Astronomical cyclic constraints on organic matter enrichment in Paleogene Weizhou Formation, Haizhong Sag, Beibu Gulf Basin

The development characteristics and enrichment patterns of organic matter are closely related to the paleo-sedimentary environment, forming a core aspect in petroleum geology research. They hold significant guiding value for the efficient evaluation and deployment of oil and gas exploration in basins with low to medium exploration maturity in marine areas. Based on cyclostratigraphy and geochemical analysis techniques, the astronomical cycles of the Paleogene Weizhou Formation in Haizhong Sag, Beibu Gulf Basin, were identified and divided, a high-frequency sequence stratigraphic framework and an astronomical time scale were constructed. The enrichment patterns of organic matter at different cyclic scales were explored by analyzing the parameters of paleosalinity, paleo-oxygen facies, paleo-water depth, paleoclimate, and paleoproductivity ("five-paleo"). The results showed that: (1) driven by astronomical orbital cycles, 3.5 cycles of 2.4 Myr ultra-long eccentricity, 7 cycles of 1.2 Myr ultra-long obliquity, 21 cycles of 405 kyr long eccentricity, 68 cycles of 125 kyr intermediate eccentricity, 88 cycles of 96 kyr short eccentricity, 163 cycles of 52 kyr obliquity, 212 cycles of 40 kyr obliquity, and 369 cycles of 23 kyr precession were recognized within the Weizhou Formation. (2) The analysis of the "five-paleo" environmental parameters indicated that organic matter enrichment periods in the Weizhou Formation developed in a sedimentary environment characterized by freshwater, weak oxidation, warm and humid conditions, shallow water, and high productivity. (3) The "five-paleo" environment and organic matter enrichment of the Weizhou Formation were regulated by astronomical orbital cycles, exhibiting synergistic evolutionary characteristics. The degree of organic matter enrichment was highest when the ultra-long eccentricity and the long eccentricity reached their maximum values. Based on this, four intervals for organic matter favorable enrichment were identified: the upper part of the upper sub-member of the second member of the Weizhou Formation, the lower part of the lower sub-member of the second member of the Weizhou Formation, the upper part of the upper sub-member of the third member of the Weizhou Formation, and the upper part of the upper sub-member of the fourth member of the Weizhou Formation. If adjacent reservoirs have good trapping conditions, the oil and gas charging conditions will be more advantageous. Under the background of strong near-source charging, these intervals could become favorable targets for future exploration. This study establishes the high-frequency sequence stratigraphic framework and the astronomical time scale. Additionally, the paleoclimate evolution characteristics of the Weizhou Formation under the regulation of astronomical orbital cycles and their constraining mechanisms on organic matter enrichment patterns are revealed, thereby providing references for future oil and gas exploration in the study area.