湖相碳酸盐岩中自生白云石碳氧同位素特征及成岩意义—以渤海石臼坨凸起古近系沙河街组为例

Characteristics and Diagenetic Significance of Carbon and Oxygen Isotopes in the Authigenic Dolomites of Lacustrine Carbonate Rocks—A Case Study of the Paleogene Shahe Jie Formation of the Shijiutuo Uplift in Bohai Sea

摘要: 渤海石臼坨凸起古近系沙河街组湖相碳酸盐岩中自生白云石主要有孔隙衬垫、孔隙充填和交代状3种产出形式，运用激光碳氧同位素分析方法对不同产状自生白云石碳氧同位素特征及其所反映的成岩意义进行研究，结果表明：1) 自生白云石形成于咸化的成岩流体中；2) 氧同位素计算温度分析表明，自生白云石共发育3期，对应三个主要古埋深区间分别为75~550 m、700~950 m和1400~2300 m；3) 第一期自生白云石高δ¹³C正值表明其碳来源于原始沉积碳酸盐；第二期自生白云石较高且偏正的δ¹³C表明其碳来源仍以原始沉积碳酸盐为主，受低成熟阶段细菌作用的影响；第三期自生白云石δ¹³C较低且偏负，说明有机质热降解过程产生的富含¹²C的CO₂对其有重要影响；4) 氧同位素分析可划分出高温与低温白云石，其中低温白云石形成于近地表–浅埋藏回流–渗透白云石化过程，高温白云石形成于中–深埋藏封闭的成岩环境。

Abstract: Authigenic dolomites mainly deposited like pore-lining, pore filling and metasomatic replacement in the lacustrine carbonate rocks of the Shijiutuo Uplift in Bohhai Sea. Using the laser isotope analysis method to study the characteristics and diagenetic significance of the oxygen and carbon isotopes in the authigenic dolomites with different occurrences, the research results are present as follows: 1) the authigenic dolomites formed in the salty diagenetic fluid; 2) the oxygen isotope temperatures showed that the authigenic dolomites developed in 3 periods, corresponding to the three main paleoburial depth intervals such as 75 - 550 m, 700 - 950 m and 1400 - 2300 m; 3) the earliest authigenic dolomite with the highest δ¹³C indicates that the carbon 13 originated from the primary sedimentary carbonates; the second earliest authigenic dolomite with higher and positive δ¹³C indicates that the carbon source is still dominated by the original sedimentary carbonate and affected by the bacterial degradation during the shallow burial stage; the latest authigenic dolomite with the lowest and negative δ¹³C shows that the CO₂ rich in carbon 12 produced by organic matter degradation played an important role in the carbon source; 4) the authigenic dolomites can be divided into the low temperature dolomite and the high temperature dolomite through the oxygen isotope analysis, and the low temperature dolomite formed in the reflux dolomitization process in the near surface to shallow burial stage and the high temperature dolomite formed in the middle to deep burial diagenetic environment.