Geochemical Characteristics of Crude Oil in Jigeda Sag of Yingen–Ejin Basin and Its Significances

There is a dispute on the oil–source of Carboniferous and Permian system in Yingen–Ejin basin, which restricts the process of oil and gas exploration in this area to a certain extent. In order to clarify the oil origin and source and provide basis for oil & gas exploration in this area, research of the basic physical properties and geochemical characteristics of the crude oil from well Mengecan 3 in Jigeda Sag of Juyanhai depression was conducted. The results show that the physical properties of the crude oil in the Variscan intrusive rocks are basically the same as those in the dolomite sandstones of the Permian system for well Mengecan 3. The oil is low–density, low–sulfur and normal medium crude oil, composition of which is characterized as high saturated hydrocarbon content, low non–hydrocarbon and asphaltenes content, and high ratio of saturated to aromatic hydrocarbons, and the saturated to aromatic hydrocarbons ratio of the lower crude oil is much higher than that of the upper crude oil. The gas chromatography characteristics of saturated hydrocarbon, carbon isotope characteristics and biomarkers of the crude oil in the Variscan intrusive rocks and Permian sandstones of Jigeda Sag have both similarities and differences, As the aforementioned, the results indicate that the parent materials of hydrocarbon were derived from reducing and high paleo–salinity conditions in the marine, and phytoplankton was the main source, while the proportion of terrestrial higher plants is higher for the oil source in Permian sandstone. Oil source comparison shows that the characteristics of biomarkers of Variscan intrusive rocks and Permian sandstone crude oil in well Mengecan 3 are significantly different from those of cretaceous Bayingebi formation, and it is closely related to the Carboniferous–Permian source rocks. The crude oil from Permian sandstones show favorable correlations with the Permian source rocks, while the crude oil from Variscan intrusive rocks show more closely correlations with the source rock of upper Ganquan formation. The discovery of Carboniferous Permian oil source has expanded the new series of oil and gas exploration in Yingen–Ejin basin, and has a good guiding role in oil and gas exploration of Paleozoic in North China.