| The genetic model and the controlling factor on the development of Ordovician dolomite reservoir are not clear, which seriously restricted the effective prediction of high quality dolomite reservoir, and the further exploration and discovery of the natural gas in the western margin of Ordos Basin. Controlled by the evolution of Helan depression in Early Ordovician, the western margin of Ordos Basin is located in a gentle slope or the weakly-rimmed platform of open sea with a certain barring of the inner gentle slope, middle gentle slope and the platform edge belt, where carbonate rocks extensively developed. The observation of rock and thin section shows that the Ordovician dolomites in the western margin of the basin are mainly composed of granular dolomite with clear residual particle structure, and crystalline dolomite including micritic-powder crystalline dolomite, coarse powder crystalline dolomite, powder-fine crystalline dolomite, fine crystalline dolomite, fine-medium crystalline dolomite, medium-coarse crystalline dolomite. The geochemical analysis of dolomite shows that the oxygen isotope δ18O values of the Ordovician dolomite in the western margin of the basin are generally negative, the carbon isotopes δ13C values are more positive, the trace elements are characterized with low Na, K and higher Fe content, and the strontium isotope 87Sr/86Sr values are obviously larger than those of the seawater in the same period, and the REE distribution curve is right deviation model of negative Eu anomaly. The cathodoluminescence shows orange, dark red and red, and the homogenization temperature of inclusions is generally high. According to comprehensive analysis of petrological characteristics, regional tectonic and sedimentary evolution, climatic environment and geochemical characteristics, the Ordovician micritic-powder crystalline dolomite in the western margin of the basin was formed by evaporation pump dolomitization in the paracontemporaneous period, and the fine crystalline-granular dolomite was formed by reflux infiltration dolomitization in shallow burial period. The inner gentle slope, middle gentle slope and the granular shoals on the edge of the platform have high primary porosity, which are more easily dolomitization, and the distribution of the Ordovician dolomite is discontinuous and uneven. After dolomitization, the primary pore of graualr shoal in contemporaneous period was preserved because of the strong anti-compacting ability of dolomite. Later, further dissolution expanded to form the most important reservoir space in the Ordovician dolomite reservoir. |
