In this study, pyrolysis experiments were conducted with a saturate-rich Tertiary source rock-derived oil from the South China Sea basin using a fixed-volume pressure vessel at temperatures from 350 to 425 degrees C for 24 h (0.92-1.85% Easy R-0) to investigate pressure effects up to 900 bar on the generation and stable carbon isotopic fractionation of light hydrocarbons in the C-6-C-7 range. The results demonstrate that the pressure retards oil cracking to light hydrocarbons, but the retardation depends on the thermal evolution. In the peak oil to early wet gas stage (350 and 373 degrees C, 0.92-1.15% Easy R-0), the light hydrocarbon generation is low but it is still suppressed by increasing pressure. In the late stages of the wet gas window (390, 405, and 425 degrees C, 1.35-1.85% Easy R-0), the light hydrocarbon generation is suppressed significantly from 200 to 470 bar, followed by promotion and promotion-suppression as pressure is increased up to 900 bar. Meanwhile, the distributions of branched alkanes, cycloalkanes, and aromatic hydrocarbons are pressure-dependent. The medium to high pressures result in increasing Mango K-1 values and toluene/n-C-7 ratios and decreasing n-C-7/methylcyclohexane ratios, suggesting that pressure benefits the occurrence of cyclization and aromatization during oil cracking, probably involving bimolecular reaction pathways. Preferential aromatization and isomerization with increasing pressure lead to significant carbon isotopic fractionations of aromatic hydrocarbons and branched alkanes as up to 4 parts per thousand and 2 parts per thousand, respectively. However, stable carbon isotopic compositions of cycloalkanes show almost no fractionation under pressurized cracking. Therefore, caution must be taken with respect to the application of light hydrocarbon-derived parameters in deep petroleum reservoirs usually at high temperatures and pressures. The carbon isotopes of branched alkanes and aromatic hydrocarbons could be potential measures to identify the pressure effects, while carbon isotopes of cycloalkanes could be an effective index for oil-oil/oil-source correlations.