Distribution and isotopic compositions of phenanthrene and methylphenanthrenes in crude oils: Implications for source input delineation and grouping oil families

This study aimed to identify the origins of crude oils by analyzing the delta C-13 values of phenanthrene (Phen) and methylphenanthrenes (9-MP and 1-MP) in some selected oils from the Tarim Basin (NW China), Termit Basin (Niger), Bongor Basin (Chad), and Fushan Depression (South China Sea). The study showed that oils with low 9-MP/1-MP ratios (<2.0), phenanthrene (Phen) delta C-13 values below -30.0 parts per thousand, and Delta C-13((9MP-Phen)) and Delta C-13((1MP-Phen)) values above zero are marine-originated oils, while oils with high 9-MP/1-MP ratios (>2.0), phenanthrene (Phen) delta C-13 values above -20.0 parts per thousand, and Delta C-13((9MP-Phen)) and Delta C-13((1MP-Phen)) values below zero are terrestrial-originated oils. Oil samples with high 9-MP/1-MP ratios (>2.0), phenanthrene (Phen) delta C-13 values ranging from -30.0 parts per thousand to -20.0 parts per thousand, and Delta C-13((9MP-Phen)) and Delta C-13((1MP-Phen)) plots below zero are mixed-originated oils with higher terrestrial contribution. In contrast, oils with low 9-MP/1-MP ratios, phenanthrene (Phen) delta C-13 values ranging from -30.0 parts per thousand to -20.0 parts per thousand, and plotting above and below Delta C-13((9-MP-Phen)) and Delta C-13((1-MP-Phen)) vice versa, are mixed-originated oils with higher contribution from marine sources.In combination with the geological settings, the results indicated that the oils from the Tarim Basin have three distinct origins. The majority of the oils are marine-sourced oils derived from the Lower-Paleozoic marine carbonate rocks. Oil samples *T2, *T3, and *T4 are mixed-originated oils with higher terrestrial contribution generated from the Mesozoic lacustrine shales/mudstones in the Tarim Basin. The oil samples *T1 (XH1) and *T9 (S94) are from a mixed origin with higher marine material, are derived from Upper-Paleozoic marine shale source rocks in the Tarim Basin. In Termit Basin, most of the selected oils are mixed oils with higher marine organism contribution from YSQ(1) and YSQ(2) source rocks from the Mesozoic Yogou Shale Formations, except for *T27 oil sample (Douwani-1), which is a mixed oil with higher terrestrial contribution from YSQ(3) source rocks of the Mesozoic Yogou Shale Formations. As for the selected oils from Bongor Basin and Fushan Depression, they are mixed oils with higher contributions from aquatic organisms generated from the Mesozoic and Cenozoic lacustrine shale rocks, respectively.This approach allowed to identify for the first time the mixed origin with higher contribution of marine organic matter of the oil samples *T1 (XH1) and *T9 (S94) from the Tarim Basin, and the mixed origin with higher contribution of terrestrial organic matter of the oil sample *T27 (Douwani-1) from the Termit Basin.