Microstructure characteristics and quantitative analysis on porosity evolution of ultra-low sandstone reservoir

The average porosity of sandstone reservoir in Jiyuan area of Ordos basins is 11.68%,average permeability is 0.58×10-3-m2,belonging to the ultra-low permeability and low porosity reservoir.Ultra-low permeability sandstone reservoir has complex micro-structure,and research on porosity evolution is a hot-spot currently.Using casting thin sections,scanning-electron micro-scopes,granularity analysis and datum form high-pressure mercury injection and X-ray,five porosity types and four throat types were analyzed and summarized.Pores evolution was researched by qualitative analysis and quantitative calculation lastly.The dominating pore type of Chang 4+5 reservoirs is inter-granular pore and feldspar-dissolved pore.The throat types are point-shaped throat,necked throat,flake throat.The micro-fractures improve permeability and become the best channel in the reservoir.Using statistical relationship between original porosity and sorting coefficient,the reservoir sandstones original are restore calculated and the average is 34.60%.Dynamics has a major impact to characteristics of pore evolution.The impact of compaction is greatest and its loss-rate of porosity is 49.4%,and the loss-rate of porosity of cementation is 28.7%.The dissolution of the reservoir sandstone improves porosity about 3.75%.Quantitative calculation result and qualitative knowledge have consistency,and are inosculated to experimental data.It is shown that the research methods and results are reliability,and have significance to similar reservoir.