Abstract:

Abstract:  Timing of oil or gas emplacements is a new subject in isotopic geochronology and petroleum geology. The principle of the illite KAr age as expounded by Hamilton et al. is that the illite is often the last or one of the latest mineral cements to form prior to hydrocarbon accumulation, and since the displacement of formation water by hydrocarbons will cause silicate diagenesis to cease, KAr ages for illite will constrain the timing of this event and also constrain the maximum age of formation of the trap structure. In this study, the possibility of authigenic illites 40Ar39Ar dating has been investigated. The illite samples were separated from the Tertiary sandstones in three rich oil reservoir belts within the Huizhou sag by cleaning, fractured by coolingheating cycling，Soxhletextracting with solvents of benzene and methanol and separated with centrifugal machine. If oil is present in the separated samples, ionized organic fragments with m/e ratios of 36 to 40 covering the argon isotopes will be yielded by the ion source of a mass spectrometer, resulting in wrong argon isotopic analyses and wrong 40Ar39Ar ages. The preliminary experiments of illite by heating did show the presence of ionized organic fragments with m/e ratios of 36 to 44. In order to clean completely up the organic gases and obtain reliable analysis results, a special purification apparatus has been established by Qiu et al. and proved valid by the sequent illite analyses. All the illite samples by 40Ar39Ar IRlaser stepwise heating yield stairup age spectra in lower laser steps and plateaux in higher laser steps. The youngest apparent ages corresponding to the beginning steps are reasonable to be interpreted as the hydrocarbon accumulation ages. The weighted mean ages of the illites from the Zhuhai and Zhujiang Formations are 12.1±1.1 Ma and 9.9±1.2 Ma respectively. Therefore, the critical emplacement of petroleum accumulation in Zhujiang Formation in Huizhou sag took place in ca 10 Ma. Later fault activity strengthened the entrance of hydrocarbon fluids into the oil systems based on the data of the fault evolvement history, petroleum system combinations and homogeneous temperatures of fluid inclusions.

Key words:

Key words: 40Ar39Ar dating of authigenic illite; timing of petroleum entrance; fluid inclusions; homogeneous temperature; sandstone reservoir; Pearl River Mouth Basin; Neogene