川西北地区固体沥青和油砂的有机地球化学特征与成因

Organic geochemical characteristics and origin of solid bitumen and oil sands in northwestern Sichuan

王广利 王铁冠 韩克猷 王兰生 师生宝

Wang Guangli Wang Tieguan Han Keyou Wang Lansheng Shi Shengbao

油气资源与探测国家重点实验室, 中国石油大学, 北京 102249 2. 中国石油大学 地球科学学院, 北京 102249 3. 中国石油 西南油气田分公司 勘探开发研究院, 成都 610051

State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China 2. College of Geosciences, China University of Petroleum, Beijing 102249, China 3. Institute of Petroleum Exploration and Development Research, Southwest Petroleum Division of CNPC, Chengdu, Sichuan 610051, China

川西北龙门山前山带不同时代固体沥青和油砂的烃类组成表现出一系列共有的地球化学特征。其碳同位素值小于-32‰,指示前寒武系来源。C29甾烷相对含量大于C27和C28甾烷,推测与蓝细菌的贡献有关,三芳甾烷的分布具有类似的特征。出现较为丰富的24-正丙基胆甾烷和24-异丙基胆甾烷,分别来自远洋藻类和海绵动物的贡献。高丰度的孕甾烷系列化合物,表征还原和静滞的水体环境。重排甾烷的缺失,高丰度的30-降藿烷、C35藿烷和C24四环萜烷,以及丰富的二苯并噻吩类化合物,进一步反映出缺氧环境和较高的碳酸盐岩含量。本区固体沥青和油砂所拥有的特征,表明其来源于震旦系陡山沱组烃源岩,并与国外来源于同一时代的原油具有可比性。除此之外,UCM和25-降藿烷的存在,表明古油藏保存和破坏过程中曾经遭受了生物降解作用。古油藏是陡山沱组烃源岩在生油窗早期的产物(Rc为0.57%~0.84%),构造运动所引起的持续抬升以及之后的风化剥蚀和生物降解作用可能是古油藏破坏的主要原因。

Hydrocarbon compositions extracted from a set of solid bitumen and oil sands collected in the northern Longmenshan Mountain, the northwestern Sichuan, South China, are unique and consistent. It is suggested that the bulk δ13C values (<-32‰) for the extracts and each fraction show overall depleted values, which is typical of pre-Cambrian source. The high concentrations of C29 steranes relative to C27 and C28 steranes can be explained by the contribution of cyanobacteria, and the distribution of triaromatic steranes has similar characteristics. 24-n-propylcholestane and 24-isopropylcholestane are abundant, which should be derived from marine chrysophyte algae and demosponges. The high abundance of pregnane and homopregnane could be associated with such specific depositional environment as anoxic to euxinic bottom waters. The absence of rearranged steranes, abundant 30-norhopane, C35 and C24 hopane tetracyclic terpanes, and a wealth of dibenzothiophene compounds, reflect the hypoxic environment and higher carbonate rock content. The features of solid bitumen and oil sands in the northwestern Sichuan indicate that they originated from the Doushantuo Formation of Sinian, and can be compared with foreign crude oils which generated during the same period. The presence of UCM and 25-norhopane proved biological degradation during the preservation and destruction processes of ancient reservoir. The hydrocarbons were generated by sulfur-rich kerogens at the early oil windows with Rc values in the range of 0.57%-0.84%. Tectonic uplift and the following erosion and biodegradation might explain the destruction of ancient reservoir.

国家自然科学基金项目(41172126),中国石油大学(北京)科研基金项目(LLYJ-2011-05)和中国石化海相前瞻性研究项目(YPH08024)联合资助。

https://doi.org/10.11781/sysydz201406731