Acta Petrologica et Mineralogica

The Ordovician weathered crust reservoir in the Ordos Basin is affected by calcites of different stages,so clarifying the genesis and influence of calcites can help to understand the mechanism of reservoir formation. Minera-logical,geochemical and isotope chronological results of calcites from the Ordovician weathered crusts in the Fuxian area of the Ordos Basin demonstrate that these calcites can be divided into white primary calcite, red secondary calcite and white secondary calcite. The white primary calcite filled in the pores and crevices of the paste mold,whereas the cleavage of calcite in the dissolved pores is much more developed than those in the dissolved seams. Geochemistry shows obvious negative Ce anomalies, a few calcites filled in the pores have positive Eu anomalies. They have red and orange-red cathodoluminescence, and their U-Pb ages are 333.3±3.3 Ma and 332.8±3.5 Ma, respectively, indicating that this type of calcite was formed in the early Carboniferous near surface environment. Gypsum was dissolved by atmospheric freshwater to form Ca²⁺, which reacted with atmospheric freshwater rich in CO^2-₃ at the gypsum mold pores and dissolution fractures. Red calcite is the secondary calcite of metasomatic dedolomitization, with dolomite crystal type mostly and orange-red cathodoluminescence. It is formed by metasomatic dolomite crystals in the atmospheric freshwater equimolarly from surface to near-surface environment, with high Mn,low Sr,high Sr isotopes and negative C and O isotopes. White secondary calcite is poikilitic,and medium-grained crystals,with developed cleavage, orange-red cathodoluminescence, high Sr concentration and high Sr isotopes, negative Ce and C anomalies, relatively stable O isotope,and U-Pb age of 212.0±19.0 Ma. The results indicate that this type of calcite was formed by the reaction of acidic fluids with Ca²⁺ during the Late Triassic hydrocarbon expulsion period. All kinds of calcite are the products of different diagenetic fluids in different diagenetic stages, which are easy to develop in the relative low-lying stagnant areas and result in the reduced rock porosity. Therefore, more attention should be paid to the high parts of paleo-karst geomorphology for effective reservoirs.

secondary calcite; dedolomitization; karst; weathered crust reservoir; calcite dating