Geochemical Characteristics and Source of Hydrocarbon Inclusions in Zhiluo Formation Sandstone in Dongsheng Area, Northern Ordos Basin

Key words: 
• hydrocarbon inclusions /
• GCMS /
• biomarker /
• source rock /
• Ordos basin

Abstract: First of all, the researchers observed and described hydrocarbon inclusions in Middle Jurassic Zhiluo Formation sandstones in Dongsheng area, northern Ordos basin under microscope and UV fluorescence, and then, carefully selected the sandstone samples containing hydrocarbon inclusions with high abundance. Subsequently, these samples were washed, crushed, extracted for hydrocarbons and analyzed for GC and GCMS and compared with oils extracted from Cretaceous sandstone, Triassic sandstone and source rock, respectively. The results show that C29 ααα sterane 20S/(20S+20R) and C32 αβ hopane 22S/(22S+22R) ratios of the inclusion oils are close to respective equilibrium values. Vitrinite reflectance values were calculated based on methylphenanthrene index(MPI) and have values from 0.64％ to 0.82％, suggesting that the inclusion oils are near mature to mature. Among C27, C28 and C29 ααα20R steranes, C27>C28<C29. C25/C26 tricyclic terpane and regular steranes/17αC29～33 hopanes ratios are relatively low. The values together with those of gammacerane index and homohopane index consistently indicate that the inclusion oils were derived from humicsapropel type organic matter under poor reducing freshwater to semisaline environment. The features are similar to those of organic matter extracted from Triassic sandstone and source rock, but are different from that of Cretaceous sandstone. Thus, it can be concluded that the inclusion oils were mainly derived from Triassic lacustrine facies source rock. 