Oil-source rock correlation for the Chang-8 oil reservoirs in the Yanchang Formation, Fuxian region, Ordos Basin

Geochemical correlation of the crude oil and source rocks from the Chang-8 to the Chang-9 oil reservoirs in the Yanchang Formation, Fuxian region, Ordos Basin indicates that there occur the Chang-7 and Chang-9 effective source rocks with higher organic matter abundances. The organic matter in the Chang-7 source rocks belongs to the I-II₁ types, and that in the Chang-9 source rocks belongs to the II₁-II₂ types, both of which have higher Ts contents than Tm contents, suggesting a mature stage of the source rocks. In the Chang-7 source rocks, C₃₀ hopane abundances in pentacycloterpane are larger than C₂₉ hopane abundances and the C₃₀RH/C₂₉Ts ratios <2, indicating a reduction environment of the Chang-7 source rocks. In the Chang-9 source rocks, a general similarity in C₃₀ hopane abundances and C₂₉ hopane abundances and C₃₀RH/C₂₉ Ts ratios >2 indicates the weak oxidation-weak reduction environments of the Chang-9 source rocks. The saturated hydrocarbons/aromatic hydrocarbons ratios are lower in the Chang-6 source rocks than those in the Chang-8 source rocks, indicating different sources of the crude oil from the Chang-6 and Chang-8 source rocks. As for the biomarkers, the C₂₇-C₂₈-C₂₉ αααR distribution patterns of regular steranes in the Chang-7 source rocks display the "L" type, those in the Chang-9 source rocks display the "V" or inverse "L" types, and those in the Chang-8 source rocks display the "V" type. The crude oil in the Chang-8 oil reservoirs mainly stemmed from the Chang-9 source rocks in the study area.