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Pore structure is the key factor to control the quality and fluid flow of ultra-tight reservoir. The relationship between rock types, pore types, and pore sizes of ultra-tight sandstone reservoirs was identified by observing microscopic characteristics under a polarizing microscope and scanning electron microscope (SEM) and quantitively assessed using high-pressure mercury intrusion (HPMI) and fractal theory. By a comparative analysis of lithic sandstone and feldspar lithic sandstone reservoirs of Upper Triassic Xujiahe Formation of the Northern Sichuan Basin, we found that the pore type of the ultra-tight sandstone reservoir was linked to the rock type. Argillaceous lithic sandstone minly generates fracture pores, while fine-grained lithic sandstone and feldspar lithic sandstone formed dissolved pores by dissolution of carbonate cement, rock debris, and feldspar particles. Meanwhile, clay minerals such as kaolinite and chlorite in the reservoir possess pores between clay platelets and intraplatelet pores within clay aggregates, which are important pore types for ultra–tight sandstone reservoirs. A method to divided intervals of pore size distribution was proposed using high-pressure mercury intrusion (HPMI), and three intervals of pore diameter distribution were observed: 10–40 nm, 40 nm–1 μm, and 1–100 μm, which is largely controlled by pore type. The reservoir with only fracture pores is a one–interval reservoir with pore diameter in the range of 1–100 μm, while the reservoirs with dissolved pores and clay mineral-related pores have the above three intervals. Meanwhile, the fractal of pores in different scale range is different, and the larger the pore size, the stronger is the reservoir heterogeneity. A treelike pore structure model can be utilized to define pore development features in ultra–tight sandstone reservoirs. Micron–scale pores with diameters of 1–100 μm is mostly made up of fractures, cement and particle dissolution pores. However, nano–scale pores in the range of 40 nm–1 μm and 10–40 nm are predominantly pores formed between clay platelets and intraplatelet pores within clay aggregates, respectively. • Rock type, pore type, and pore size of ultra-tight sandstone reservoirs have a close relationship. • Pores between clay platelets and pores within clay aggregates are essential pore types in ultra-tight sandstones. • Three intervals of pore distribution were divided using high-pressure mercury intrusion (HPMI). • The heterogeneity of pores in ultra-tight reservoirs is mostly dictated by pore size and fractures. [ABSTRACT FROM AUTHOR]

Pores and throats are the key parameters for determining the pore structure and fluid flow behavior in tight sandstone reservoirs. In this paper, pores and throats of the tight sandstone reservoir of the Upper Triassic Xujiahe Formation in the northern Sichuan Basin were investigated using classical petrography, scanning electron microscopy, and confocal laser scanning microscopy. In addition, a method of pore as well as throat identification and quantitatification using high-pressure mercury injection (HPMI) measurements was worked out. The results show that clay mineral-related pores are well developed in lithic sandstone and feldspathic lithic sandstone in addition to pores produced by dissolution of calcite cement and clastic grains. Intergranular pores are connected by lamellar, curved lamellar, and narrow pore throats. The pores and throats associated with intragranular dissolution and clay minerals are mostly tube shaped. The rapid-increasing and slow-increasing profiles of the mercury injection curves represent the dominance of pores and throats, respectively, allowing to differentiate pores and throats. Calculations show, in addition to fractured lithic sandstone, that has micron-sized pore–throat systems, the other lithic sandstones possess dominatly nano-sized pore–throat systems. In case of feldspathic lithic sandstones, micron-sized and nano-sized pore throat systems are equally important. In micron-sized pore throat dominated reservoirs, the pore volume is slightly larger than the throat volume, whereas for the nano-sized pore throat system, the pore volume is considerably larger than the throat volume. The key factors that are favorable for the formation of lithic sandstone pore throat systems are early calcite dissolution, lithic fragments dissolution, and early chlorite cementation. In contrast the key factors that are favorable for the formation of feldspathic lithic sandstone pore throat systems are dissolution of feldspars and lithic fragments. • A new method to distinguish pores and throats within tight sandstone reservoirs is proposed. • Lithic and feldspathic lithic sandstone reservoirs host different pore and throat types. • Pore volume is slightly larger than throat volume in the micron-size pore network. • Pore volume is considerably larger than throat volume in the nano-size pore network. • Compaction, cementation, and dissolution directly affected the development of pore throat systems. [ABSTRACT FROM AUTHOR]