Astronomical cycles analysis and paleolake level evolution characteristics of Paleogene upper Niubao Formation: A case study of the Ni-1 well in Tibetan Plateau

The Nyima Basin in the central Tibetan Plateau is a Cenozoic terrestrial rift basin located over the Bangong-Nujiang suture zone. The Ni-1 well is the first geological exploration well in the Nyima Basin. Based on the spectrum analysis data of the natural gamma logging curves of the Paleogene upper Niubao Formation from the Ni-1 well, it is confirmed that the significant influence of astronomical orbital periods on the deposition of lacustrine strata in the Nyima Basin. Filter analysis shows that the upper Niubao Formation has preserved 41 short eccentricity (~100 kyr) cycles, 76 obliquity (~54 kyr) cycles and 178 precession (~23 kyr) cycles, with an average sedimentation rate of 92.03 m/Ma and a sedimentation time limit of ~4.1 Myr. In addition, combined the change trends of Fischer diagram and total organic carbon contents, it is suggested that the lake levels of the upper Niubao Formation experienced once ascending-descending fluctuations. It is concluded that the formation of high-quality hydrocarbon source rocks in the Nyima Basin is mainly controlled by changes of lake levels induced by astronomical cycles.