Characteristics and origin of dolomite reservoir in the fifth member of the Ordovician Majiagou Formation in the Ordos Basin, China

The dolomite reservoir in the 5th member of the Ordovician Majiagou Formation (M5) in the Ordos Basin can be divided into two types based on whether or not gypsum is present, i.e., gypsum-bearing and gypsum-free reservoirs. The gypsum-bearing reservoir mainly develops in the 1–4th sub-members of M5 and is composed of micritic to silt-sized dolomite. The main storage space is provided by mold pores formed through the dissolution of gypsum precursors. The content of Fe in the matrix dolomite ranges from 583 to 3810 ppm; δ 13 C V-PDB and δ 18 O V-PDB values of the dolomite are in the range of − 3.5 to + 1.4‰ and − 6.5 to − 8.0‰, respectively. These characteristics support that the dolomite was possibly formed by syngenetic or penecontemporaneous Sabkha dolomitization, while the reservoir space in the dolomite mainly developed during the karst stage. Mold pores in the reservoir are generally filled by minerals such as calcite, dolomite, and quartz, impairing its storage capacity. The gypsum-free reservoir occurs mainly in the 5th sub-member of M5, consisting of silt-sized to fine-grained dolomite. Gypsum is absent in the reservoir and its main storage space is provided by intergranular pores and irregular vug. The content of Fe in the matrix dolomite is comparatively low, ranging from 233 to 2010 ppm. The δ 13 C V-PDB and δ 18 O V-PDB values of the dolomite are in the range of − 0.8– − 0.4‰ and − 6.6– − 5.3‰, respectively, close to those of the Ordovician seawater. This indicates that the dolomite was possibly formed through reflux dolomitization during the burial stage. On the other hand, the formation of the reservoir space in the dolomite was mainly associated with the high-energy sedimentary environment and the dolomitization process.