Change in hydrogen isotope composition of n-alkanes, pristane, phytane, andaromatic hydrocarbons in Miocene siliceous mudstones with increasing maturity

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The variation in the hydrogen isotope composition (D/H ratio values) of n-alkanes, pristane, phytane, and aromatic hydrocarbons with increasing maturity was investigated using a set of immature to mature Miocene siliceous mudstones. The D/H values of aromatic hydrocarbons change at the earliest stage (vitrinite reflectance, Ro = 0.4–0.5%), by way of higher values in immature rocks. A dramatic increase in values is observed for pristane and phytane during the mature stage (Ro = 0.6–0.7%), attributable to hydrogen exchange with sediment porewater. The decrease in the values of short chain n-alkanes and the disappearance of carbon number predominance in n-alkane D/H values with increasing maturity is a result of the thermal cracking of aliphatic structures in bitumen and kerogen. The long chain n-alkanes derived from higher plant wax can maintain their initial values, even in the zone of extensive oil generation (Ro < 0.7%).