Movable oil and its controlling factors in tight sandstones: A Case study of the Triassic Chang7 reservoir, Yanchang Formation, Ordos Basin

Tight oil in place in China has been estimated over 10 billion tons, but it is difficult to define JPthe availably producible reserves. Several efficient ways, including perfecting Quartz Crystal Microbalance with Dissipation (QCM-D) to evaluate adsorption ratio per unit in subsurface cores, applying mercury injection capillary pressure(MICP) and nitrogen adsorption to define the lower oily limit and the effective specific surface area, and combining oil testing results in confined cores, were utilized to evaluate the movable oil in the Triassic Chang7 reservoir, Yanchang Formation, Ordos Basin. The results indicated that the thickness of oil adsorbed on tight sandstone surfaces was 15.6 nm with an intensity around 13 mg/m², the lowest oily limit was around 18 nm, the effective specific surface area was around 1m²/g, the porosity was around 6%-10%, and the ratio of free oil was around 14.3%-48.6%. All these results were comparable with the NMR experiments. Several factors controlled the movable oil amounts in tight sandstones, including crude oil density and viscosity, effective specific surface area, effective porosity volume and mineral constituents. For specific region or layer, the amounts of movable oil is parametrically controlled by pore surface area/effective porosity volume.