116 / 2022-02-22 10:53:12

Reservoir quality difference research of tight sandstones on Shihezi Formation, Ordos Basin

reservoir quality; diagenesis; Shihezi Formation; Ordos Basin

The reservoirs in LX Area, the northeast margin of Ordos Basin, are characterized by tightness and strong heterogeneity, whose porosity is generally lower than 10% and permeability lower than 0.1mD. Although the reservoir physical property is generally poor, the high-quality reservoirs with relatively high porosity and high permeability are developed. Their porosity mainly concentrates in 10%- 15% and permeability is higher than 0.1mD, and the natural gas mainly distributes in the relatively high-quality reservoirs, Therefore, it is very important for natural gas exploration to clarify the origins of reservoir tightness and the formation mechanism of relatively high-quality reservoirs in LX Area.

To illustrate the formation mechanism of relatively high-porosity and high-permeability reservoirs in tight sandstones, this paper focused on He4 and He8 reservoir in Shihezi Formation of LX Area, and systematically studied the petrological characteristics, pore structures and diagenesis of the reservoir based on the core observation, analysis data including casting thin section, scanning electron microscope, X-ray diffraction etc.. The densification origins of the He4 and He8 reservoir as well as the formation mechanism of relatively high-quality reservoir were analyzed. The results show that the reservoir quality is mainly controlled by sedimentation and diagenesis. The tight reservoir mainly occurs in the thin-bedded and fine-grained sandstones deposited in low energy environment, such as the edge of distributary channels; during the burial history, porosity loss resulting from strong compaction accounted for more than 50% of the primary porosity, and carbonate cementation and siliceous cementation further enhanced the densification of the reservoir. The relatively high-quality reservoir mainly develops in the coarse-grained sandstones in the main body of distributary channels developed in high energy environment. It is found that the formation mechanism of He8 reservoir is different from that of He4 reservoir. The He4 reservoir is characterized by the early cementation of chlorite rims and the lower plastic component content, which are conducive to resisting the compaction. Therefore, more primary pores are preserved to form the He4 high-quality reservoirs. The dissolution of feldspar occurs in the He8 sandstones and forms the secondary pores, which composing the main pore types of the He8 high-quality reservoirs.