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The porosity and permeability of tight oil reservoirs are quite low. The occurrence of oil in tight oil reservoirs, which are composed of a micrometer-sized pore network system, is one of the most important scientific issues to be solved in tight oil research. Oil was removed from two tight sandstone core samples (with diameter 3 mm and 5 mm) which were saturated with KI and oil and monitored by core flooding system using X-ray Micro-CT, to display the occurrence and accumulation characteristics of oil in the micrometer-sized pore network. Pores with radius larger than 10 μm have good connectivity and comprise the dominant pore network system for oil accumulation, while 62.9%-84.1% of the oil accumulates in pores with radius from 10-60 μm. Pores with radius smaller than 10 μm are huge in number; however, most of them are isolated in space, the oil saturation of these pores is low and they only gathered 6.8%-20.0% of oil in tight sandstone. The oil saturation of tight sandstone increases in three steps with pore radius in the range of 10%-40% for pores with radius smaller than 10 μm, 30%-75% for pores with radius 10-60 μm, 40%-75% for pores with radius 60-80 μm. The occurrence of oil in micrometer-sized pore network systems in tight sandstones is controlled by the size, origin and spatial distribution of pores.