LAUSR

The Tazhong Low Rise has abundant hydrocarbon resources in the Yingshan Formation of Lower Ordovician, where dolomite has been demonstrated to be a significant target for carbonate hydrocarbon exploration. Using optical observation and geochemical tests, three types of dolomite can be used as replacement and cement products. D1 dolomite is silt or fine crystalline, and has the fluid inclusions of low Th (71.4 °C) and corresponding salinity (9.89 wt% NaCl). In addition, geochemical data indicate that D1 dolomite was formed with shallow burial fluids and was intensely affected by weathering karst in the middle Ordovician, which served as a mixing-zone for dolomitisation. D2 dolomite of medium-coarse grain has similar geochemical characteristics as D1 dolomite except that it has higher diagenetic temperature and negative δ18O, suggesting that it generally developed in a deep burial environment. The cathodoluminescence showed two zonal structures in D1 dolomite, while at least three zonal structures in D2 dolomite were influenced by multiple burial diagenetic fluids. The D3 dolomite has a saddle structure with medium-coarse crystalline, high contents of Mn, Ba, Fe, and Sr, high Th (140 °C), high salinity (20.36 wt% NaCl), and negative δ18O (–10.1 to–7.8‰). Meanwhile, D3 dolomite is distributed near faults and fractures that are accompanied by hydrothermal minerals and is obviously influenced by hydrothermal fluids.