Origin of extra-low permeability and controlling factors of the Chang-6 reservoirs in the Yongning prospect area, Zhidan Oil Field, Ordos Basin

The present paper gives a detailed description of the origin of extra-low permeability and controlling factors of the Chang-6 reservoirs in the Yongning prospect area, Zhidan Oil Field, Ordos Basin on the basis of well logs, cast sections, SEM, X-ray diffraction and grain-size analysis. The results of research show that both sedimentation and diagenesis, especially the sedimentary microfacies have an important effect on extra-low permeability of the Chang-6 reservoirs. In general, the porosity and permeability in the subaqueous distributary channel sandstones are better than those in the channel mouth bar sandstones, which in turn are better than those in the interdistributary bay sandstones. The compaction and cementation tend to be destructive to the development of the porosity and permeability, whereas diagenetic dissolution, alteration and fissures tend to be constructive to the improvement of the porosity and permeability of the reservoirs.