Authigenic Illite ⁴⁰Ar/³⁹Ar Dating Of Hydrocarbon Accumulation Ages In Baiyun Deep-water Area, South China Sea

The Baiyun deep-water area in the north of the South China Sea, is an important area for oil and gas exploration in recent years. Determining the period of oil and gas accumulation could help us to understand the burial history of the basin and the distribution of the gas and oil. However, it is a difficult issue to date of the petroleum emplacement with the traditional isotopic geochronological methods.Based on the presupposition that authigenic illite clay minerals were commonly the last minerals prior to the hydrocarbon accumulation, authigenic illite clays can precipitate as a result of the maximum age of hydrocarbon emplacement. However, a lot of technical problems need to be solved for the authigenic illite dating, mainly includes:（1） separating and extracting authigenic illite from sandstones;（2） purification of the organic matters from the oil-field;（3） evaluation of the ³⁹Ar recoil.In this study, authigenic illite samples separated from the sandstones of the Zhuhai and Zhujiang formations in the Baiyun Sag, and Wenchang and Enping formations of the Huizhou Sag, in the north of the South China Sea, are analysed by the ⁴⁰Ar/³⁹Ar laser stepwise heating technique. The main points of the investigation in this study include:（1） Under the guidance of Prof. Qiu, I have improved the Organic Gas Removal System（OGRS）, greatly promoting its purification capabilities. The OGRS can effectively remove the organic gases to obtain pure noble gases for the mass spectrometer analyses. The sandstones samples from oil-field contain the organic matters, which could splitted into fragments covering all the argon isotopes. This system is a key core technique for us to obtain reliable authigenic illite ⁴⁰Ar/³⁹Ar ages.（2） The grain sizes of authigenic illite are in nanometer, they release gases at very low laser outputs. In order to obtain reliable authigenic illite ⁴⁰Ar/³⁹Ar ages of the hydrocarbon accumulation in detail, we have a new laser-controlling board with laser output precision of 0.1% to replace the old one of 1%. We also have a Zn S filter of 7 mm in thickness to reduce 40% laser energy reaching to samples. Therefore, we can have more precise laser output for high-precision illite ⁴⁰Ar/³⁹Ar dating. This is an important technique improvemen.（3） Using the UV-laser to weld and seal the aluminum sample tubes under ultrahigh vacuum, the samples were effectively prevented from air and comtaminations during neutron irradiation, which promote the sample irradiation quality.（4） In order to interpret the ⁴⁰Ar/³⁹Ar dating results, authigenic illite samples were also analysed by scanning electron microscope（SEM） and X-ray diffraction（XRD）. The morphology of the clay minerals is different separated from different depths. The percentage of the illite increases with the burial depth, but irrelevant with the strata.（5） The clay samples from the Baiyun and Huizhou Sags have high LREE/HREE ratios and weak negative Eu anomalies, which indicate that they might have the same genetic.（6） All the illite samples from oil-saturated sandstone reservoirs in the Zhuhai, Zhujiang and Enping formations in the Baiyun Sag, in the Enping and Wenchang formations in the Huizhou Sag, yield gradually increasing age spetra by ⁴⁰Ar/³⁹Ar laser stepwise heating. Two illite samples of BY39 from the Zhujiang Formation and BY53 from the Zhuhai Formation in the Baiyun Sag, in the deepest collection sections with best illite growth, yield “age plateaus” in the low laser output steps covering more than eight contiguous steps with more than 4% of ³⁹Ar released. Their plateau ages are 15.0 ± 0.3 Ma（1σ,MSWD = 0.12） and 16.5 ± 0.3 Ma（1σ,MSWD = 0.3） respectively, corresponding to concordant isochron ages of 15.2 ± 1.4 Ma and 16.6 ± 1.3 Ma on the reverse isochron diagrams of 36Ar/40 Ar vs 39Ar/40 Ar.（7） Aussuming that the ³⁹Ar recoil caused 30% ³⁹Ar loss during irradiation, the correct ages of above two illite samples are 10.6 ± 0.3 Ma and 11.4 ± 0.3 Ma respectively, which might be the best estimation of the maximum ages of the hydrocarbon accumulation in the Baiyun Sag.（8） The pleatue ages of the illite samples in mid-high temperature steps reperesent the ages of the source detrital feldspars. The detrital source in Baiyun sag are diversity, due to the Zhujiang and Zhuhai formation experienced complex geological process, which may be affected by multiple source regions. However, the detrital sources in Huizhou sag were simple because the Wenchang and Enping formation were deposited in the depression periods.