The timing of petroleum emplacement is crucial in understanding petroleum-forming processes and predicting new prospecting regions. This study investigates the possibility of determining the age of petroleum emplacement by an illite 40Ar/39Ar laser stepwise heating technique. A specially designed apparatus has been used to clean up the organic gases in samples contaminated with residues from oil or gas fields, resulting in credible 40Ar/39Ar data. The sandstone samples are from the Zhuhai and Zhujiang formations in the Huizhou sag in the Pearl River Mouth Basin, South China Sea. Illite 40Ar/39Ar laser stepwise heating consistently yields gradually rising age spectra at the low-temperature steps until reaching age plateaus at high-temperature steps. The first-step apparent ages of the six samples from the Zhujiang formation give a weighted mean age of 11.4 ± 0.5 Ma (1σ), in agreement with an age of 12.1 ± 1.1 Ma in the first three steps of the sample from the Zhuhai Formation. These dates are interpreted as being caused by the formation of fine authigenic illite and therefore indicative of the maximum age of petroleum emplacement in the Huizhou sag because illite is commonly one of the latest forming minerals prior to hydrocarbon accumulation. The plateau ages ranging from 58 to 276 Ma in high laser-output steps are interpreted as being caused by variable contributions from detrital K feldspar in the sandstones. The older total ages based on the illite 40Ar/39Ar stepwise heating results are equal to the K-Ar ages and are not usable in delineating petroleum emplacement because they represent mixtures with variable contributions from authigenic illite and detrital K feldspar. The 40Ar/39Ar stepwise heating technique thus can be applied to resolve important petroleum emplacement age information from illite samples that otherwise cannot be provided by traditional K-Ar methods.