Diagenetic differentiation in the Ordovician Majiagou Formation, Ordos Basin, China: Facies, geochemical and reservoir heterogeneity constraints

Diagenesis is a key process that affects carbonate reservoir formation and evolution. This paper presents a detailed study of a carbonate shoal reservoir in the Ordovician Majiagou Formation, Ordos Basin, China. This reservoir records different diagenetic features that are related to reservoir heterogeneity. Petrographic observations and geochemical data reveal that the two facies present in a single shoal exhibit differences in rock and pore textures, spatial distributions, and reservoir properties. The shoal margins consist of grain dolomite with few intergranular pores and pervasive cement infilling, whereas the shoal core is dominated by crystalline dolomite with abundant intercrystalline pores. These two types of reservoir experienced multi-stage and different diagenetic pathways. The shoal deposits were pervasively dolomitized during the very early diagenesis with the dolomitizing fluid of highly saline seawater due to evaporation. The shoal margins were then cemented due to the inflow of hypersaline seawater and gypsum precipitation during penecontemporaneous to shallow burial, which resulted in significant reservoir degradation. However, this densification and relatively poor reservoir properties of the shoal margins protected the shoal core from cementation and alteration by hypersaline seawater. During deep burial, continued diagenetic modification and recrystallization occurred preferentially in the shoal core as the diagenetic fluid flowed and migrated into the layers with relatively high porosity-permeability. This resulted in intense lithological and porosity transformation within the shoal core. Multiple stages of diagenetic differentiation affected the formation and preservation of dolomite reservoir, which provide new insight into the carbonate reservoir heterogeneity. The results have implications for future oil-gas exploration in high-quality reservoirs developed in old superimposed basins, such as the Ordos Basin and those in a similar geological setting.