Abstract: The Ordovician dolomite in Ordos basin is an important natural gas reservoir. Studying the genesis of dolomite and reservoir development is of great significance to the development of the deep carbonate exploration theory. Based on observations of a large number of cores and thin sections, combined with geochemical analysis of stable carbon and oxygen isotopes, rare earth elements, order degree of dolomite, formation water and inclusions, the developmental characteristics and formation mechanism of two Ordovician type dolomite reservoirs in the basin were studied in detail. The analyses reveal that the dolomicrite containing anhydrite nodules or columnar crystals is mainly sabkha genesis in an evaporitic environment of tidal flat. The development of its large-scale reservoir was mainly controlled by the long-term unconformity surface, below which plenty of dissolved mold holes of the anhydrite nodules and columnar crystals were developed. The granular beach dolomite is mainly genesis result of seepage reflux and the development of its reservoir pores is primarily affected by the high-frequency fluctuations of the sea level, generating multi-cycle intergranular (intercrystalline) porous reservoirs. Although the dolomitization fluids of both types of dolomite were concentrated seawater, the original pore types and characteristics of the two dolomites are different due to their different mechanisms and rates of dolomitization. In the subsequent diagenetic transformation and evolution, the two types of dolomite pores show both similarities and obvious differences. This finding will provide useful guidance for the exploration of deep dolomite reservoirs.

Key words: Ordos Basin, Ordovician, dolomite, genesis of reservoir, anhydrite, mold pores, granular beach