碳酸盐岩烃源岩不同热模拟方式下气体碳同位素演变特征

Carbon isotopic evolution of hydrocarbon gases generated from carbonate source rocks via different thermal simulation methods

我国海相烃源岩普遍处于高—过成熟阶段,现有的烃气碳同位素指标不能直接应用于判识海相碳酸盐岩天然气成因类型以及进行油气源对比。利用云南禄劝Ⅱ_(1)型低成熟海相碳酸盐岩烃源岩,开展了不同热模拟方式下的系列热解生烃实验,对收集的气体产物进行碳同位素分析。结果表明:①在整个热演化阶段,干酪根碳同位素值随成熟度变化不大,而甲、乙烷碳同位素值均随成熟度增加先变轻再变重,具有相似的演变特征,在油气生成的主要阶段,明显小于其母质干酪根碳同位素值,在过成熟阶段,乙烷碳同位素变重的趋势明显加快,甚至大于其母质干酪根的碳同位素值,呈现出“煤型气”特征,故单纯地采用甲、乙烷碳同位素值来判识天然气类型时需要慎重;②在成熟度相同时,半封闭—半开放体系模拟实验所得气体碳同位素值相比封闭体系模拟实验的要轻,这指示同一烃源岩排出烃气所形成的常规天然气藏,其烃气碳同位素值与滞留在源内的页岩气碳同位素值存在一定的差异,显示出似乎“不同源”的特征,在利用碳同位素模版或回归公式开展气源对比时也需要注意;③两种热模拟方式下、同一种烃源岩在全演化阶段,甲烷碳同位素值总是比乙烷的要小,这表明由单一烃源岩直接供气形成的常规天然气藏,不会发生甲、乙烷碳同位素的“倒转现象”。

Chinese marine source rocks are generally at the stage of high-to over-mature,there are still some uncertainties about whether the existing hydrocarbon isotopic indexes can be directly employed to distinguish the genetic types of marine carbonate natural gas and consequently,gas-oil source correlation can be carried out.In this paper,a series of pyrolysis experiments for hydrocarbon generation with different conditions were carried out using the low-maturity marine carbonate source rocks of LuquanⅡ1 type in Yunnan province to analyze the carbon isotope of the collected gas products.The results showed that:①During the entire thermal evolution stage,the carbon isotope values of kerogen did not vary significantly with the increasing of maturity,while the carbon isotope values of methane and ethane both decreased and then increased with the increase of maturity,showing similar evolution characteristics.In the main oil and gas generation stage,it is significantly smaller than the isotope value of its parent material kerogen.In the over-mature stage(VR o≥2.2%),the tendency of carbon isotope of ethane to become heavier is obviously accelerated,even greater than the carbon isotope value of its parent material kerogen, showing the characteristics of “coal-type gas”. This suggested that we need to be cautious when simply using the carbon isotope values of methane and ethane to identify the type of natural gas. ②At the same maturity, the carbon isotope values obtained from the pyrolysis experiment of semi-closed and semi-open system are lighter than those obtained from the pyrolysis experiment of closed system. This indicates that conventional natural gas reservoirs formed by the discharge of hydrocarbon gas from the same source rock have a certain difference between the carbon isotope values of hydrocarbon gas and the carbon isotope values of shale gas retained in the source, showing the characteristics of “different sources”. Attention should also be exercised to the use of carbon isotope templates or regression formulas to carry out gas-source comparisons. ③With the two pyrolysis methods, the carbon isotope value of methane is always lighter than that of ethane in the full evolution stage of the same source rock. This indicated that conventional natural gas reservoirs formed by direct gas supply from single set of source rock do not have “inversion” phenomenon of methane and ethane carbon isotopes.