Research On Microscopic Pore Structure And Porosity Evolution In Low-permeability Sandstone Reservoirs

Taking three low-permeability reservoirs with different palaeogeothermal background, burial history and different sandstone classifications as the research object, which is Funing formation of Early Paleogene in Shanian, North Jiangsu basin, Xishanyao formation of Middle Jurassic in Niujuanhu, Santanghu basin, and Shihezi formation of Middle Permian in Yan’an, Erdos basin, with the technique of casting sections, staining, scanning electron microscopy, liquid nitrogen adsorption method micropore testing, grain size analysis, X-diffraction, electron microprobe analysis, energy dispersive analysis, mercury penetration and rate-controlled mercury penetration, the microscopic pore structure of low-permeability sandstone reservoirs is studied. Base on the accurace method of model analysis, the porosity evolution pathways of the three low-permeability reservoirs are studied, and their influential factors are analysed.The conclusions are in following:(1) There are pores and pore-throats of various genetic types in s low-permeability reservoirs with different sandstone classifications, and their radius changes in a large scale. The quantities of relatively large pore-throats are usually small, which is the main reason for their low-permeability. The results of rate-controlled mercury penetration show, the permeability of sandstone reservoirs is mainly controlled by the quantity and radius of relatively large pore-throats. The micropores occupy a considerable proportion of porosity in low-permeability reservoirs, and their proportions are obviously mproving with reservoir physical property being worse. As a sort of effective accumulating space of natural gas, micropores are important part of porosity in ultra low-permeability natural gas reservoirs.(2) There are some differences of original porosity in different sandstone reservoirs, which are lower than today. The major porosity losses of most reservoirs are caused by compaction, except those being cemented by carbonate in early phase. Different sandstone reservoirs suffered dissolution more or less, which is controlled by organic acid concentration and filtrate capability of pore network. There are great differences of cementational porosity losses in sandstone reservoirs, all cement will fill pores and block pore-throat to make reservoirs densifying, except carbonate cement in early phase.(3) The reservoir of arkose sandstone with the relatively high palaeogeothermal, short period of large buried-depth, week-alkaline environment in early diagenetic phase, is offen cemented by carbonate in the smaller depth, where the porosity declined sharply with depth. The carbonate cementation can retard compation, and provides dissolution mass. In the relatively high palaeogeothermal condition, organic acid supplying by organic maturation dissolved carbonate and framework grains, and formed substantial secondary porosity. The products of dissolution of aluminum-silicate framework grains, are precipitated in the pore network as an authigenic kaolinite, what results in a significant decrease in permeability of reservoir.(4) The reservoir of lithic sandstone with relatively low palaeogeothermal, moderate period of large buried-depth, acidic diagenetic environment (coal measure strata) in early diagenetic phase, is offen free of carbonate cements. The ductile lithic strongly deformed in compacting process, and the porosity decreased sharply with depth.In the relatively low palaeogeothermal condition, organic acid supplying by organic maturation is limited, the dissolution and cementation are both relatively week in the more closed pore network caused by intensive compaction.(5) The reservoir of quartzose sandstone with moderate palaeogeothermal, long period of large buried-depth, because of the late time of quartz overgrowth formation, porosity decreased sharply with depth. The quartz and authigenic clay mineral are precipitated from the dissolution mass of framework grains, which makes intergranular pores consuming drasticly, and filled part of secondary porosity. In the condition of substantial porosity loss in compacting process, the quartz overgrowth and clay mineral filled primary and secondary porosity severely, what makes the reservoir be ultra-low-permeability usually.