Method and application of hydrogen isotope analysis of n-alkanes

The source rock and crude oil from the Shahejie Formation in the Dongying Sag of the Bohai Bay Basin were studied. Hydrogen isotopic compositions of n-alkanes from different sedimentary environments were determined using a combination of gas chromatograph, GC/TC interface and an isotope mass spectrometer. In the upper section of the fourth member of Shahejie Formation, the hydrogen isotopic composition of source rocks was heavy, ranging from -161‰ to -111‰. While the hydrogen isotopic composition of source rocks in the lower section of the third member of Shahejie Formation was relatively lighter, ranging from -186‰ to -134‰. The results suggested that from brackish water to saline water sedimentary environment, the hydrogen isotope value shifted toward heavier composition following the increasing of salinity in water. The difference of hydrogen isotopic composition was relatively smaller for samples with large maturity differences. The results indicated that the influence of maturity on hydrogen isotopes of a single hydrocarbon was relatively smaller in the range of the normal oil generation window, which was mainly related to the source of parent material and the sedimentary environment. The characteristics of hydrogen isotopic composition of n-alkanes could provide an important scientific basis for the correlation of oil and source rock and the identification of parent material source and paleosedimentary environment.