Microstructural Characterization And Grading Evaluation Of Tight Sand-conglomerate Reservoir

In tight sandstone reservoir microstructure,pore dominated and complicated variable nano-submicro pore-throat structure was developed,which determines the oil and gas large-scale accumulation of dense sandstone,local ’deserts’ enrichment and high production,and also leads to the poor correlation between macroscopic physical properties(porosity,permeability,mobility).It’s of great significance to the exploration and development of tight oil and gas reservoirs that microstructural characterization and grading evaluation.Based on the tight sand-conglomerate reservoir with great potential for exploration and development but not enough research of Lower Cretaceous Shahezi Formation in Xujiaweizi Depression,Songliao Basin,the paper analyzed the microstructural characterization experiments and evaluated microscopic pore-throat structure and establishes the standard of grading evaluation of tight reservoir,which can provide practical guidance for future exploration and arrangement and enrich the theories and methods of microscopic evaluation of tight reservoir.The pore of Shahezi Formation were divided into intergranular pore,dissolution pores,inter-crystal pores and micro-cracks and the main pore types were dissolution pores and inter-crystal pores,which were mainly connected with the bent sheet and tube-shaped throat.The size and distribution and connectivity of pore throat were characterized by cryogenic nitrogen adsorption experiment,mercury injection and nuclear magnetic resonance(NMR).The range of pore size has grown from a handful nm to hundreds nm and 80% were under 1μm in tight sand-conglomerate reservoir of Shehezi Formation.The separating of the pore and throat is poor and the throat radius is mainly less than 0.5μm in the tight reservoir,whose major storage space belongs to micro-pore and micro-throat.The analysis of the rate-controlled mercury penetration data prove that the range of pore distribution,volume,peak value were basically the same,and the pore size was mainly between 100 and 200μm.The permeability is controlled by throat size,the lager permeability,the wider range of throat size.The upper limits of physical properties of tight sand-conglomerate reservoir were determined by balance method and the throat radius was 2μm,and the according upper limits of physical properties was 0.6mD and 8.5% of glutenite,0.6mD and 10.5% of sandstone.The lower limits were determined by measuring the thick of irreducible water film and the throat radius was 22 nm,and the according lower limits were 0.015 mD and 2.2% of glutenite,0.015 mD and 3.5% of sandstone.Based on the result of combining movable inflection point and gas testing,the tight reservoir were divided into type I and type II and the according throat radius was 0.4μm and the grading limits were 0.06 mD.The grading limits of type I was greater than 0.06 mD and the lower limits were 5.6% of glutenite and 7.5% of sandstone.The grading limits of typeⅡwas 0.02-0.06 mD and the limits of porosity were 2.7%-5.6% of glutenite and 4.5%-7.5% of sandstone.