Origin and reservoir characteristics of Ordovician dolostones in the Ordos Basin

Dolomite is one of the most important rock types for the development of the Ordovician effective reservoirs in the Ordos Basin. Its genesis remains controversial due to its complex and variable rock texture and occurrence. In this paper, through analysis of macroscopic regional geological background, microscopic rock texture, mineralogy, and geochemistry, the Ordovician dolomites in this area are divided into three types, i.e., micrite-fine-powder crystal, coarse-powder crystal, and fine (medium) crystal. It is pointed out that they were formed in three different dolomitization diagenetic environments, namely, penecontemporaneous dolomitization by evaporative pumping, mixture dolomitization of freshwater and magnesium-rich brine, and seepage-reflux dolomitization, respectively. In terms of horizon and spatial distribution, they represent the features of “strata-controlled” and “region-specific”. The analysis of genesis provides the following findings. First, the three types of dolomitization are correlated to certain extent in terms of temporal-spatial evolution. In other words, they were all originated along with the deposition of gypsum mineral under evaporation background. Second, the main pore types, i.e., dissolved pores, intercrystalline pores, and organic framework, were respectively formed in three types of dolomites. They generally distributed in “specific” type of dolomite, which indicates that the pore genesis is closely related to dolomitization diagenesis environment. Third, the development and distribution of effective dolomite reservoirs is mainly subject to three types of elements, such as primary sedimentary facies belts, diagenetic environment controlling regional dolomitization, and sequence boundary caused by variation of relative sea level.