Abstract: The marine crude oil in Tarim Basin is considered to be mainly originated from two sets of source rocks, i.e., Cambrian-Lower Ordovician and Middle-Upper Ordovician, but which is a dominant contributor has been in dispute for a long term. Based on previous research works, through a comprehensive study based on the comparison of biomarkers, fraction carbon isotopes, n-alkane carbon isotopes and the mass balance simulation of carbon isotope, the marine crude oil in Tabei area is confirmed to be mainly contributed by the Middle-Upper Ordovician source rocks (up to 88%), and different regions make different contributions. However, confirmed by current drillings, the high-abundance Middle-Upper Ordovician source rocks show limited distribution, based on which it is difficult to explain the scale of hydrocarbon resource from the giant marine oil field widely distributed in Tabei area. Based on the geological and geochemical evidences including the kerogen elements changing with burial depth, the variation of hydrocarbon generation potential and the paleo-TOC recovery, it is validated that a large scale of hydrocarbon generation and expulsion have occurred in the deep present-day low-TOC source rocks of the platform area, Tarim Basin. In addition, the effective present-day low-TOC source rocks are comprehensively identified according to the hydrocarbon expulsion threshold, source rock geochemistry and logging features. On this basis, the hydrocarbon-generation potential method is adopted to evaluate the hydrocarbon expulsion quantity in the Middle-Upper Ordovician high-abundance and effective low-abundance source rocks of various accumulation systems in the platform area, Tarim Basin. The hydrocarbon expulsion quantity of Middle-Upper Ordovician low-abundance source rocks in the platform area is 494.4×10⁸t, and that from the high-abundance source rocks is 1 353×10⁸t. The relative contribution ratios of high-abundance and low-abundance source rocks are 73.2% and 26.8% respectively. Among them, the hydrocarbon expulsion quantity of Middle-Upper Ordovician source rocks in Lunnan-Yingmaili accumulation system, Tabei uplift is 327.7×10⁸t, and the relative contribution ratios of high-abundance and low-abundance source rocks are 59.8% and 40.2% respectively, indicating that the low-TOC source rocks make an important contribution to the formation of giant marine oilfield discovered in Tarim Basin.

Key words: Tarim Basin, marine crude oil, relative contribution, present-day low-TOC source rock, Middle-Upper Ordovicion, hydrocarbon accumulation

摘要： 塔里木盆地海相原油被认为主要来自寒武系-下奥陶统和中-上奥陶统2套烃源岩，但何者为主长期以来存在争议。在前人研究工作的基础上，通过生物标志物对比、馏分碳同位素对比、单体烃同位素对比和碳同位素质量平衡模拟的综合研究，证实了塔北地区海相原油以中-上奥陶统烃源岩贡献为主，最大可达88%，不同地区贡献不同。然而，目前钻井证实的高丰度中-上奥陶统烃源岩分布局限，难以解释塔北地区已发现的广泛分布的特大型海相油田油气资源规模。通过干酪根元素随埋深变化、生烃潜力变化、古TOC恢复等多方面地质地球化学证据证实，塔里木盆地台盆区深部低TOC烃源岩发生过大量生排烃，并根据排烃门限、烃源岩地球化学和测井特征综合判识了有效低丰度烃源岩。在此基础上，采用生烃潜力法评价了塔里木盆地台盆区各成藏体系中-上奥陶统高丰度和有效低丰度烃源岩的排烃量。台盆区中-上奥陶统低丰度烃源岩排烃量为494.4×10⁸t，高丰度烃源岩排烃量为1 353×10⁸t，高、低丰度烃源岩的相对贡献比率分别为73.2%和26.8%。其中，塔北隆起轮南-英买力成藏体系中-上奥陶统烃源岩排烃量为327.7×10⁸t，高、低丰度烃源岩的相对贡献比率分别为59.8%和40.2%，表明低TOC烃源岩对塔里木盆地发现的特大型海相油田的形成具有重要贡献。

关键词: 塔里木盆地, 海相原油, 相对贡献, 低丰度烃源岩, 中-上奥陶统, 油气成藏