Shale rocks are associated with nanopores as small as 2 to 20 nm, wherein the strong pore-wall attraction induces alterations in the density distribution of hydrocarbons. Based on their density values, confined hydrocarbons can be categorized into two phases: adsorbed and free phases. Researches has been suggest that the density value of free phases is equal to that of bulk phase. In this study, molecular dynamic simulations (MDs) were employed to investigate the density distribution and potential energy of hydrocarbons within nanopores. Results revealed that under identical temperature and pressure conditions, the density of free phases is lower than that of bulk phases. This discrepancy in densities observed in free phases can be attributed to a reduced potential energy resulting from weaker hydrocarbon-hydrocarbon interactions compared to bulk phases. Neglecting these differences in densities between free and bulk phases may lead to an overestimation of hydrocarbons-in-place (HCIP) for shale reservoirs. Furthermore, we analyzed the effects of temperature, pressure, and pore type on variations in density. The findings in this paper provide insights into the distribution characteristic of hydrocarbons within nanopores of shale formations from the perspective of nanoscale.