Characteristics and origins of the Chang-6 reservoir rocks with extra-low and super-low permeability in the NX area, Ordos Basin

According to the data of physical properties, cast thin section, SEM, X-ray diffraction analysis, mercury injection test, the present paper deals with the characteristics and origins of the Chang-6 reservoir rocks with extra-low and super-low permeability in the NX area, Ordos Basin. The Chang-6 sandstone reservoirs have low porosity, and extra-low and super-low permeability. The porosity types consist of residual intergranular pores and secondary solution openings. The pore throat types contain mesopore-fine throats and micropore-fine throats. The early diagenetic compaction and cementation are major influencing factors that lead to the formation of poor physical properties of the Chang-6 sandstone reservoirs. The porosity loss caused by mechanical compaction ranges from 45.9% to 65.6%, with an average of 56.5% of the total porosity, and that caused by cementation ranges from 21.8% to 46.4%, with an average of 28.3% of the total porosity. In the cements, clay minerals and carbonate minerals occur as main cements that may reduce the reservoir quality of the sandstones. The dissolution of feldspar, laumonite and lithoclasts may largely improve the physical properties of the Chang-6 sandstone reservoirs, including the increase of the porosity from 22.2% to 71.4%, with an average of 44.2% of present-day porosity. The Chang-6 sandstone reservoirs with extra-low and super-low permeability were developed during the latest Cretaceous, and from that time on, the physical properties cited above remain unchanged.