Geochemistry of saturated hydrocarbons and thermogenic hydrocarbon input in seabed sediments from the south central Laoshan uplift in South Yellow Sea Basin

The geochemical characteristics of saturated hydrocarbon in the surface sediments of the southern central Laoshan uplift in the South Yellow Sea Basin were studied by organic geochemistry method and the relationship between saturated hydrocarbon and deep oil and gas was analyzed.The n-alkane chromatograms of seabed sediments from 25 sites show two types of distribution:front peak and back peak in the south central Laoshan uplift of the South Yellow Sea Basin.Some of chromatograms exist distinct "UCM" bulges, which indicate that there may be theromgenic hydrocarbons imported into seabed sediments and suffer biodegradation in these sites.In m/z191 mass spectrogram, the hopane series of seabed sediments exhibit geological, biological and mixed configurations, in which Ts/Tm and C₃₁-litre hopane 22S/(22S+22R)indicate high thermal maturity of organic matter, also mean the importation of exogenous thermogenic hydrocarbons with high thermal maturity.The 20S/(20S+20R)-C₂₉ and αββ/(ααα+αββ)-C₂₉ steroids parameters of seabed sediments at some sites reflect that the maturity of organic matter has reached maturity, which is obviously deviated from the immature-low maturity characteristics of modern sediments, and also indicates the importation of exotic high maturity thermogenic hydrocarbons.The three main sources of organic matter including terrigenous organic matter, marine endogenous organic matter and thermogenic hydrocarbon input in seabed sediments in the south central Laoshan Uplift of the South Yellow Sea.The vertical seepage of deep reservoirs along boundary faults and unconformity in strata towards the seafloor is the main mechanism of thermogenic hydrocarbon input of seabed sediments.