Dolomitization of Ordovician Kelimoli carbonates linking to shale oil Formation, western Ordos Basin: new insights from magnesium and strontium isotopes

Xianghui Jing ¹ JINGCHUN TIAN ^1* Lei Zhang ^2* Fengjie Li ^2* Han Li ^2*

• ¹ Chengdu University of Technology, Chengdu, Sichuan Province, China
• ² PetroChina Changqing Oilfield Company, Xi'an, Shaanxi Province, China

Deciphering dolomitizing process has great significance for high-quality hydrocarbon reservoir prediction in carbonate successions. The Ordovician Wulalike Formation provides shale oil in western Ordos basin, while lateral-contact marine dolostones of Kelimoli Formation contribute major reservoir for extra hydrocarbons. Nonetheless, the origin and occurrence of dolostones are underexplored. Coupled with petrographic and lithologic analyses, this study attempts to investigate dolomitizing fluid pathways and dolomitization pattern of Ordovician Kelimoli carbonates based on elemental and isotopic geochemistry. Extremely low Rb concentrations (i.e., less than 0.1) and Mn/Sr ratios (i.e., less than 2) of carbonates with micropores indicated that they are valid proxy for geochemical signatures of coeval seawater. By contrast, dolostones developing vuggy pores showed pronouncedly higher 87 Sr/ 86 Sr composition and Mn contents than other dolostone types, revealing that vuggy dolostones experienced meteoric water leaching and were altered geochemically. Quantitative calculation of 87 Sr/ 86 Sr ratios and fluid-inclusion microthermometry revealed that the Kelimoli dolostones (i.e., dolograinstones and crystallized dolostones) were formed before meteoric water leaching influences at a deepburial environment under seawater derivatives and sealed brine water. In evaporitedolograinstone successions, increasing of magnesium isotopic composition (δ 26 Mg) with increasing burial depths indicated that the dolomitizing fluid migrated downwardly.Comprehensive isotopic evidences of 87 Sr/ 86 Sr, δ 18 O, and δ 13 C suggested that dolomitizing fluid was derivatives of coeval seawater. In crystallized dolostone successions, upwardly heaving of δ 26 Mg ratios revealed that the dolomitizing fluid moved upwardly. The 87 Sr/ 86 Sr, δ 18 O, and δ 13 C and microthermometric evidences indicated that these dolostones were formed at a deep-burial, high-temperature environment and the dolomitizing fluid was derived from sealed brine water. Based on above investigation, a comprehensive dolomitizing pattern was proposed for the studied section of Ordovician Kelimoli Formation.