## 1 Introduction  
With the continuous deepening of exploration, China's onshore oil and gas exploration has shifted its focus to deep and ultra-deep exploration. In the exploration of ultra-deep carbonate rocks, the discovered oil and gas fields have the characteristics of large-scale, deep burial, and high oil and gas production capacity of single wells. Carbonate reservoirs belong to the focus of oil and gas exploration and development practice and are the hot spot of basic theoretical research at home and abroad. Taking the Middle East as an example, its abundant oil production accounts for about two-thirds of the world's total production, of which 80% comes from carbonate reservoirs [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. There are abundant biological and lithofacies types in the basin, and there are also multiple tectonic units, forming a complex tectonic-lithofacies system. These geological conditions provide a good foundation for prospecting. The marine carbonate rocks of the Lower Paleozoic Ordovician Majiagou Formation are rich in natural gas resources. It is one of the important areas for the exploration of large-scale natural gas reserves in the Ordos Basin [15, 16, 17, 18], indicating that the Lower Paleozoic carbonate rocks in the Ordos Basin have great exploration potential. The Ordovician Majiagou Formation in the Ordos Basin develops a dolomite reservoir, which has good reservoir performance and a good relationship between source and reservoir configuration and can form natural gas industrial accumulation [19; 20; 21; 22]. At present, important discoveries have been made in the dolomite in the central basin, and several exploration wells have obtained industrial gas flow, which has a good exploration prospect. However, due to the low degree of exploration in this field and the lack of drilling coring, there is still a lack of in-depth understanding of the genetic mechanism, distribution law, and main controlling factors of this kind of dolomite reservoir. Therefore, this study provides an important theoretical basis for the future exploration and production of Ordovician cream and salt in the basin [23; 24; 25].  
Previous studies on the Ordovician in the Ordos Basin mainly focused on lithofacies paleogeography, and there are relatively few studies on the Ordovician reservoirs [26; 27; 28; 29; 30; 31; 32]. The controlling factors of oil reservoir development are still unclear, which makes it difficult to meet the regional evaluation and target optimization and restricts the next exploration process. Based on the theories of sedimentology, sequence stratigraphy, reservoir geology, and petroleum geology, this study comprehensively applied the analysis and testing methods of cast flakes, carbonates and oxygen isotopes, scanning electron microscopy, cathodoluminescence, trace elements, rare earth elements, and fluid inclusions, and, combined with the structural background, sedimentary microfacies and diagenesis data. The genetic mechanism of dolomite in the subsalt paste formation of the Ordovician Majiagou Formation in the Ordos Basin is systematically studied. It provides important theoretical support for the exploration and production of Ordovician gas under paste salt in the Ordos Basin.