Timing of petroleum emplacement is one of the important scientific problems in petroleum geology. This study investigates the possibility to determine the age of petroleum emplacement by authigenic illite 40Ar–39Ar laser stepwise heating technique. The sample is from the reservior sandstone of the Tertiary Zhuhai Formation in the Pearl River Mouth Basin. The sandstone was gently disaggregated by repeated freezing-heating in order to reduce the detrital feldspar content. A special purification apparatus is made to clean up the organic gases which will affect the argon isotope analyses with m/e ratios from 36 to 40. The released gases were first cleaned up through the special apparatus, and then further purified by two Saes NP10® Zr/Al getters. An increasing ladder-shapped age spectrum is obtained for the illite 06ZJ26I 40Ar–39Ar laser stepwise heating. The weighted mean age (12.1 ± 2.2) Ma (2σ) of the three initial lowest laser-power steps is interpreted as the minimum age of authigenic illite, which indicates the maximum age of petroleum emplacement. A plateau of (98.0 ± 0.9) Ma is yielded in high laser-power steps. The plateau age might represent the age of the terrestrial detrital feldspar of the sandstone. The contribution to the age spectrum of different mineral components with different activation energies might be recognized by 40Ar–39Ar stepwise heating technique. Compared with the traditional K-Ar method, the 40Ar–39Ar stepwise heating technique can obtain more information from the illite sample.