Biomarker characteristics of Lower Paleozoic source rocks in Chezhen Sag, Bohai Bay Basin

The biomarkers of the Lower Paleozoic carbonate rocks in the Chenzhen Sag of Bohai Bay Basin were stu-died using GS-MS. Their sedimentary environment, organic matter origin and thermal evolution degree were discussed, and compared them with the Eogene source rocks. The normal alkanes showed a single-peak distribution, with the main peak carbon varying between n-C₁₆ and n-C₁₈. Odd/even carbon number preference was not obvious, and Pr/Ph ratio ranged from 0.37 to 1.2, showing a weak phytane dominance or a balance between pristane and phytane. Hopane was the main terpane compound, with low tricyclic terpanes, high tetracyclic terpane, normal homohopane and low gammacerane contents. The C₂₇, C₂₈, C₂₉ regular steranes showed an asymmetrical "V" type distribution, with low rearranged sterane and high 4-methyl rearranged sterane contents. Phenanthrene and tris-fluorene series were the main aromatic compounds, lack of biphenyl and other higher plant imported biomarkers. The naphthalene series was abundant while 1,2,5-trimethylnaphthalene and 1,2,5,6-tetramethylnaphthalene had a low abundance. Dibenzothiophenes were rich and dibenzofurans were poor. Aromatic steranes were mainly triaryl steranes, and mono-aromatic steranes were absent. These biomarkers indicated that the Lower Paleozoic source rocks were deposited in a reductive shallow sea environment, mainly originated from fungus, algae and lower aquatic plankton, and were highly mature and over-mature. Four saturated hydrocarbon biomarkers and five aromatic hydrocarbon biomarkers were chosen to distinguish source rocks in the Lower Paleozoic and the Eogene, which provided evidence for oil and source correlation.