Saturated hydrocarbon geochemical characteristics of the oil sand from YD1 well in Jinyang Basin and its significance for oil and gas exploration

Recently researchers from Shenyang Geological Survey of CGS made several new discoveries in geological well YD1 in Jinglingsi-Yangshan basin (Jinyang Basin). During the drilling of YD1 the authors detected several oil and gas showings in the fractures of the drilling core in Mid-Jurassic Tiaojishan Formation (J₂t) which was mainly composed of intermediate igneous rocks. The characteristics of biomarkers from YD1 oil sand were analyzed by the method of gas chromatography and gas chromatography mass spectrometry (GC, GC-MS). The results show that the two oil sand samples have similar geochemical characteristics:the n-alkane distribution exhibits one sole peak pattern, and the dominant peaks are nC₂₁ and nC₂₂ separately; there is no clear odd or even carbon number predominance for the n-alkanes, and thus the carbon preference index (CPI) is only slightly higher than 1; the samples have low Pr/Ph ratios. The terpene is characterized by the normal distribution of C₂₃ tricyclic terpene and similar abundance of C₂₄ tetracy-clic terpene and C₂₆ tricyclic terpene, with a relatively abundant value of gammacerane. The regular steranes are dominated by the C₂₇-steranes, and their relative abundance is in the order of C₂₇ > C₂₉ > C₂₈, exhibiting the "V" distribution pattern. These biomarkers' characteristics indicate that the source rocks are matured and predominated mainly by planktonic/algal organic matter. The source rocks were deposited in a lacustrine environment with a high-salinity stratified water column and anoxic conditions. The oil-source correlation results showed that the crude oil from the samples was possibly derived from the source rock in Early Jurassic Beipiao Formation (J₁b). The results indicate that the source rock from Beipiao Formation is widely spread in Jinyang Basin and has good oil and gas potential, and hence Jinyang Basin is a petroliferous basin with good potential for oil and gas exploration.