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The tight carbonate reservoir was controlled by various geological factors, and such factors played different roles in buried depths and formations. Therefore, studies related to the factors controlling carbonate reservoir distribution are of great significance for the prediction and evaluation of high-quality dolostone reservoirs. In this paper, we focus on the controlling factor of the submember Ma55 dolostone reservoir in the western Ordos Basin. The main rock types, reservoir pores, physical properties, and pore structure characteristics of the reservoir were analyzed by thin section identification, physical property analysis, and mercury injection, respectively. Then, the main controlling factors of reservoir development were comprehensively analyzed from the perspectives of palaeostructure, lithofacies palaeogeography, diagenesis, and diagenetic facies. The results show that two kinds of dolostone reservoirs in the submember Ma55 developed in the western Ordos Basin, including intercrystalline pore-type and dissolution pore-type. The former reservoir is primarily characterized by powder-fine dolostone with residual structure, dolomite intercrystalline pore, and micropore with porosity ranging from 2% to 11%. There are three types of pore structures developed in it, such as macropore-medium throat-single peak (MAMS), macropore-fine throat-single peak (MAFS), and medium pore-fine throat-single peak (MEFS). The latter reservoir is mainly featured by powdery crystalline dolostone with gypsum and halite dissolution, moldic pore, and dissolved pore between breccias with a porosity greater than 5%. It consists of two types of pore structures, such as macropore-fine throat-single peak (MAFS) and medium pore-coarse throat-multipeak (MECM). The intercrystalline pore-type dolostone reservoir is mainly controlled by the lithofacies palaeogeographic environment and diagenesis. In specific, the shoal microfacies at the edge of the platform and the active reflux seepage dolomitization are the basic sedimentary environment conditions for reservoir formation and the key to reservoir formation, respectively. The dissolution pore-type dolostone reservoir is primarily influenced by both paleostructure and diagenesis. The relatively high part of the paleostructure provides favorable conditions for the formation of evaporate minerals, and early freshwater dissolution is the key to reservoir formation. This research will provide a theoretical basis for forecasting the favorable distribution areas of different types of dolostone reservoirs. [ABSTRACT FROM AUTHOR]

Diagenetic fluids is one of the most important reservoir modifiers, their differences can result in various petrography, storage capacity, and geochemical characteristics, so clarifying the diagenetic fluids is vital for understanding dolostone origin. Dolomitization is an important genetic type of dolostone reservoir, the fluid properties differ in different dolomitization, which may lead to reservoir storage capacity changes. Therefore, the identification of dolomitization fluid properties is critical for deeply understanding of dolomitization process and predicting storage capacity. Two types of dolostone are developed in submember Ma55 of Majiagou Formation in northwestern Ordos Basin, with obviously different petrological characteristics and reservoir properties. On the basis of petrological studies such as core and casting thin section observation and cathodoluminescence analysis, the diagenetic fluid properties of these two dolostone are characterized by geochemical analytical methods such as major and trace element tests. The results show that Type-1 dolostone is mainly composed of micritic dolomite, showing micritic structure and algae-rich lamina structure, and accompanied by evaporite minerals and moldic pores. This type of dolostone has a various Mn content, weak to medium cathodoluminescence intensity, high contents of TiO2, Al2O3, K2O+Na2O, Li, and U, and lower content of TFe2O3. The type-2 dolostone is composed of fine-grained dolomite with obvious residual texture of primary limestone, clear brim, and cloudy center structure, accompanied by the existence of intergranular pores. Most of this kind of dolostone have medium-strong cathodoluminescence, higher TFe2O3 content and lower TiO2, Al2O3, K2O+Na2O, Li, and U contents. Moreover, both the two types of dolostone have similar Fe/Ca and Mn/Ca ratios, and a low and concentrated CaO content, whose composition is similar to that of stoichiometric dolomite. The comprehensive analysis shows that the diagenetic fluid of Type-1 dolostone is mainly a high salinity fluid existed in plaster and calcareous sediments in a near surface environment with low temperature. The diagenetic fluid of Type-2 dolostone may be a high salinity brine formed by evaporation and concentration of seawater with normal salinity. The research results will provide a significant theoretical basis for the evaluation of dolostone reservoir quality and the prediction of favorable areas of reservoir distribution. [ABSTRACT FROM AUTHOR]