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马五段是鄂尔多斯盆地中部气田下奥陶统主力气源岩中的主力生烃层段。通过对比４口井２３个样品下奥陶统马五段气源岩氯仿沥青“Ａ”、氯仿沥青“Ｃ”含量，族组成及饱和烃馏分生成物标记化合物地化特征，认为该段气源岩氯仿沥青“Ａ”和氯仿沥青“Ｃ”具同源性，但前者成熟度略高于后者，前者含量及其总烃含量随样品有机碳值增高而增大；相反，后者含量及其总烃含量随样品有机碳值增大而减小；二者的沥青转化率和烃转化率都随ＴＯＣ值增大而变小。根据二者含量、总烃含量以及沥青转化率和烃转化率与样品有机碳关系的综合研究，提出马五段碳酸盐岩气源岩有机碳含量下限值可确定为０．０４％。最后认为，评价高、过成熟阶段碳酸盐岩大中型气田气源岩不仅要依据有机质丰度的高低，氯仿沥青“Ａ”的数量和质量，还必须考虑到氯仿沥青“Ｃ”的数量和质量，此外还要结合沥青转化率和烃转化率加以综合判识。
The Ma-5 Member is the main hydrocarbon generating layer in the main source rocks of the Lower Ordovician in the Ordos Basin gas field. By comparing the geochemical characteristics of the “A”, “C” content of chloroform bitumen and the composition of the labeled compounds in the hydrocarbon composition of the saturated hydrocarbon fraction with 23 samples of the Lower Ordovician from the 4 samples in the Lower Ordovician, However, the former has a slightly higher degree of maturity than the latter, and the former and the latter increase with the increase of organic carbon in the samples. On the contrary, the content of the latter and The total hydrocarbon content decreases with the increase of organic carbon value of the sample. Both the asphalt conversion and the hydrocarbon conversion decrease with increasing TOC. According to the comprehensive study of the content of both, total hydrocarbon content, bitumen conversion and hydrocarbon conversion, and the relationship between the organic carbon content and the sample, the lower limit of organic carbon content in the source rocks of the Ma5 Member carbonate gas source can be determined to be 0.04%. Finally, it is concluded that the gas source rocks in high and medium mature carbonate rock gas fields not only depend on the abundance of organic matter and the quantity and quality of chloroform bitumen “A” but also the amount of “C” Quality, in addition to be combined with asphalt conversion and hydrocarbon conversion to be comprehensively identified.