Geochemical features and sedimentary environment of terrestrial crude oils

Marine and terrestrial oils are formed in various types of sedimentary environments and their compositions are affected by the paleoclimate and the regional geological environment. Terrestrial oils formed in different environments show specific composition. Based on hydrocarbon composition, isotope composition, and biomarkers, terrestrial oils can be divided into four types and the detailed geochemical characteristics of each type have been studied. The following three main oil-forming environments are summarized: (1) humid and semi-humid climate, fresh-brackish water lacustrine; (2) arid and semi-arid climate, salinized water or salt lacustrine; (3) humid climate fresh water lacustrine-bog or river-marsh. C 27 -steranes and gammacerane are abundant in the second environment, but oleanane, C 29 -sterane, benzohopines, nor-benzohopane and sometimes 4-methyl steranes are important indicators of the third environment. We proposed a preliminary discrimination diagram: taking the 13 C value of crude oil as abscissa and logarithmic Pr/Ph ratio as ordinate, four oil-generating environments can be distinguished with this mode: (1) fresh-brackish water lake; (2) saline water lake; (3) river-marsh; (4) peat-bog and paralic swamp.