Simulating Experiments Combined With GC-IRMS: Application To Effective Source Rock Research

It is generally accepted that source rock is a key factor controlling the formation of oil and gas reservoirs. A conception that effective source rock is dynamic is proposed in this paper in order to study a specific oil/gas reservoir. With the extent of investigation increasing, some static parameters or indexes shouldn't be trusted without limit, and also shouldn't be simply applied to the oil/source and gas/source correlation, as well as the evaluation of effective source rock. In contrast, a perfect target is that the whole process of hydrocarbon generation will be accurately described through setting up a set of mathematical models. Thus, combined with an actual geological background, the identification and evaluation of effective source rock can be achieved quantitatively. This study is being an attempt advancing towards the above-mentioned destination.Based on the actual geological cases, three different pyrolysis system were selected in the study. Then the carbon isotopic variations of individual hydrocarbons from pyrolysates were revealed by the simulating experiments and the GC-IRMS (gas chromatography-isotope ratio mass spectrometry) analysis. The results derived from the experiments provide a lot of important evidences for the oil (gas)/source correlation and the identification and evaluation of effective source rock. Main conclusions are summarized as follows: 1) Simulating Experiment on Hydrocarbon Generation and Expulsion of Source Rocks from the Liaohe oilfield shows that hydrocarbon expulsion has no considerable effect on the carbon isotopic composition of the liquid n-alkanes. This provides an experimental evidence for the application of the liquid hydrocarbon 6 I3C in the oil/source correlation. During the early hydrocarbon generation, the isotopic compositions of liquid hydrocarbons have no obvious variation with increasing temperature, therefore, they can be used into the oil/oil and oil/source rock correlations. However, at the peak of hydrocarbon generation, the second cracking of those heavy hydrocarbons formed at the early stage, such as asphaltene, NSO fraction, and n-alkanes with high carbon number,leads to the remaining n-alkanes markedly riched 13C in carbon isotope, ranging from l%o to 4%o. The variation should be considered in the oil/oil and oil/source correlations. 2) Three potential source rocks from the Ying-Qiong basins were pyrolyzed at different temperatures. The carbon isotopic composition of individual light hydrocarbons from pyrolysates indicate that, for the natural gases from the Ying-Qiong basins, it is difficult to further identify the origin of gas in terms of carbon isotope data alone. Light hydrocarbons from pyrolysates can be divided into two main generation stages: at the lower temperature stage, light hydrocarbons are formed directly from primary cracking of kerogen, and their carbon isotopic distributions accord with isotope kinetic fractionation; at the higher temperature stage, isotopic distributions of light hydrocarbons become more complex due to the mixing of light hydrocarbons with different generation mechanisms, e.g. secondary cracking of heavy hydrocarbons. In addition, experimental results also prove that an "accumulative" effect may be one of major factors that cause the part reversal in carbon isotope ratios among light hydrocarbon components from the Ying-Qiong basins. 3) Kinetic simulating experiment combined with GC-IRMS analysis indicates that the experimental results can be reliably extrapolated to geological conditions through the kinetic parameters derived from the experiment. On the one hand, the kinetic simulating experiment is usually applied to obtain the kinetic parameters of hydrocarbon generation, and further to recover the hydrocarbon generating history of potential source rock. So it can provide a quantitative evaluation for effective source rock. On the other hand, effective source rock will be identified in terms of the relationship between the isotopic composition (5l3Ci) of methane in natural gases and methane conversion ra...