Characteristics and genesis analysis of dolomite in Lower Ordovician Penglaiba Formation in Tahe area

The dolomite of the Lower Ordovician Penglaiba Formation is an important successor field for oil and gas exploration in the Tahe oilfield. Clarifying the sedimentary characteristics and genetic mechanisms of the dolomite is of great significance for oil and gas exploration and development in the Tahe oilfield. Based on this, the geological and geochemical characteristics and genetic mechanisms of the dolomite in the Lower Ordovician Penglaiba Formation of the Tahe Oilfield were systematically studied through core description, thin section identification, scanning electron microscopy, cathodoluminescence, and geochemical analyses. The results showed that: (1) The study area was dominated by crystalline dolomite, followed by residual granular dolomite. Mud-microcrystalline dolomite was mainly anhedral. Silty-medium crystalline dolomite was mostly subhedral to euhedral. Medium-coarse crystalline dolomite was predominantly xenomorphic. Residual granular dolomite was mainly composed of silty-fine crystalline dolomite, showing residual intraclastic texture. (2) Cathodoluminescence showed that the mud- microcrystalline dolomite emitted dull blue-purple or dark brown luminescence. Silty dolomite emitted dull purple-red luminescence. Fine-medium crystalline dolomite had a purple core with a bright orange-red rim-shaped band. Medium-coarse crystalline dolomite exhibited purple-red and a bright brown-red rim-shaped luminescent band. (3) Mud-microcrystalline dolomite was mainly enriched in heavy rare earth elements (REE), with large δ¹⁸O fluctuations and ⁸⁷Sr/⁸⁶Sr ratios close to those in the Early Ordovician seawater, indicating quasi-syngenetic dolomitization by high-salinity evaporative seawater. Residual granular dolomite was mainly characterized by residual intraclastic texture and its genetic mechanism was burial dolomitization. The REE distribution patterns of silty crystalline dolomite and fine-medium crystalline dolomite were similar to those of contemporaneous micrites, which were enriched in heavy REEs with large δ¹⁸O fluctuations. The ⁸⁷Sr/⁸⁶Sr values were partially higher compared with the strontium isotope values in contemporaneous seawater, indicating products of shallow-medium burial dolomitization superimposed with hydrothermal fluids. Medium-coarse crystalline dolomite was relatively enriched in light rare earth, with negative δ¹⁸O values and ⁸⁷Sr/⁸⁶Sr values higher than those in contemporaneous seawater, indicating formation by hydrothermal metasomatism related to clastic rock layers.