Characteristics And Applications Of Carbon And Hydrogen Isotopic Compositions Of Individual N-alkanes In Lower Palaeozoic Carbonate Rocks In The Huanghua Depression

The fact that the Lower Palaeozoic carbonate rocks in the Huanghua Depression have hydrocarbon potential and can generate commercial reservoir has been proved by many years' exploration practice. However, its high maturity-over maturity has enormous influence on the use of conventional biomarkers. In this paper, the characteristics of n-alkanes in bitumen A and bitumen C of the Lower Palaeozoic carbonate rocks from the Huanghua Depression, including biomarkers, carbon and hydrogen isotopic compositions, were determined to reveal the main characteristics of the early organic matter, to restore the paleoenvironment and provide basis for potential indigenous oils and gases in the Lower Palaeozoic.The organic geochemical analysis of n-alkanes in bitumen A was carried out. The results show that the carbonates with high maturity and over maturity were deposited in a weakly oxidizing-weakly reducing environment, and its organic matter is mainly of bacterial and algae bio-input. In addition, the study of the free carbon and hydrogen isotopes in n-alkane indicates that there are three different sedimentary environments and some samples are infected by organic compounds of terrestrial source rocks. It also shows that hydrogen isotope cannot match carbon isotope at identifying marine subfacies for the reason that it is dramatically influenced by theδD of environmental water. However, hydrogen isotope has an incomparable advantage over carbon isotope in distinguishing marine environment from terrestrial environment.Compared with bitumen A, the study about the organic geochemical characteristics of n-alkanes in bitumen C indicates that its organic matter is mainly of bacterial, algae and zooplankton bio-input. They have similar environment but bitumen C has lower maturity than bitumen A. Furthermore, the carbon and hydrogen isotopic compositions of individual n-alkanes in bitumen C are also detected. The results show that most samples have the same distribution pattern, which proves that the carbon isotopic compositions of individual n-alkanes in bitumen C is not dramatically affected by geologic functions after the diagenesis. Meanwhile, it is suggested that the hydrogen isotope property of n-alkane elements in bitumen C depends mostly on the sedimentary environment and the nature of organic matter. TheδD of some samples has marked even/odd predominance in the low molecular weight n-alkane range. This indicates that their organic matter maybe from different original bodies. The results above show that the main characteristics of the early organic matter are still reserved in bitumen C by reason of the crystal-enclosed function. Bitumen C has lower maturity than bitumen A, and its carbon and hydrogen isotope properties of n-alkane elements can be used to recover paleoenvironment and identify potential indigenous oils and gases in the Lower Palaeozoic carbonate rocks in the Huanghua Depression.