The Geochemical Characteristics and Oil Source Correlation of Chang 8 Crude Oil in Huanxi Area, Ordos Basin

In recent years, breakthroughs have been made in the Chang-8 member of the western part of the Tianhuan Depression in the Ordos Basin. The unclear understanding of the oil source of Chang8 crude oil in the Huanxi region has constrained subsequent oil exploration. To solve this problem, the characteristics and sources of crude oil were studied by physical property analysis, crude oil chromatography and crude oil chromatography-mass spectrometry. The analysis results show that the crude oil is light or medium quality. The main peak carbon of the total hydrocarbon chromatogram of crude oil is C14 to C15 with odd-even dominance value (OEP) ranging from 0.94 to 1.09, pristine/phytane (Pr/Ph) ranging from 1.16 to 1.14, and C21−/C21+ranging from 2.00 to 3.73. The distribution and main parameters of steroterpane biomarkers are characterized by low relative abundance of rearranged hopanes, low gammacerane, low C30rearranged hopanes/C30 hopanes (C30*/C30) ratio (0.07-0.33), ααα- 20R configuration steranes are distributed in a “V” shape. The crude oil parent material comes from aquatic organisms and is formed in a fresh water, weak reduction and weak oxidation sedimentary environment. The oil source comparison shows that the Chang 8 crude oil in the Huanxi area comes from the Chang 7 black shale inside the Basin. The Huanxi area is adjacent to the high-quality source rock in the eastern lake basin. It is speculated that the oil generated from the Chang 7 black shale in the accumulation period is driven by the hydrocarbon generation pressurization, and is laterally channeled along the fault, unconformity surface and sand body, and accumulates in favorable positions to form reservoirs. The Chang 8 oil-bearing formation in the Huanxi area shows good potential for reservoir formation and exploration.