The coexisting free gas, solution gas, and oil in a tight oil reservoir, constrained by the capillary force of the micrometer/nanometer pore/pore-throat system, are of crucial importance to the sweet spot of a tight oil reservoir. Equations of modified total capillary force for the liquid phase are proposed to indicate the tight oil mobility in situ, and are derived from the Kelvin capillary force equation, the Peng-Robinson (PR) equation of state (EOS) (Robinson et al. 1977), and the Van-Laar (VL) equation (Renon and Prausnitz 1968). In addition, relationships between the vapor/liquid state, phase-flow state for production, and gas/oil ratio (GOR) are established in correspondence with the production data of North American and Chinese (Yanchang) tight oil plays. The following conclusions can be drawn.

• We proposed a modified total capillary force function to indicate the tight oil mobility in situ, using the PR EOS method and the EOS+λ (VL equation) method.
• We identified five phase stages according to the gas volume ratio ( y_i) in situ: the single-phase oil flow stage, the multiphase transient-flow stage, the multiphase stable-flow stage, the multiphase supercritical-flow stage, and the single-phase gas flow stage.
• Tight oils are more mobile in three stages: near the critical point of y_{i-critical(1)}, such as in the Yanchang and Bakken tight oils; at the transition zone between oil and wet-gas zones before y_{i-critical(2)}, such as in the Eagle Ford tight oil; and between the y_{i-critical(2)} and y_{i-critical(3)} in a gas-condensate state, which is at the transition zone between wet-gas and dry-gas zones.