Carbon and Oxygen Isotopic Composition of Saline Lacustrine Dolomite Cements and Its Palaeoenvironmental Significance: A Case Study of Paleogene Shahejie Formation, Bohai Sea

• Published in: Minerals (Basel) Vol. 9; no. 1; p. 13
• Main Authors: Meng, Hailong, Lv, Zhengxiang, Shen, Zhongmin, Xiong, Chenhao
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.01.2019
• Subjects: Bohai Bay Basin; Carbon; Carbon 13; Carbon dioxide; carbon-oxygen isotope; Carbonate minerals; Carbonates; Cathodoluminescence; Cements; Composition; Crystals; Diagenesis; Dolomite; dolomite cements; Dolomitization; Dolostone; Electrons; Fossils; High temperature; Hydrocarbons; Isotopes; lacustrine carbonates; Lake basins; Lakes; Lasers; Low temperature; Oxygen; Oxygen isotopes; Paleogene; Phase transitions; Seawater; Sedimentation & deposition; Seepage; Shahejie Formation; Spatial discrimination; Spatial resolution; Temperature; Thermometers; United States > US; Japan; Bohai Sea
• ISSN: 2075-163X; 2075-163X
• DOI: 10.3390/min9010013

The dolomite reservoirs in the Paleogene Shahejie Formation in the Bozhong area of the Bohai Bay Basin contain a large amount of dolomite cement. Petrologic and mineralogic studies have shown that the dolomite cements can be divided into three types according to their occurrence: coating dolomite (CD), pore-lining dolomite (LD), and pore-filling dolomite (FD). The laser microsampling technique was used to analyze the C and O isotopes in the carbonate minerals. This method is an effective way to produce CO2 gas from a particular carbonate structure in a thin section, and it has a spatial resolution of 20–50 µm and an optimal precision of approximately ±0.22σ for δ13C and δ18O in carbonate standard materials. The carbon and oxygen isotopic compositions and the oxygen isotopic geothermometer results showed that the dolomitization fluid is mainly low temperature fluid, the lake basin environment is relatively closed, and the salinity index Z value is greater than 120, which indicates the invasion of seawater. CD and early-stage LD crystals were mainly very fine crystals with faint cathodoluminescence, which indicates the early formation of diagenesis. The high temperatures of late-stage LD and FD measured by oxygen isotope thermometers indicates that they formed at a deeper depth. The dolomite cements in the study area may have formed in two stages: seepage-reflux dolomitization during the penecontemporaneous period and burial dolomitization.