低TOC碳酸盐岩在高-过成熟阶段的生气潜力与乙烷δ13C研究

鄂尔多斯盆地奥陶系马家沟组碳酸盐岩有机碳含量多偏低，热演化程度高，奥陶系风化壳储层天然气乙烷δ13C既有油型气也有煤成气特征，使鄂尔多斯盆地中部大气田气源和成因确定，一直困扰着奥陶系天然气勘探。依据低TOC烃源岩热解生烃实验和热模拟实验高温下乙烷δ13C变重的启示，本项目拟以低TOC含量的低成熟-成熟海相碳酸盐岩为研究对象，对原始样品和抽提可溶有机质后的样品，进行高温条件下的温压共控、热解-色谱-质谱和热解-色谱-同位素质谱生气热模拟实验及相关分析，研究低TOC含量碳酸盐岩在高-过成熟热演化阶段的生气量，评价其生气潜力；阐明不同TOC含量碳酸盐岩在高-过成熟热演化阶段的主控生气母质（干酪根或可溶有机质）；明确乙烷δ13C与母质和热演化程度的关系。本研究对认识低TOC含量碳酸盐岩在高-过成熟热演化阶段是否可成为有效气源岩具有理论意义，对推动鄂尔多斯盆地奥陶系天然气勘探具有现实意义和应用价值。

The natural gas exploration of the Ordovician paleo-weathering crust of the giant gas fields, Ordos Basin, is perplexed by the key issue of determination of the gas source and generation. This issue resulted from two principle factors, one is that with the totally low carbon contents and high maturation, the capacity of carbonate rocks of the Ordovician Majiagou Formation to generate giant gas fields were questioned, the other is that the ethane δ13C value of the gas are partly featured by oil-type and partly featured by coal-type gases. Based on the earlier study of hydrocarbon generation pyrolysis experiment on low TOC source rocks, which showed that low TOC source rocks mainly generate short chain alkanes without alkenes, while high TOC source rocks mainly generate long chain alkanes as well as alkenes at 450℃, we think that liquid hydrocarbons generated by low TOC source rocks at mature stage are not sufficient to discharge and preserved in source rocks, and at higher maturation stage, gas generation from kerogen come into end while gas generation from liquid hydrocarbons start. Thermal simulation experiments with sapropelic source rocks and oils from other researchers showed that ethane δ13C value become heavier than -28‰ at temperature over 500℃. Therefore, we consider that low TOC carbonate rocks of the Ordovician Majiagou Formation could be effective gas source rocks. Based on the above consideration, this item intend to target the low TOC and low-mature to mature carbonate rocks, to make a series of thermal simulation experiments at higher temperature on original and abstracted carbonate rocks, respectively. The experiments include the temperature and pressure joint controlled thermal simulation experiments, the thermal pyrolysis/gas chromatography/mass spectrometry experiments, and thermal pyrolysis/gas chromatography/isotope spectrometry experiments. The destination of this item are to estimate the gas generation at high to over mature period and to evaluate the gas producing potential of low TOC carbonate rocks; to expound the master organic matter (kerogen or dissolved organic matter) of carbonate rocks with different carbon contents at high to over mature period; to clear and definite the relationship between ethane δ13C value and its sources (kerogen or dissolved organic matter), and ethane δ13C value and thermal evolution level. To put this item into effect is theoretically significant for understanding whether low TOC carbonate rocks prove to be effective gas source rock or not at high to over mature period, and have practical value for propelling natural gas exploration of the Ordovician Majiagou Formation in Ordos Basin.