Dedolomitization from the meteoric environment to the burial environment of Ordovician dolostones (China): results from petrographic and geochemical analyses

Calcitized Ordovician dolostones in the Yichuan-Huanglong area of the Ordos Basin (China) are discussed and evaluated by utilizing petrographic, mineralogical and geochemical data. Symbiotic dedolomites with red and white colors are separated and studied. The new data demonstrate that the red dedolomites (Dd1) occurred as the result of the migration of karst-related meteoric water that can be characterized by karst breccias, low Sr and high Mn contents, low δ13C values, high 87Sr/ 86Sr ratios and pure liquid–fluid inclusions. The occurrence of the white dedolomites (Dd2) around stylolites or pores, their low Mn contents, 110–120 °C homogenization temperature and hydrocarbon inclusions, associated with less depletion of δ13C than Dd1, suggest that Dd2 were formed under a burial hydrocarbon expulsion environment. Due to an insufficient amount of Ca ions from the dissolution of residual gypsum, the dedolomitization of Dd1 is a mole per mole replacement process along the rim interfaces, which retains the original pore system and provides pathways for hydrocarbon migration. The formation of Dd2 is a result of the dissolution/precipitation process of Dd1 and/or dolostones by the acidic fluids of mature hydrocarbons in the burial stage that completely convert the rock. The calcitized Ordovician dolostones in Ordos Basin is the result of continuous dedolomitization in an evolving environments from meteoric to burial.