Microscopic characteristics and formation mechanism of Upper Triassic Chang 7 tight oil reservoir in the southwest Ordos basin

The petrologic feature, diagenesis, pore types and controlling factors of reservoir quality of Chang 7 formation in Longdong area, Ordos basin were studied using casting thin section, scanning electron microscopy, cathodoluminescence, porosity and permeability data. The results show that the diagenetic processes of Chang 7 tight oil reservoir are different from conventional reservoir and the reservoir quality is a product of many factors which are controlled by tectonic and sedimentary conditions, petrology, burial process and diagenesis. Chang 7 sandstone is mostly feldspathic litharenite and lithic arkose, with an average porosity of 10. 1%and permeability of 0. 18 mD. Intense mechanical compaction occurred during the early rapid burial process. However, the dissolution of feldspar was still common due to the abundant acid fluid from adjacent source rocks and dissolution pore is one of the major pore types. The reservoir quality was severely affected by illite, ferrocalcite and ankerite cementation. Illite mainly occurred in early stage and was formed by deposition, alteration of mica, smectite and matrix, and by feldspar dissolution, albitization in late stage. Ferrocalcite and ankerite cementation formed after acid dissolution of debris and caused further porosity loss in mid-late stage.