GEOCHEMICAL CHARACTERISTICS AND ORIGIN OF INCLUSIONS AND ADSORBED HYDROCARBONS IN YAOJIA FORMATION SANDSTONES IN QIANJIADIAN AREA, THE KAILU BASIN

Inclusion hydrocarbons and adsorbed hydrocarbons were observed under microscope and UV fluorescence in Lower Cretaceous Yaojia Formation sandstones in Qianjiadian area,the Kailu Basin.Subsequently,these sandstone samples were washed,crushed,extracted for hydrocarbons and analyzed by GC and GC-MS.The results showed that ratios of C₂₉ ααα sterane 20S/(20S+20R) and C₃₁ αβ hopane 22S/(22S+22R) of the hydrocarbons were closed to respective equilibrium values,suggesting that the hydrocarbons were marginally mature to mature.Among C₂₇,C₂₈ and C₂₉ ααα20R steranes,C₂₇>C₂₈29.Ratios of C₂₅/C₂₆ tricyclic terpane and regular steranes/17αC_29-33 hopanes were relatively low.The values together with those of gammacerane index and homohopane index consistently indicated that the inclusion oils and adsorbed hydrocarbons were derived from humic-sapropel type organic matter under poor reducing freshwater to semi-saline environment.The features were similar to those of organic matter extracted from the Upper Jurassic Jiufotang Formation source rocks in the Kailu Basin.Thus,it could be concluded that the inclusion oils were mainly derived from the Upper Jurassic Jiufotang Formation source rocks of lacustrine facies.