This study proposes a new method for estimating tight oil resources in a closed petroleum system using an alternative mass balance model. The proposed approach primarily derives resource estimates based on a large volume of Rock-Eval pyrolysis data. In addition to leveraging the cost-effective, straightforward, and rapid process, the proposed method has the flexibility to assess the potential of different types of resources, including organic-rich tight (shale) oil and organic-lean tight oil. The method is validated by assessing the oil resource potential of the middle Permian Lucaogou Formation (P2l) in the Jimusaer depression, Junggar Basin, China. Results indicate that the P2l contains 1.62 × 109 t (11.91 × 109 bbl) of organic-rich and 2.88 × 109 t (21.17 × 109 bbl) of organic-lean tight oil. Of the organic-rich tight oil, 21% comprises free oil resources, amounting to 0.343 × 109 t (2.521 × 109 bbl). The proposed approach deals sufficiently with the different resources in a petroleum system well and provides a useful tool for estimating the resource potential of an unconventional play.