Characteristics of tight oil sandstone reservoirs: a case study from the Upper Triassic Chang 7 Member in Zhenyuan area, Ordos Basin, China

Tight oil reservoirs account for a large portion of hydrocarbon resources in continental basins. The Chang 7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin is a well-known high-quality source rock in China and considerable petroleum resources have been found in its tight oil sandstone reservoirs. This study uses thin sections, constant-rate mercury injection, mercury injection, scanning electron microscopy, and the fractal dimension to study its mineral compositions and pore structure and to discuss the effects of cement mineralogy and pore structure on reservoir physical property. Tight oil sandstone reservoirs in Chang 7 Member mainly consist of gray and grayish green medium-fine-grained lithic arkose, feldspathic litharenite, and litharenite. Feldspar dissolved pores are the most developed pores in the reservoirs, followed by intergranular pores and lithic dissolved pores. Throats of the reservoirs are mainly in pore shrinkage type, neck type, sheet type, and bent sheet type. The tight oil sandstone reservoirs have complex pore structure and with the increasing radius, the structure of throats becomes simpler, while the structure of pores becomes more complex. According to median radius of pore-throat, the maximum mercury intrusion saturation, and extrusion efficiency, the reservoirs can be divided into three types: micro throat-high mercury intrusion saturation type with high physical property, adsorptive throat-medium mercury intrusion saturation type with medium physical property, and micro adsorptive throat-low mercury intrusion saturation type with low physical property. Ferrocalcite and illite are the main cements reducing reservoir physical property, while chlorite cement can protect the reservoir physical property to some extent. In microscopic pore structure parameters, except pore-throat skewness, median radius of pore-throat, pore-throat ratio, pore-throat sorting coefficient, and the fractal dimension of big pore-throats, can affect reservoir physical property.