag revealed that the development of the thin coal seams had probably been affected by short eccentricity and precession factors. The formation process of coal seams was determined to have been affected by high seasonal contrast, precipitation, and insolation. During the periods with high values of short eccentricity, the seasonal contrasts tended to be high. During those periods, fluctuations in the precession controls resulted in periodic volume changes in precipitation and insolation of the region, resulting in the development of thin coal seams. It was also found that the periods with low precession were the most conducive to coal seam development. On that basis, combined with such factors as sedimentary environmental conditions conducive to the development of thin coal seam groups, this study established a theoretical model of the comprehensive influences of short eccentricity and precession on the development and distribution of Paleogene thin coal seam groups in offshore lacustrine basins. The patterns of the Paleogene astronomical periods and paleoclimate evolution, along with the control factors which impacted the development of thin coal seam groups in offshore lacustrine basins, were revealed.