The Research On Cand H Isotopic Compositions Of N-Alkanes In Crude Oils And Extracts Of The Upper Cretaceous From The Southern Songliao Basin

Paleoclimate and paleoenvironment have remarkable influences on the development of source rocks, so it is significant to reveal the favorable depositional paleoenvironment of excellent source rocks for evaluating source rocks and forecasting their distributions. Biomarker is a main research aspect in organic geochemistry, and plays an important role on reconstructing the paleoclimate and depositional paleoenvironment of organic matter accumulation. With the development of new techniques and methods, carbon and hydrogen isotopic compositions of individual compounds become a more and more important field in organic geochemistry. At present, the researches on carbon isotopic compositions of individual compounds are more, and relatively mature. In contrast, determining hydrogen isotopic compositions of individual compounds is a new technique in oil/gas geochemistry, less reports and more controversies although it has obvious predominance on some aspects, such as identifying the source of organic matter, revealing the depositional environment and the interaction between formation water and rocks. Therefore, it is necessary to develop a great number of basic work on hydrogen isotopic composition of individual compounds. This study is an attempt to apply the new tool into oil and gas geochemistry on the basis of the information of biomarkers and carbon isotopic compositions.The Songliao Basin is one of most important petroliferous basin in China. It is significant for further oil/gas exploration in the Songliao basin periphery to ascertain whether marine transgressional event is a control factor on development of the source rocks in this area. In this paper, biomarkers, n-alkanes carbon and hydrogen isotopic compositions of the upper Cretaceous crude oils and the extracts of potential source rocks from the southern Songliao basin have been determined by GC-MS (gas chromatography mass spectrometry), GC-C-IRMS (gas chromatography combustionisotope ratio mass spectrometry) and GC-TC-IRMS (gas chromatography - thermal conversion - isotope ratio mass spectrometry) respectively. The investigation indicates that: 1) Semi deep-deep lacustrine is the favorable paleoenvironment for the excellent source rock development in the Songliao basin based on the biomarker distributions and carbon isotopic compositions of individual n-alkanes. 2) Effect of thermal maturity on 8D values of individual n-alkanes in source rock extracts is not significant during the stages from immaturity to early maturity (Ro<0.8%), and it can't conceal the differences from source of organic matter and depositional environment. So hydrogen isotopic compositions can be used as an useful auxiliary tool for making the oil/source correlation and reconstructing the depositional paleoenvironment. 3) The hydrogen isotopic signatures of H-alkanes in source rock extracts from the Qingshankou and Nenjiang formations of the Upper Cretaceous in the southern Songliao basin display obvious characteristics of terrestrial organic matter. Relatively Large variations of ?-alkanes hydrogen isotopic compositions occur among source rock extracts of Nenjiang formation, which is possibly related to a great change of depositional water body. More work need to be carried out in order to testify the relationship between the marine transgressional event and the hydrogen isotopic composition variation for Nenjiang formation. The carbon and hydrogen isotopic compositions of w-alkanes in source rock extracts from the Qingshankou formation can't appear to surrpport the existence of marine transgressional event, in other words, the southern Songliao Basin had not probably been affected by marine transgressional event during the period of the Qingshankou formation deposition. 4) Chemicai degradations of kerogen can release abundant dissoluble organic matter from the kerogen great molecule, which can reliably reveal the paleoenvironment of organic matter accumulation. Therefore, it is significant to further discuss the source of organic matter and reconstruct the depositional paleoenvironment.