内蒙古西部额济纳旗及邻区石炭系—二叠系烃源岩干酪根C同位素的特征和影响因素

Kerogen carbon isotopic features and influencing factors of CarboniferousPermian hydrocarbon source rocks in Ejina Banner and its vicinities,western Inner Mongolia,China

许多研究者认为,烃源岩干酪根C同位素组成主要受干酪根类型的影响,可以作为评价烃源岩类型、沉积环境和进行油气源对比的重要指标,并得到广泛应用。对额济纳旗及邻区典型剖面石炭系—二叠系烃源岩有机碳(TOC)的丰度、地球化学特征、热演化特征和干酪根C同位素分布的研究表明,研究区石炭系—二叠系烃源岩为浅海陆棚相沉积环境形成的泥质岩,TOC含量中等,以Ⅱ类干酪根为主,烃源岩演化进入成熟-过成熟阶段,干酪根C同位素具有显著偏重的特点。并且干酪根C同位素组成明显与热演化程度和有机碳丰度有关,随着烃源岩演化程度的不断提高,重碳同位素不断富集,在成熟—过成熟阶段,对C同位素的影响可达4‰～6‰。随着TOC含量的增加,干酪根C同位素显著偏轻,影响值可达4‰以上。由于研究区烃源岩已进入成熟—过成熟阶段,干酪根C同位素的分布在演化过程中已经发生较大变化,干酪根类型的影响相对降低或已不显著,不能再作为评价烃源岩干酪根类型的指标。

Many researchs indicate that the carbon isotopic composition of kerogen in hydrocarbon source rocks is mainly affected by kerogen types, and the composition can be used as an important evaluation index of hydrocarbon source rocks types, sedimentary en- vironment and oil and gas source correlation. This method has been widely applied. Through the research of organic carbon abundance, geochemical characteristics, thermal evolution characteristics and carbon isotopic composition of kerogen in Carboniferous- Perrrdan hydrocarbon source rocks of typica/sections in Ejina Banner and its vicinities, it is showed that the Carboniferous- Permian hydrocarbon source rocks in the area are the mud rocks which formed in shallow marine shelf facies, and the source rocks have the characteristics of middle organic carbon, type II kerogen and mature-overmature evolution stage. The carbon isotopic composition of kerogen is remarkable heavier, and the distribution of carbon isotopic composition of kerogen is mainly affected by the thermal evolution and organic carbon abundance. The heavy carbon isotope is continuously concentrated, and the effects of carbon isotope is 4‰- 6‰ during the mature-overmature stage. With the increase of organic carbon, the kerogen carbon isotopic composition is remarkable lighter, and the influence value can reach more than 4%0. Because the source rocks have already entered into the mature-overmature stage in this study area, the kerogen carbon isotopic had already been greatly changed. The effect of kerogen types is relatively reduced, so the kerogen carbon isotopic composition can not be used as the evaluation index of source rocks kerogen types.