Lacustrine shale oil is typically heavier and more viscous than marine shale oil, impeding its exploration and exploitation. Thus, identification of its origin is key to elucidating its fluidity. Here, the middle Permian Lucaogou Formation of the Jimusar sag, Junggar Basin, China, was used as an example to investigate this issue. The main controlling factor for the characteristics of crude oil in the Lucaogou Formation is organic matter precursor inputs and their proportions (based on coevolution of biology and environment) caused by different depositional paleoenvironments between the upper and lower sections of the formation. The higher β-carotane abundance and C₂₈/C₂₉ sterane ratio than the average for contemporary Permian organic matter indicate that the organic matter precursor of the Lucaogou Formation was a type of salt-tolerant planktonic green algae, Dunaliella. The physical properties of crude oil from this unit (density, 0.87–0.92 g·cm⁻³; viscosity, 33.9–551.8 mPa·s) result from high resin and asphaltene content, typical of oils generated from Dunaliella-rich organic matter. During deposition, higher water salinity in the lower section resulted in a higher abundance of Dunaliella. For this reason, crude oil from the lower section is heavier than that from the upper section, despite the lower section being more thermally mature. The organic matter precursor in the Lucaogou Formation resulted in high-density crude oil, which is difficult to process. However, light crude oil still exists at high-maturity stages. The understanding in this study is general for the formation of heavy oils in saline lacustrine basins.