湘西地区下寒武统牛蹄塘组页岩沉积环境与有机质富集

Sedimentary environment and organic matter enrichment of the Lower Cambrian Niutitang Formation shale, western Hunan Province, China

祝庆敏 卢龙飞 潘安阳 陶金雨 丁江辉 刘旺威 黎茂稳

ZHU Qingmin LU Longfei PAN Anyang TAO Jinyu DING Jianghui LIU Wangwei LI Maowen

中国石化 油气成藏重点实验室, 江苏 无锡 214126 页岩油气富集机理与有效开发国家重点实验室, 江苏 无锡 214126 中国石化 石油勘探开发研究院 无锡石油地质研究所, 江苏 无锡 214126

Key Laboratory of Petroleum Accumulation Mechanisms, SINOPEC, Wuxi, Jiangsu 214126, China State Key Laboratory of Shale Oil and Gas Accumulation Mechanism and Effective Development, Wuxi, Jiangsu 214126, China Wuxi Research Institute of Petroleum Geology, SINOPEC, Wuxi, Jiangsu 214126, China

下寒武统牛蹄塘组是我国中上扬子地区发育的一套极为重要的海相页岩层系。为进一步明确我国南方下寒武统牛蹄塘组页岩有机质的富集环境与条件，以湘西沅陵地区牛蹄塘组页岩为研究对象，开展了岩石学、有机地球化学和元素地球化学分析等系统研究。结果表明：牛蹄塘组富有机质页岩沉积期处于干冷型气候向暖湿型气候转换期，从沉积早期到晚期其生物生产力水平、水体还原程度和热液作用强度表现为低-高-中高的变化趋势。湘西地区牛蹄塘组页岩有机质的富集并非受控于单一因素，而是古气候、生物生产力、水体氧化-还原性质、沉积速率和热液活动等多个要素相互配置与耦合的结果。牛蹄塘组页岩沉积早期水体较浅，整体处于偏氧化环境，初级生产力水平较低，不利于有机质富集；沉积中期早寒武世发生的大规模海侵使水体加深，伴生的上升洋流携带大量营养盐类进入表层水体，促使藻类大量勃发，同时底层水体缺氧和硫化的环境使有机质大量保存；晚期尽管水体氧化程度有所增加，但受华南持续拉张作用的影响，大陆边缘较强的热液活动提供了丰富的营养物质，生产力仍保持较高水平；且具有相对较高的沉积速率，从而使有机质得以快速埋藏、保存和富集。

The Lower Cambrian Niutitang Formation shale is an extremely crucial marine source rock developed at the mid-upper Yangtze block of China. To clarify the paleoenvironment and conditions which are responsible for organic matter enrichment of the Niutitang Formation shale in South China, a systematic study of petrology, organic geoche-mistry and elemental geochemistry was carried out by focusing on the Niutitang Formation shale in Yuanling, western Hunan Province. Results showed that the paleoclimate was transforming from dry-cold to warm-wet during deposition of Niutitang Formation. The level of biological productivity, the redox properties of water and the intensity of hydrothermal activity showed a consistent trend of low-high-medium-high from the early to the late stage of deposition. Rather than being controlled by a single factor, the enrichment of organic matter in the Niutitang Formation shale was the result of the mutual configuration and coupling of multiple factors such as paleoclimate, biological productivity, water redox properties, deposition rate and hydrothermal activity. The water was shallow and oxic with a low-level of productivity in the course of deposition of the lower Niutitang Formation, which was not conducive to the enrichment of organic matter. A large-scale of Early Cambrian transgression occurred during deposition of the medium Niutitang Formation, and the associated upwelling carried extensive substances such as nutrients and sulfates entered the surface water, which promoted the blooming of algae. The anoxic-euxinic environment at the bottom water was conducive to the preservation of organic matter. Although the degree of water oxidation increased during deposition of the upper Niutitang Formation, the strong hydrothermal activity triggered by the continuous extension of South China provided rich nutrients, which kept a high-level biological productivity. Meanwhile, the relatively high deposition rate led to organic matter insufficiently degraded, resulting in rapid burial, preservation and enrichment.

国家自然科学基金 企业创新发展联合项目U19B6003-01和面上项目41972164

https://doi.org/10.11781/sysydz202105797