Heterogeneity of dolomite-spotted limestone reservoirs of the sub-salt fourth member of the Ordovician Majiagou Formation in the Ordos Basin and development environment of high-quality reservoirs

The sub-salt fourth member of the Ordovician Majiagou Formation in the Ordos Basin is a regional exploration hotspot, and dolomite-spotted limestones form important petroleum reservoirs. Currently, reservoir heterogeneity, high-quality reservoirs and their controlling factors have been poorly understood, limiting in-depth comprehensive evaluation and exploration and development in future. Therefore, with an integrated method of petrography, sedimentology, ichnology and reservoir geology, this paper studies heterogeneity characteristics of the reservoirs and reveal high-quality reservoir types and development environments. The results show that dolomite-spotted limestones in the study area can be classified into two types: three-dimensional box structure with Glossifungites ichnofacies and two-dimensional horizontal maze structure with Cruziana ichnofacies. Six types of reservoir spaces develop in the dolomite-spotted limestone: matrix micro-pores, residual mud micro-pores, dolomite intercrystalline pores, dolomite intracrystalline micro-pores, dissolution pores and fractures. The effective reservoir space is dolomite intercrystalline pores inside burrows and matrix pores are isolated, these lead to strong heterogeneity of dolomite-spotted limestones and sample size dependence of physical property measurement. Modeling analysis of full-size samples reveals that porosity and permeability of the dolomite-spotted limestone with three-dimensional box structure are 3.20% and 1.03 mD, respectively, while those with two-dimensional maze structure are 0.55% and 0.18 mD, respectively. The former has better physical properties and pores are well connected in all directions, making it a high-quality reservoir type. Further comprehensive analyses of ichnofacies and depositional sequences indicate that the depositional environment where this high-quality reservoir develops is the high-energy intertidal to shallow subtidal zone.