青藏高原古近系牛堡组三段天文周期旋回分析及古湖平面演化研究：以尼玛盆地尼1井为例

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

关键词:  尼玛盆地  米兰科维奇旋回  频谱分析  湖平面演化  烃源层分布

Key words:  Nyima Basin  milankovitch cycles  spectrum analysis  lake level evolution  source rock layers distribution

摘要:

青藏高原中部尼玛盆地是一个形成于班公湖-怒江缝合带之上的新生代陆相裂谷盆地。交通不便和勘探程度较低等因素导致其高精度沉积旋回特征和地层学一直鲜有研究。尼1井是盆地内第一口地质探井，通过对该井古近系牛堡组三段的自然伽马测井曲线进行频谱分析，证实了天文轨道周期对盆地内湖相地层的沉积具有显著影响。滤波分析显示牛堡组三段保存了41个短偏心率(～100 kyr)周期，76个斜率(～54 kyr)周期和178个岁差(～23 kyr)周期，平均沉积速率为92.03 m/Ma，沉积时限～4.1 Myr。此外，结合Fischer图解与总有机碳含量的变化趋势，确定牛堡组三段湖平面经历了1次长周期的升-降变化过程。综合分析，认为尼玛盆地优质烃源岩的形成主要受天文旋回周期影响下的湖平面变化所控制。

Abstract:

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.