Hydrocarbon potential and biomarker assemblages of Paleogene source rocks in the Cangdong sag, Bohai Bay Basin, China

The organic geochemistry of the Paleogene-aged second and first members of the Kongdian Formation (Ek₂ and Ek₁ ) and the third member of the Shahejie Formation (Es₃) in the Cangdong sag, Bohai Bay Basin, China, are characterized and the hydrocarbon generating potential is evaluated. The Ek₂ source rocks have a good to very good oil-generating potential, as indicated by the high abundance and good quality of organic matter within a mature stage. Although the Ek₁ mudstones have entered the oil window, they are organic lean and are dominantly composed of Type III kerogen. Accordingly, the Ek₁ mudstones are considered to be poor source rocks. The Es₃ member is thermally mature and contains several intervals that have high contents of good-quality organic matter (mixed Type I and II kerogens), suggesting that these intervals also have a good potential for oil-generation. Given this potential, new exploration plays sourced from the Es₃ member could be possible in the Cangdong sag. Biomarker distributions in the Ek₂ and Es₃ source rocks are both characterized by a dominance of medium- to high-molecular-weight n-alkanes (n-C₁₈ to n-C₂₉), a low sterane/hopane ratio, and a low diasterane abundance. The Ek₂ samples have low Pr/Ph ratios (<1.0), relatively high gammacerane/C₃₀ hopane ratios (>0.1) and β-carotane, trace amounts of C₃₀ 4α-methyl-24-ethlycholestanes, and a predominance of C₂₉ regular steranes. Collectively, these characteristics indicate that abundant prokaryotic biomass mixed with some terrigenous organic matter was deposited under reducing conditions within a stratified water column. In contrast, the Es₃ source rocks are characterized by variable Pr/Ph ratios, low gammacerane and β-carotane, comparable C₂₇ and C₂₉ regular steranes, and relatively high C₃₀ 4α-methyl-24-ethlycholestanes. These properties indicate that the Es₃ source rocks contain significant contributions from prokaryotic and algal sources, and were deposited under reducing to suboxic conditions within a relatively less stratified water column.