Abstract:

This study focuses on the control of astronomical orbital forcing climate change on sea level and sedimentation rate during the Oligocene in the northern South China Sea,which in turn affects the generation and preservation of organic matter. Using natural gamma logging data from drilling Wells L9,C6,and Y9 in the study area,together with Palynology,foraminiferal,and organic matter data,cyclostratigraphic analyses are applied to reveal the control mechanism of astronomical orbital cycles on hydrocarbon source rock development. The 2.4Ma ultra-long eccentricity and 1.2Ma long obliquity cycles were identified in the northern part of the South China Sea. During high-eccentricity and high-obliquity phases,the climate was warm and humid,sea level was relatively high,biological productivity increased,and the depositional environment became more reducing,all of which favored the accumulation and preservation of organic matter. Under these conditions,the abundance of organic matter ranged from 1.5% to 4%,with amorphous organic matter content increasing by 14.2%. In contrast,during low-eccentricity and low-obliquity phases,the climate was cold and arid,sea level was low,biological productivity decreased,and the depositional environment became more oxidizing. In this case,the organic matter abundance ranged from 0.3% to 1.5%,with vitrinite-rich(coaly)components accounting for 66.4% to 95.1%. Furthermore,it was found that when the sedimentation rate was less than 9cm/ka,the organic matter abundance could reach~3.5%,whereas at rates greater than 12cm/ka,the abundance decreased to 0.3%~0.7%. On this basis,a depositional model for Oligocene marine hydrocarbon source rocks in the northern South China Sea under astronomical forcing was established,providing insights into the spatial and temporal distribution of high-quality source rocks and offering a scientific reference for future deep-time and deep-Earth hydrocarbon exploration.

Key words: northern South China Sea, astronomical orbit drive, Oligocene, hydrocarbon source rocks, palaeoclimate

摘要：

重点探讨了南海北部渐新世天文轨道强迫气候变化对海平面和沉积速率的控制,进而影响有机质的生成与保存。利用研究区L9、C6、Y9等钻井自然伽马测井、孢粉、有孔虫和有机质数据,运用旋回地层学分析方法,以揭示天文轨道周期对烃源岩发育的控制机制。在南海北部地区识别出2.4Ma超长偏心率及1.2Ma长斜率周期。认为偏心率和斜率高值时期,气候温暖湿润,海平面处于高位,生物生产力提高,沉积环境还原性增强,有利于有机质堆积保存,有机质丰度介于1.5%~4%之间,其中无定形有机质含量增加14.2%;在偏心率和斜率低值时期,气候寒冷干燥,海平面处于低位,生物生产力降低,沉积环境氧化性增强,有机质丰度介于0.3%~1.5%之间,其中煤质有机质组分占比为66.4%~95.1%;且研究发现沉积速率小于9cm/ka时,有机质丰度达3.5%,沉积速率大于12cm/ka时,有机质丰度为0.3%~0.7%;在以上研究基础上,总结出天文周期控制下渐新世南海北部海相烃源岩的发育模式,有助于预测优质烃源岩的时空分布规律,为深时深地油气勘探提供科学依据。

关键词: 南海北部, 渐新世, 天文轨道驱动, 烃源岩, 古气候

CLC Number: 

• P534.614