Abstract:

Sandstone reservoir characteristics of the Upper Paleozoic coal-bearing strata in Luyi Sag are not yet clearly understood, and this hinders the relevant research and exploration deployment of natural gas in this area. In this study, we use analytical methods such as core observation, thin section identification, scanning electron microscopy, mercury intrusion testing, and acoustic emission testing to investigate the characteristics of sandstone reservoirs and further identify the factors affecting the reservoir development. The study results show that the sandstone reservoirs are dominated by fine and medium-grained and fine lithic quartz sandstone. There are various types of diagenesis, among which compaction and cementation are the ones mostly damaging the reservoirs, while dissolution and fracturing contribute more to the damaging. The reservoirs are tight ones with ultra-low porosity and ultra-low permeability, and the reservoir spaces are dominated by dissolved pores, intergranular micropores, fractures, with minor residual primary pores and mold pore. The reservoir pores can be divided into three categories based on the structures. Type I are good in structures and physical property, but with limited development; type Ⅱ and type Ⅲ are widely developed, but are poor in structures and physical properties. The reservoir development can be affected by many factors and the tectonic setting affecting the reservoirs are in a moderate-depth buried diagenetic environment for a long time; the sedimentary microfacies control its position of the development. The compaction reduces the primary porosity by 22.01%. The organic acid-bearing diagenetic fluids are the main factor promoting the dissolution of unstable components to form the secondary pores; fracturing greatly improves the reservoir permeability but its development scale is limited. This understanding is of great significance to the study of unconventional gas reservoirs in coal-bearing strata, and it provides a key reference for the deployment of relevant exploration in this area.

Key words: reservoir characteristics, densification, main controlling factor, coal-bearing stratc, Luyi sag