Characteristics and porosity development of the karst dolostone reservoirs in the fifth member of the Ordovician Majiagou Formation in the Daniudigas field,northern Ordos Basin,northern Shaanxi

The present paper gives a detailed description of the characteristics and porosity development of lowporosity and low-permeability karst dolostone reservoirs in the fifth member of the Ordovician Majiagou Formation in the Daniudi gas field,Ordos Basin. According to core observation,thin section examination and scanning electron microscopy analysis,the emphasis here is laid on dolostone types,porosity types and genesis of the karst dolostone reservoirs. The dolostone types include gypseous dolostones,microcrystalline dolostones,fine-grained dolostones and algal-laminated dolostones. The porosity types contain solution openings developed during the epidiagenetic periods, intercrystal pores generated by dolomitization, and residual intercrystal pores of algal-laminated dolostones. The solution openings and microfissures of gypsum,solution openings and microfissures,intercrystal pores of dickite and intercrystal pores of dolostones are interpreted to be valid reservoir spaces of the dolostone reservoirs. The development of intercrystal pores in the dolostones tend to be governed by the contents and structures of dolomites,and the residual intergranular pores of algal dolostones are protected by residual organic matter in the pore spaces of the algal dolostones. The dolomitization may be responsible for the low porosity and low permeability of the karst dolostone reservoirs. The dolomites generated during the penecontemporaneous-shallow burial stages generally have small,subhedral to anhedral dolomite crystals,microporosity and narrow pore throat radius. Therefore,both the microcrystalline dolostones and fine-grained dolostones have low porosity and low permeability in the fifth member of the Middle Ordovician Majiagou Formation. The karstification may give rise to the heterogeneity of the dolostone reservoirs. The filling of cracks,caves and larger dissolved pores by argillaceous calcitemay also reduce the porosity and permeability of the karst dolostone reservoirs. The gypseous microcrystalline and fine-grained dolostones with low porosity and low permeability are not easily filled by the mud carried by the surface water,which,in turn,may result in the decrease of the porosity and permeability of the karst dolostone reservoirs in the study area.