Abstract:

The core of well Pu-115 was systematically sampled to investigate the sedimentary and geochemical characteristics of the three types of rocks of the upper section of Mb Ⅲ of the Shahejie Formation: siliciclastics, carbonates and mixed rocks. The result indicates that the carbonate components of the target formation are proven to be authigenic. Carbon and oxygen isotopic compositions of the carbonates are significantly different from that of the carbonate components of clastic rocks. When siliciclastics are the dominant deposition, δ¹⁸O is depleted and does not covariate with δ¹³C, which suggests a relatively open system with minor evaporation and probably abundant inflow of fresh water. When carbonates deposite, oxygen isotope becomes heavier and carbon isotope excurses negatively. The positive trend of oxygen isotope may indicate a heavy evaporation or less inflow of fresh water, and the depletion of carbon isotope may denote the oxic condition of the bottom water. The frequent change of the carbon and oxygen isotopic composition may reflect a rapid shift of the redox state of the lake bottom, and the decrease of clastic components may indicate a fall of the lake water level and the lake was relatively isolated and closed during the deposition of the target formation.

Key words: Dongpu sag, carbonate, mixed sediments, carbon and oxygen isotope

摘要：

通过对典型井位濮115井沙三段地层进行岩芯观察和薄片鉴定, 结合碳、氧稳定同位素和元素地球化学特征研究, 对东濮凹陷沙河街组三段碳酸盐岩及陆源碎屑与碳酸盐的混合沉积作用机理和成因进行讨论。结果表明: 研究井段地层中碳酸盐组分为自生矿物, 碳酸盐岩和碎屑岩中碳酸盐的碳、氧同位素分异明显, 碳酸盐岩中δ¹⁸O偏正, δ¹³C偏负; 碎屑岩中δ¹⁸O偏负, δ¹³C偏正; 混积岩为二者的线性混合。同位素特征指示碳酸盐岩在强蒸发、弱淡水注入的条件下形成, 湖水相对封闭, 水体较浅, 底层水体可能呈氧化状态; 碎屑岩形成于弱蒸发、多淡水注入的相对开放湖体中, 湖平面较高, 底层水体可能缺氧; 混积岩则形成于两者的过渡阶段。频繁变动的碳、氧同位素组成可能指示对应沉积期的气候条件和湖平面, 并指示湖盆底层水体氧化还原状态的频繁变动。研究井段陆源碎屑含量逐渐减少, 可能指示湖平面整体上处于下降期, 与之相对应的沉积期湖盆相对封闭的阶段占主导。

关键词: 东濮凹陷, 碳酸盐岩, 混合沉积物, 碳、氧同位素