Dolomitization of the Middle Ordovician Ma55 Sub-member of the Majiagou Formation and implications for hydrocarbon exploration in the northern Ordos Basin, NW China

The Middle Ordovician Majiagou Formation in the northern Ordos Basin consists of a sequence of subtidal and peritidal carbonates that are extensively dolomitized. The diagenetic evolution of the Ma5(5) Sub-member was investigated to determine the factors controlling reservoir quality in the northern Ordos Basin. Dolomite in the Ma5(5) Sub-member is classified into three main types on the basis of crystal size: dolomicrite (D1), micritic-silty and fine silty dolomite (D2), and silty (and coarser) dolomite (D3). The Dolomite D1 was produced by dolomitization in a low-salinity diagenetic environment and transformed by near-surface freshwater leaching during shallow burial. The Dolomite D2 developed as a result of post-penecontemporaneous seepage-reflux dolomitization and was affected by later burial dolomitization. The Dolomite D3 was produced during burial dolomitization. Although the original porosity of the Dolomite D1 was low, freshwater diagenesis increased the porosity significantly. As a result of the transformation of montmorillonite to illite, there was a deficiency in the Mg2+ necessary for Dolomite D2 formation, resulting in the nonuniform dolomitization of this type of dolomite. The intercrystalline pores of the Dolomite D2 are mostly filled with montmorillonite and illite, reducing its porosity and permeability and consequently making the Dolomite D2 a poor reservoir. The Dolomite D3 has a higher porosity and contains various types of dissolution and intercrystalline pores, generating a higher permeability. This dolomite is a potential reservoir in the study area and a possible target for oil and gas exploration. Overall, the results of the study suggest that the quality of a potential dolomite reservoir is strongly controlled by the diagenetic history of the host carbonates.